The PHOIBOS mission Probing Heliospheric Origins with an Inner Boundary Observing Spacecraft A proposal to the (ESA) for a probe of the Solar Corona and inner in response to the call for “Cosmic Vision”

Co-Proposers

Milan Maksimovic LESIA & CNRS Observatoire de Paris Place Jules Janssen 92195 Meudon cedex France Tel : 331 4507 7669 Fax : 337 4507 2806 Email: [email protected]

Marco Velli Jet Propulsion Laboratory, USA & University of Firenze, Italy 4800 Oak Grove Drive Pasadena, California 91109 USA Tel : 1 818 354 4369 Email: [email protected]..gov [email protected]

The general scientific objectives of the PHOIBOS mission are being supported by a large scientific community distributed throughout the world. The PHOIBOS mission concept is proposed and sup- ported by the following people :

Vassilis Angelopoulos28, Thierry Appourchaux1, Bruno Bavassano2, Stuart D. Bale3, Matthieu Ber- thomier4, Lapo Bettarini33, Douglas Biesecker38, Lars Blomberg5, Peter Bochsler6, Volker Both- mer7, Jean-Louis Bougeret8, Andrew Breen39, Carine Briand8, Roberto Bruno9, Vincenzo Car- bone10, Patrick Canu4, Thomas Chust4, Jean-Marc Defise11, Thierry Dudok de Wit12, Luca Del Zan- na33, Anders Eriksson13, Silvano Fineschi35, Lyndsay Fletcher14, Keith Goetz15, Roland Grappin41, Antonella Greco10, Shadia Habbal36, Don Hassler16, Bernd Heber17 , Petr Hellinger18 , Tim Hor- bury19 , Karine Issautier8, Justin Kasper20, Ludwig Klein8, Craig Kletzing21, Säm Krucker3, Vladi- mir Krasnoselskikh12, William Kurth21, Rosine Lallement22, Philippe Lamy23, Hervé Lamy30, Simone Landi33, Olivier Le Contel4, Fabio Lepreti10, Dominique LeQuéau24, Robert Lin3, Milan Maksimovic8, Francesco Malara10, Ian Mann25, Ingrid Mann26, Lorenzo Matteini33, William Mat- thaeus37, Dave McComas27, Ralph McNutt43, Nicole Meyer-Vernet8, Zoran Mikic42, Michel Mon- cuquet8, Neil Murphy28, Zdenek Nemecek29, Emanuele Pace33, Filippo Pantellini8, Viviane Pier- rard30, Jean-Louis Pinçon12, Elena Podlachikova31, Raymond Pottelette4, Lubomir Prech29, Ondrej Santolik29, Robert Rankin32, Franco Rappazzo28, Marco Romoli33, Alain Roux4, Jana Safrankova29, Fouad Sahraoui4, Edward C. Sittler40, Charles W. Smith44, Luca Sorriso-Valvo10, Jan Soucek18, Pavel Travnicek18, Andris Vaivads13, Marco Velli28,33, Andrea Verdini33, Nicole Vilmer8, Robert Wimmer-Schweingruber17, Gaetano Zimbardo10, Thomas Zurbuchen34

1IAS, France; 2IFSI-INAF, Italy; 3University of California Berkeley, USA; 4CETP, Vélizy, France; 5KTH, Stockholm, Sweden; 6University of Bern, Switzerland; 7University of Goettingen, Germany; 8LESIA, Observatoire de Paris, France; 9IFSI-CNR, Italy ; 10Università della Calabria, Italy; 11CSL, Belgium; 12LPCE, Orléans, France; 13IRFU, Uppsala, Sweden; 14University of Glasgow, UK; 15University of Minnesota, USA; 16SWRI Boulder, USA; 17University of Kiel, Germany; 18IAP- CAS, Prague, Czech Republic; 19Imperial College, London, UK; 20MIT, Cambridge, USA; 21University of Iowa, USA; 22SA-IPSL, France; 23LAM, Marseille, France; 24CNRS, Paris, France; 25University of Alberta, Canada; 26University of Kobe, Japan; 27SWRI, San Antonio, USA; 28JPL, Pasadena, USA; 29Charles University, Prague, Czech Republic; 30IAS Brussels, Belgium; 31Royal Observatory, Brussels, Belgium; 32University of Alberta, Canada; 33University of Firenze, Italy; 34University of Michigan, USA; 35 Istituto Nazionale di Astrofisica, Torino, Italy; 36University of Hawaï, USA; 37University of Delaware, USA; 38NOAA Space Environment Center, Boulder, USA; 39University of Wales, UK; 40GSFC, USA, 41Luth, Observatoire de Paris, France, 42SAIC, San Diego, USA, 43APL, Baltimore, USA,44University of New Hampshire, Durham, USA .

TABLE OF CONTENTS TABLE OF CONTENTS...... 3 Executive summary...... 4 1) Introduction ...... 6 2) Scientific Objectives...... 6 2.1) Explore the fundamental processes underlying coronal heating and acceleration .7 2.2) Determine magnetic field structure and dynamics in the source regions of the fast and slow solar wind...... 10 2.3) What mechanisms accelerate, store and transport energetic charged particles? ...... 13 2.4) Explore dusty plasma phenomena and their influence on the solar wind and energetic particle formation...... 15 3) Mission Profile ...... 16 4) Payload description...... 19 4.1) Overview of all proposed payload elements...... 19 4.2 Baseline Payload...... 21 4.2.1 Fast Plasma Instrumentation...... 21 4.2.2 Ion Composition Analyzer (ICA)...... 21 4.2.3 Energetic Particle Instrument...... 22 4.2.4 DC Magnetometer...... 23 4.2.5 Radio and Plasma Wave Instrument (RPWI)...... 23 4.2.6 Neutron/Gamma-Ray Spectrometer (NGS)...... 24 4.2.7 Coronal Dust Detector (CD)...... 25 4.2.8 Hemispheric Imager (HI)...... 25 4.2.9 Polar Source Region Imager (PSRI)...... 26 4.2.10 Common Data Processing Unit (CDPU)...... 26 5) Spacecraft description...... 26 5.1) Spacecraft architecture...... 26 5.2) Key factors for power management...... 28 5.3) Spacecraft mass budgets...... 29 6) Science Operations and Archiving ...... 30 7) TRL & Key technology areas...... 30 7.1) Spacecraft TRL & Technology areas ...... 30 7.2) Payload TRL & Technology areas ...... 30 8) International partnership and Costs ...... 31 8.1) International partnership...... 31 9) Communications and Outreach ...... 31 Acknowledgements :...... 31 References...... 31 ANNEXE: SUPPORT LETTERS ...... 33

Sun's outer atmosphere, within the first few solar Executive summary radii. In this regards, there are both theoretical limi- Fifty years after the Sputnik launch and the begin- tations (in the understanding of the physics of the ning of the Space Physics era the time has come for coupling between photons and plasma) and experi- the in-situ exploration of one of the last frontiers in mental limitations (limited number of observables the solar system – the solar corona and inner helio- such as spectral lines or the hardly solvable inverse sphere. PHOIBOS (Probing Heliospheric Origins problem of the line of sight integration). On the other with an Inner Boundary Observing Spacecraft) is a hand solar wind in-situ measurements have had ac- mission of exploration and discovery designed to cess to the very detailed state of the local plasma make comprehensive measurements in the never- properties (full particles velocity distribution func- observed region of the heliosphere from 0.3 AU to tions, observations of the electromagnetic plasma as close as 3 solar radii from the ’s surface. fluctuations over a huge frequency range ) but at The primary scientific goal of Phoibos will be to locations far from the corona and the solar wind determine how magnetic field and plasma dy- acceleration region. namics in the outer solar atmosphere give rise to the corona, the solar wind and the heliosphere. To understand therefore the engine at the source Reaching this goal is a Rosetta-stone step for all of the solar wind one must probe the region from of astrophysics, allowing the understanding not the to about 20 Rs, where internal, only of the plasma environment generated by electric, magnetic and turbulent energy in the our own sun, but also of the space plasma envi- coronal plasma is channelled, through mecha- ronment of much of the universe, where hot nisms that are not completely understood, into tenuous magnetized plasmas transport energy bulk energy of the supersonic solar wind flow (see and accelerate particles over a broad range of Figure 1). scales. Moreover, by making the only direct, in-situ measurements of the region where some of the deadliest solar energetic particles are energized, PHOIBOS will make unique and fundamental contributions to our ability to characterize and forecast the environment in which future space explorers will work and live.

Scientific objectives The existence of the solar wind has been estab- lished for over forty years now, and abundant data has been accumulated concerning its average prop- erties. More recently, a number of satellite mis- Figure 1 : Model profiles of the solar wind speed (U) and sions carried out both in-situ measurements of the the Alfvén speed (Va) with distance from the sun. The solar wind in the region beyond 0.3 AU, as well as vertical bar separates the source region of the wind from remote-sensing measurements probing the solar the supersonic wind flow. PHOIBOS will be the first mis- atmosphere from the photosphere to the outer co- sion to fly in the source region of the wind. rona in the visible, and x-ray bands, with Understanding the solar wind is of fundamental nearly continuous coverage. This has led to major significance to all of astrophysics since it is the advances in our understanding of how the solar prototype for all stellar winds and related magnetic field and the magnetic activity cycle astrophysical flows. Approaching as close as three shape the solar wind’s structure and influence its solar radii from the surface will allow direct overall dynamics. detection of the plasma physical processes at work in coronal heating. The scientific objectives of the The fundamental mystery of how the solar corona PHOIBOS mission therefore fit perfectly within the is heated and the solar wind is generated remains scientific themes defined in the ESA Cosmic Vision unsolved, however, because of the major gap in our 2015-2025 program and more precisely the question knowledge of the sub-mercurean region of the “How does the Solar System work?“ heliosphere. Remote-sensing strategies have probed the coronal properties by analysing the International partnership photons emitted, scattered, or absorbed by the

The international scientific community has been for NASA. In such a situation, the PHOIBOS launch dreaming for a mission to explore the corona for a could occur in December 2018, as discussed in sec- long time. In the past the European Space Agency tion 3, or every 1.6 years later following the mission and European scientists have already been involved launch window. in several studies such as the Vulcan ESA As an alternative option, in the case there were an SCI(88)7 initiated by Giuseppe Colombo or the opportunity for an earlier launch and an ESA/NASA SCP proposal in 1993 for an M3 mission by E. agreement, PHOIBOS could be envisage as an M Marsch and A. Roux. On the NASA side, the num- mission with a cost sharing of 1/3 for ESA & mem- ber of past studies and reports that have been car- ber states and 2/3 for NASA. The launch could occur ried out in the last thirty years is even greater. The in this case in October 2015 or May 2017. most accomplished study is the 2005 Solar Probe with a Science & Technology Definition Team The PHOIBOS mission profile (STDT) chaired by D.J. McComas NASA/TM— The mission concept proposed here differs from the 2005 - 212786; (http://solarprobe.gsfc.nasa.gov/ present NASA scenario and is not meant to compete solarprobe_news.htm). These past efforts have with that Solar Probe: its purpose on the contrary is a led to the final conclusion that a probe to support collaboration with NASA in the event the nature of mankind’s first mission into the Solar Corona is the solar probe mission were to change. In particular technically feasible. were NASA were not to implement Solar Probe in its present STDT/2005 form, the PHOIBOS concept While the scientific team supporting this proposal could provide an interesting alternative scenario, in strongly hopes that NASA’s STDT/2005 Solar which: Probe mission will be developed and flown as soon (i) classical solar panels technology combined with as possible, the current PHOIBOS mission concept heat shield thermal generators are used as power is being proposed for a New Cosmic Vision 2015- systems instead of RTGs and 2025 study to demonstrate and emphasize the (ii) a new original trajectory combined to the use of a strong European scientific community interest in a plasmic electric propulsion system are implemented solar probe mission. With , and in order to reach the final scientific orbit instead of SOHO Europe has been involved in a leading posi- the classical Jupiter low altitude swing-by with a tion with respect to space-based solar and helio- very high C3 for escape. spheric research during the last two decades. The This final orbit is highly elliptical with a perihelion strong European participation in STEREO and a at 4 solar radii (3 radii from the surface), an aphelion timely launch of would keep some of at 3.5 AU and an inclination of 60 deg. with the the momentum of the last decades. It must be em- ecliptic plane. The important and interesting point phasized that the top priority for the European solar considering this alternative scenario is that, de- and heliospheric community is realization of the spite its apparent complexity, it will allow more Solar Orbiter within the 2015 time-frame. The flexibility in the fine adjustment of the final orbit. realization of an ambitious mission such as It may avoid the possible drawbacks of a Jupiter PHOIBOS would complement and build upon swing-by such as the high departure V∞, the risky Orbiter science to finally resolve one of the fun- low altitude Jupiter swing-by at 5.5 AU with expo- damental problems in all of astrophysics: the sure to significant radiation levels and no subsequent existence of stellar coronae and the driving of ability to change the trajectory. solar and solar-type winds. Such knowledge is also paramount to our understanding of the Earth’s With the proposed final orbit, PHOIBOS will spend space environment and the influence of space envi- up to 10 days making comprehensive measurments ronment on the Earth and space exploration. in the never-observed region of the heliosphere from 0.3 AU to 3 solar radii from the Sun’s surface and up The PHOIBOS mission is to be developed in a to about 20 hours at distances closer than 10 solar joint collaboration between NASA, ESA and radii. member states. The international partnership and sharing of responsibilities will be discussed be- The Payload concept tween the concerned space agencies. The overall Concerning the payload necessary to reach our scien- mission cost is estimated to be around 900 Meuros tific objectives, the current proposal has greatly been (see section 8.3). We propose therefore to imple- inspired by previous Solar Probe proposals and espe- ment PHOIBOS as an L mission with a cost shar- cially by the last NASA STDT/2005 report. To meet ing that could range between 1/2 to 2/3 for ESA & the PHOIBOS science objectives, we recommend a member states and consequently between 1/2 to 1/3 payload comprising both comprehensive in-situ and

- 5 - relevant limited remote-sensing instruments. The point within 30 Rs. These measurements must be PHOIBOS payload is described in section 4. done continuously in time during all phases of the near encounter period in order to ensure wave 1) Introduction mode identification or not miss brief spatial struc- tures such as discontinuities and shocks. PHOIBOS, a mission of exploration and discovery to the inner frontier of the solar system, is designed 2) Scientific Objectives to understand how the solar corona is formed and the solar wind accelerated. This mission will access The solar wind flow at solar minimum is subdivided the source regions of the heliosphere with in-situ into high and low speed streams, with speeds of measurements and possibly images of the solar around 750 km/s and 400 km/s respectively. The photosphere, the corona and the polar magnetic Ulysses mission has shown that the fast wind is the fields. With a perihelion well within the region basic outflow from the corona at solar minimum, where the magnetic field still guides the plasma while the much more irregular slow solar wind is outflow and the solar wind kinetic energy is domi- confined to the equatorial regions, presumably aris- nated by that of the sun’s magnetic field – the ing from regions adjacent or inside the streamer belt. Alfvén critical point, somewhere between 10 and As the solar cycle progresses, the streamer belt ex- 20 Rs - PHOIBOS will be able to access the regions pands in latitude so that, at activity maximum, the of the corona where the plasma turbulence is corona appears to be nearly uniformly distributed strongest. It will most probably reach, at perihelion, around the solar disk, while high speed wind streams the location of the proton temperature maximum occur over a much smaller volume. This is illustrated and probe the region where fast solar wind accel- by the ‘dial plot’ in Figure 2.1, (McComas et al. eration is occurring. At perihelion, PHOIBOS falls 2003) which depicts solar wind speed measurements within helmet streamers at solar minimum, and will as a function of latitude during the first and second probably spend a significant time within closed Ulysses orbits, near times of solar minimum and field line regions at . With the three maximum activity respectively. orbits planned, the conditions at solar maximum and solar minimum will be explored and compared. As a result PHOIBOS will determine by which physical mechanisms the outer solar atmosphere gives rise to the corona, solar wind and helio- sphere throughout the sun’s activity cycle.

The main scientific objectives of PHOIBOS are to:

1 - Explore the fundamental processes underly- ing coronal heating and solar wind acceleration 2 - Determine magnetic field structure and dy- namics in the source regions of the fast and slow solar wind. 3 - Determine what mechanisms store, acceler- ate and transport energetic particles. 4 - Explore dust and dusty plasma phenomena in the vicinity of the Sun and their influence on Figure 2.1 : Polar plots of solar wind speed as a function the solar wind and energetic particle formation. of latitude for Ulysses’ first two orbits, superposed on solar images characteristic of solar min (8/17/96, left To address these objectives experiments aboard panel) and max (12/ 07/00, right panel), EIT , LASCO C2 images from SOHO and Mauna Loa K-coronameter PHOIBOS will: (McComas et al., 2003). These images show how the solar wind characteristics measured in-situ depend strongly on Measure the plasma and its composition, elec- the solar coronal magnetic field structure, fast wind ema- tromagnetic field, energetic ions and electrons nating from coronal holes and slow wind appearing to and their properties in the region from 60 Rs to originate from the magnetic activity belt. 4 Rs; monitor neutron and gamma ray emission from the sun; image the corona in white The fast solar wind, with average speed around 750 and possibly EUV; determine the photospheric km/s, originates from regions where the coronal elec- magnetic field at high latitudes from a vantage tron temperature is lower. This inverse correlation

- 6 - between flow speed and coronal electron tempera- Several fundamental plasma physical processes – ture where the freezing in of minor ion charge waves and instabilities, magnetic reconnection, states occurs (Figure 2.2) shows that the foundation velocity filtration, turbulent cascades – operating of the original theory of the solar wind (Parker, on a vast range of temporal and spatial scales are 1958), i.e. that high coronal electron temperatures believed to play a role in coronal heating and so- and electron heat conduction drive the solar wind lar wind acceleration, but the lack of magnetic expansion, needs to be reconsidered. SOHO meas- field and detailed plasma measurements in the urements of the very high temperatures of the cor- region inside 70 Rs does not allow their validation onal ions, together with the persistent positive cor- or confutation at this time, and though Solar Or- relation of in-situ wind speed and proton tempera- biter and Solar Sentinels will move inside 0.25 AU ture, suggest that other forces, namely magnetic or 53 RS before 2020, only PHOIBOS will explore mirror and wave-particle interactions should also the critical regions within 20 Rs. contribute strongly to the expansion of the outer corona. Basic unanswered questions concern the storage, transport, and release of the mechanical energy re- quired for coronal heating; the specific mechanism(s) for the conversion of energy between the magnetic field and thermal particles; the dynamics of photo- spheric and coronal magnetic fields in the source regions of the solar wind; and the sources of high- energy particles and the mechanisms by which they are accelerated.

These questions motivate three broadly distinct but interlinked top-level PHOIBOS objectives. A fourth

top-level objective of an exploratory nature concerns Figure 2.2 : Anti-correlation of solar wind speed (dot- the source, composition, and dynamics of dust in the dashed line) with the freezing in temperatures deter- inner solar system. In the following sections, these mined from O7 to O6 abundances (blue line), and mag- four main objectives are translated into specific sci- nesium to oxygen composition ratios (red line) as a entific questions and basic measurement require- function of time during a low-latitude high-speed low- ments. speed wind crossing period in Ulysses’ first orbit. 2.1) Explore the fundamental processes underly- SOHO observations have also made important ing coronal heating and solar wind acceleration contributions to our knowledge of the slow solar wind, which is confined to regions emanating from The solar corona loses energy in the form of radia- the magnetic activity belt and seems to expand in a tion, heat conduction, waves, and the kinetic energy bursty, intermittent fashion from the top of helmet of the solar wind flow. This energy must come from streamers, seen to expand continuously, in X-rays, mechanical energy residing in photospheric convec- by . A third type of flow arises from larger tion, the solar magnetic field acting both to channel eruptions of coronal magnetic structures, or cor- and store this energy in the outer atmospheric layers. onal mass ejections (CMEs), which also lead to However, the mechanisms by which the energy is acceleration of high-energy particles. As the solar transferred and dissipated to generate the hot corona, activity cycle progresses, Ulysses has shown how solar wind, and heliosphere throughout the Sun’s the simple fast-slow structure gives way to a much activity cycle remain one of the fundamental unan- more variable, but typically slower, solar wind at swered questions in solar and heliospheric physics. activity maximum, apparently originating not only from the much more sparse coronal hole regions Remote-sensing measurements of the solar corona and quiet sun, but also from coronal active regions. and in-situ measurement of particle distribution func- tions in the fast and slow solar wind streams have The energy that heats the corona and drives the shown that the heating process is correlated with wind is believed to come from photospheric mo- magnetic structure. SOHO/UVCS observations using tions and is channeled, stored and dissipated by the the Doppler dimming technique (Li et al., 1998; magnetic fields that emerge from the photosphere Kohl et al., 1998) (Figure 2.3) and interplanetary and structure the coronal plasma. scintillation measurements (Grall et al. 1996) indi- cate that the high speed solar wind is rapidly acceler-

- 7 - ated near the Sun, reaching speeds of the order of in the slow wind. The difference between the fast 600 km/s within 10 Rs. Observations of comet and the slow solar wind extends to the shape of the C/1996Y1 confirm a most probable speed of about particle distribution functions. The fast wind exhib- 720 km/s for the solar wind at 6.8 RS (Raymond et its proton perpendicular temperatures which are al., 1998). slightly higher than the parallel temperatures. Proton distribution functions in the fast wind also present a beam accelerated compared to the main distribution by a speed comparable to the Alfvén speed, a feature shared by the alpha particles. Turbulence is also different in fast and slow streams, with fast streams containing fluctuations in transverse velocity and magnetic fields which are more strongly correlated in what is known as Alfvénic turbulence, a well- developed spectrum of quasi-incompressible waves propagating away from the sun. In the slow wind no such preferred sense of propagation is observed, while larger density and magnetic field magnitude fluctuations are present, revealing a much more stan- dard and evolved MHD turbulent state (Grappin et

Figure 2.3 : Acceleration profiles of the fast solar wind al. 1990). after SoHO: H0 (red) and O+5 (black) flow velocities from Kohl et al. (1998), OVI (green) flow velocities from Anti-correlation of wind speed with coronal (freez- Antonucci et al. (2004); Full-dashed blue lines : plume ing-in) electron temperature and the heliospheric and interplume flow velocities from Gabriel et al. (2005) distribution of the high speed wind at solar minimum (Figure 3) place the origin of the fast wind in coronal Such rapid acceleration appears to result from the holes. Measurement from the CDS -SUMER ex- extremely large and anisotropic effective tempera- periments aboard SOHO have ascertained that the tures in the lower corona, which have been meas- electron temperature is bounded by 106 K (David et ured by SOHO/UVCS in coronal holes, though not al 1998), in agreement with the brightness tempera- directly for protons, the main solar wind constitu- ture based on radio observations of the corona. This ent. These temperatures are much higher perpen- presents a discrepancy with the freezing in tempera- dicular to the magnetic field. The fast solar wind ture for different ion charge states measured in-situ measured in situ shows what may be a relic of this by the SWOOPS experiment on Ulysses, the most anisotropy, smaller than that inferred from coronal direct interpretation of which requires an electron observations, but persisting in the distance range temperature maximum of about 1.5 106 in coronal from 0.3 to 5 AU. Proton, alpha-particle, and minor holes. The discrepancy may be resolved by only by ion distribution functions in the fast wind also pre- admitting strongly non-maxwellian distribution func- sent a non-thermal beam-like component whose tions for the electrons, or large differential flow speed is comparable to the local Alfvén speed. All speeds between ions of the same charge in the co- these properties suggest that Alfvén or ion- rona, which could have strong implications on the cyclotron waves play a major role in coronal heat- structure of the fast solar wind in the acceleration ing and solar wind acceleration in high-speed wind. region. Measurements close to the sun, within the region where the solar wind becomes supersonic to Alfvén Contrary to proton distributions, observed electron waves, are necessary to remove ambiguity due to in velocity distribution functions (eVDFs) exhibit non- situ evolution and obtain direct measurements Maxwellian features whatever the type of wind, slow where the main acceleration is occurring. or fast, in which they are observed. The eVDFs per- manently exhibit three different components : a The fast solar wind flow is steady, with fluctua- thermal core and a supra-thermal halo, which are tions in radial speed of order 50 km/s, and the always present at all pitch angles, and a sharply charge-state distributions indicate a low freezing-in magnetic field aligned “strahl” which is usually an- temperature. The slow solar wind is variable, with tisunward-moving (Rosenbauer et al., 1977). Energy higher but variable freezing-in temperatures. The transport and dissipation mechanisms strongly de- composition of the fast and slow wind also differ, pend on the mean free path of particles in the coronal Mg and Fe being overabundant with respect to O in plasma, which varies drastically both with distance the slow wind. Solar wind protons and ions are from the Sun (from the base of the corona to the however typically hotter in high speed streams than - 8 - supersonic solar wind), as well as across coronal Alfvén speed, low frequencies and strong structur- structures (coronal holes to helmet streamers). This ing of the corona (Velli, 1993). Waves reaching dependence has led to the suggestion that coronal the lower corona should therefore be shear Alfvén heating arises from energy stored in non-thermal waves, although discrete coronal structures such as wings of particle distribution functions generated loops and plumes might channel surface waves between the chromosphere and transition region or, and propagate energy as global oscillations as more generally, in the region where the solar at- well. mospheric plasma changes from collisional to col- lisionless. The higher temperatures and subsequent Simulations show that, in a highly stratified at- outflows would then arise naturally through veloc- mosphere, the nonlinear interactions of Alfvén ity filtration by the Sun’s gravitational potential waves launched from the photosphere are able to (Scudder, 1994), and may even explain the exis- generate and sustain an incompressible turbulent tence of the fast solar wind (Maksimovic et al., cascade, which displays the observed Alfvénicity. 1997; Zouganelis et al, 2004). The efficiency of turbulence in transporting en- ergy to the dissipative scales is, however, still The different properties of the low-frequency elec- unclear. The spectral slope at different coronal tromagnetic field and velocity fluctuations ob- heights evolves with distance, subject to expansion served in the fast and slow solar wind are further and driving effects, affecting the radial depend- evidence of the role played by turbulence and ence of dissipation. The initial spectrum of Alfvén wave-particle interactions in coronal heating. Fast waves in the photosphere cannot be constrained by streams contain stronger fluctuations in transverse in-situ data collected in the far solar wind, since velocity and magnetic fields, and display a higher local processes contribute to its shaping there degree of correlation between the velocity and (Verdini and Velli, 2007). magnetic fluctuations (often described as a well- developed spectrum of quasi-incompressible Only observations close to the solar surface will Alfvén waves propagating away from the Sun). In help in constraining the shape of the Alfvénic the slow wind, this correlation occurs at a much spectrum with relevant implications on the role of lower level, while larger density and magnetic field turbulence in the acceleration of the solar wind magnitude fluctuations are present, indicating a and the heating of the corona (Figure 2.4). more evolved MHD turbulent state there. This dif- ference in turbulence state between fast and slow wind streams, together with the fact that slow wind distribution functions are much closer to equilib- rium, suggests that the outward propagating wave flux contributes to the heating of the steady fast wind, while the slow wind is heated much more variably. It is not known, however, how the turbu- lent activiy increases toward the Sun, whether it is sufficient to power coronal heating and solar wind acceleration, and how it is driven by time- dependent events in the photosphere, chromos- Figure 2.4 : The rms amplitudes u and b (in velocity phere, transition region, and low corona (see e.g., units) as functions of heliocentric distance for a photo- spheric Kolmogorov spectrum with u = 40 km s-1 at the Mathtaeus et al, 1999). coronal base. Far symbols indicate observational con- straints from in-situ measurements and inter-planetary Whether the solar corona is heated by low- scintillation, near symbols from remote sensing. PHOI- frequency waves resulting from motions natu- BOS should reach inside the fluctuation maximum re- rally arising in the photosphere or whether the gion, measuring spectra and correlations where the gap dominant energy source resides in the currents in the data is (Verdini and Velli, 2007). stored via slower field line motions has been the subject of strong debate. Among the MHD By providing the first in situ measurements of the waves, only Alfvén waves would appear to sur- distribution functions, waves, turbulence, and elec- vive the strong gradients in the chromosphere tromagnetic fields from 0.3 AU to 4 Rs, and by cor- and transition region, because slow modes relating them with plasma and magnetic field struc- steepen into shocks while fast modes suffer total tures, PHOIBOS will be able to answer the basic reflection. Transmitted waves propagate at large questions of how the solar corona is powered, how angles to the radial direction, due to the large the energy is channeled into the kinetics of particle distribution functions in the solar corona and wind,

- 9 - and how such processes relate to the turbulence and any latitudinal gradient in the average field at the wave-particle dynamics observed in the helio- coronal base must be washed out by transverse non- sphere. The comprehensive measurement of radial expansion closer to the Sun. Flux tube expan- plasma and electromagnetic fluctuations in the sion is a natural effect of the combined decrease in inner solar wind (<20 RS), will determine how the magnetic field and currents induced by the accelerat- energy that powers the corona and wind is dissi- ing solar wind flow, and models suggest that it oc- pated and what the dominant dissipative structures curs out to radial distances > 10 Rs. Based on Ulys- are as well as the frequency spectrum of electro- ses data, the magnitude of the average polar mag- magnetic fluctuations. Small-scale magnetic recon- netic field has been estimated to be 6 G at solar nection occupies an important place in current dis- minimum, though values up to 15 G in the photo- sipation theories of the coronal plasma (Matthaeus sphere may not be excluded. At present, there are no et al. 2003). An important set of investigations on direct measurements of the polar magnetic field be- PHOIBOS will therefore be the multi-instrument low 1.5 AU (Sittler and Guhathakurta, 2002). By detection of signatures of small-scale reconnection, measuring the radial magnetic field in situ along its such as bi-directional plasma jets, accelerated par- trajectory, and remotely sensing the polar photo- ticles, magnetic field, and velocity gradient correla- spheric field at the same time, PHOIBOS will allow tions along the trajectory. PHOIBOS’s measure- a complete description of magnetic field and solar ments of the properties of turbulence and wind expansion free from unknown parameters. nonlinear plasma dynamics in the corona and These measurements will provide both a test of exist- solar wind will be a watershed for all of astro- ing models of coronal structure and rigorous con- physics, where these phenomena are invoked straints on future coronal models. over widely different contexts, from accretion disks to the collisionless shocks occurring in The magnetic network in the quiet Sun looks re- galaxy-cluster formation. markably similar to the network in coronal holes in spectral lines formed at lower, transition region tem- Measurement Requirements peratures, while it is harder to distinguish in lines formed at 106 K. If a similar coronal heating mecha- • Magnetic field, velocity field, and density fluc- nism is at work in both the quiet Sun and coronal tuations and spectra holes, any difference in their appearance is presuma- • Particle distribution functions of protons, elec- bly related to the magnetic field topology, including, trons, alpha particles, and possibly minor ion spe- perhaps, its time dependence. The larger densities, cies; suprathermal populations apparently higher electron temperature, and different • 3-axis electric and magnetic field measurements chemical composition of the quiet Sun would then be • Coherent structure identification using electric the result of a larger filling factor of closed magnetic and magnetic wave-form data field lines compared with that in coronal holes. • Plasma wave measurements (electron density, While the imprint of the coronal holes and of the temperature, velocity) and high resolution wave- equatorial helmet streamers in the solar wind meas- form data for electromagnetic fields. ured in-situ is well visible in the form of fast and slow wind streams and embedded plasma sheet, the • High frequency measurements approaching the fate of the quiet Sun corona is unknown. Is the proton cyclotron frequency. plasma in the quiet Sun confined by closed magnetic • Electron temperature gradient, density gradient, field lines, so that the fast wind is entirely of coronal electric field/ interplanetary potential. hole origin? Or is there a mass loss from the quiet Sun as well, and if so, what is its speed and how does 2.2) Determine magnetic field structure and it merge with the surrounding solar wind? dynamics in the source regions of the fast and slow solar wind. The magnetic field in active regions above sunspots provides the strongest confinement of hot plasma in The geometry of the magnetic field expansion in the corona and is seen as bright x-ray loops, which the inner corona, from the photosphere out to a few often end in cusp-like shapes at their summit. At solar radii, plays a fundamental role in determining greater heights, these develop into streamers, which density distribution and solar wind speeds in solar at solar minimum are large and elongated and form a wind models, as the field lines define the flow belt around the solar magnetic equator. Remote sens- tubes along which mass and energy flux are con- ing observations by SOHO/UVCS of the EUV emis- served. Ulysses observations have shown that the sion lines of minor ions, combined with multi-fluid radial magnetic field component measured in the models, provide some clues about the source regions fast wind is largely independent of latitude, so that

- 10 - of the slow solar wind in coronal streamers, but the microstreams and pressure-balanced structures. magnetic field topology in these regions and the These are fluctuations in radial velocity that last role it plays in plasma outflow are unknown. about sixteen hours in the spacecraft frame and have a magnitude on the order of 50 km/s. By flying The complexity of the magnetic field structure through coronal holes over a range of distances from increases with increasing activity during the solar 30 to 8 Rs PHOIBOS will observe and cross coronal cycle. At activity maximum, disk observations plumes or their remnants, estimate their filling factor show the existence of very complicated loop struc- and contribution to the overall solar wind flow, and tures, and images of the extended corona show assess the expansion factors of the flow tubes carry- streamers protruding from the solar surface not ing the solar wind flow. These observations will only in the equatorial regions but at all latitudes make it possible to clarify how microstreams form around the disk as well. PHOIBOS will determine and evolve and to determine what their relationship where the slow solar wind forms in and around to coronal fine-scale structures is. Achieving this streamers and whether specific magnetic signa- objective will require both in-situ measurement of tures, such as embedded current-sheets, are associ- the magnetic field and plasma velocity and full dis- ated with its formation. Further, studies of solar tribution function (density temperature and composi- wind sources during periods of solar maximum tion of solar wind) to identify individual flow tubes indicate a contribution to the wind from inside and use of the tomographic reconstruction technique active regions as well. PHOIBOS will determine of the all sky white-light coronagraph, which will the topology of magnetic field lines within active provide information on the filling factor and geomet- regions that give rise to solar wind flow. rical distribution of plumes.

PHOIBOS will travel over coronal holes, the quiet The LASCO and UVCS on the SOHO Sun, and the active solar corona at distances be- mission have made important contributions to our tween 9 and 4 Rs and under both solar minimum knowledge of the origins of the slow solar wind and maximum conditions. It will the origin of streams around helmet streamers. Sequences of the fast and slow wind and correlate the flow speed LASCO difference images obtained in 1996 (sunspot with closed/open magnetic field line topologies, as minimum) give the impression of a quasi-continuous measured by photospheric field measurements and outflow of material in “puffs” from the streamer belt determined indirectly through the in-situ measure- (Sheeley et al., 1997). A quantitative analysis of ment of such parameters as electron and energetic moving features shows that they originate above the particle bi-directional streaming. Relating the in- cusp of helmet streamers and move radially outward, situ coronal observations with surface structures with a typical speed of 150 km/s near 5 Rs, increas- will require remote sensing: ecliptic viewing of the ing to 300 km/s at 25 Rs . The average speed profile white light corona to trace field lines in the plane of is consistent with an isothermal corona at the tem- the PHOIBOS orbit, tomographic images from the perature T ≈ 1.1 X 106 K (UVCS/SOHO measure- all-sky coronagraph to identify coronal structures ments indicating a temperature 1.6 X 106 K in the in the local spacecraft environment, and a polar streamer core, at activity minimum) and a critical view of the photosphere and photospheric magnetic point near 5 Rs. The ejection of material may be fields from the spacecraft perspective to identify caused by loss of confinement due to pressure-driven and locate the source region structures. instabilities as the heated plasma accumulates or to current-driven instabilities (tearing and or kink-type White light and UV coronograph-spectroscopic instabilities) in the sheared field of the streamer. observations also show coronal holes to be far from PHOIBOS will cross the paths of these ejecta from featureless as well. Bright striations, or plumes, can streamers and will ascertain whether the ejection of be traced all the way from the solar surface out to coronal material occurs in a continuous flow or 30 Rs .The relationship of plumes to the fast wind is whether the puffs are in fact disconnecting plas- poorly understood. They appear above X-ray bright moids. If the latter, PHOIBOS will determine the points in the coronal holes, and are denser than the magnetic field configuration of the plasmoid as well surrounding regions. UV lines in the plumes appear as the magnetic morphology at the point of discon- to be narrower (i.e., the plumes are cooler) than in nection in the corona. Comparison of Galileo radio the darker lanes separating them, while measure- data with UVCS/SOHO images clearly shows the ments of outflows suggest that the dark lanes are association of the slow wind with streamer stalks, preferential outflow regions (Teriaca et al., 2003). that is, with the regions above the cusps of helmet Fine-scale structures are observed in the fast wind streamers that include the current sheet (Habbal et as well as in coronal holes, including the so-called al., 1997). It is not known, however, whether there is

- 11 - a single current sheet that runs along the nearly definitely intermittent solar wind component is pre- equatorial strip of maximum brightness in the sent in the form of CMEs and the fine-scale structure white corona, i.e., along the streamer belt (as sur- of the solar wind from active regions supports at mised by Wang et al., 1997), or whether there are a least a spatially structured origin for the various flow number of stalk/sheet structures of finite longitudi- streams. More generally, smaller CME-like events at nal extent. Nor is the structure of current sheets in all scales could contribute significantly to the solar streamer stalks known. Do they have a simple wind throughout the activity cycle. structure, or are they made up of multiple sheets in a more complex magnetic field morphology, as is Direct in-situ measurements of the structure of the suggested in part by UVCS/SOHO measurements solar wind, of the ion and electron distribution func- (Noci et al., 1997) and multiple current sheet cross- tions, as well as elemental abundance variations ings in-situ (Smith, 2001). close to the sun are required to understand the source regions of the wind. PHOIBOS will directly measure As observed in-situ at large distances from the Sun, both the electron distribution function and flow the solar wind appears as a continuous plasma out- speeds of minor ions in the coronal hole, and, at peri- flow. Its quasi-steady character may be a property helion, may directly sample composition differences of the outflow at the solar source. However, the on closed and open fields. By continuous direct sam- apparently quasi-stationary wind may also result pling the plasma flow as it moves close to the Sun, from a number of spatially limited, impulsive PHOIBOS will be able to assess the space and time events that are distributed over smaller scales filling factor of the fast solar wind, while imaging (Neugebauer, 1991, Feldman et al. 1997). There is the coronal structures that it will cross in the range abundant evidence for the intermittent or “pulsed” above 10 Rs. The time-dependent variability ob- (Feldman et al., 1997) character of the high-speed served in the wind might also increase close to the wind: observations of microstreams and persistent sun, leading to effects of multiple sources observable beam-like features in the fast wind; interplanetary by PHOIBOS, for example from a multitude of scintillation measurements of field-aligned density bursty events or micro CME’s. structures having a 10:1 radially-aligned axial ratio and apparent field-aligned speeds ranging from To locate the source regions of the solar wind, in- ~400 to ~1280 km/s (Coles et al. 1991; Grall et al. situ ion, plasma and magnetic field measurements at 1996); and remote sensing observations of the high resolution are necessary covering an extended chromosphere, transition region, and corona reveal- field of view due to the contribution to aberration of ing explosive, bursty phenomena, dubbed mi- Alfvénic turbulence (max estimated to be 200 km/s, croflares, associated with magnetic activity over or ∆u/U=0.3), together with images both of the un- an extremely wide range of energy and time scales. derlying photospheric field and the quasi- Feldman et al. (1996, 1997) have interpreted the simultaneous coronograph all sky images as well as fine-scale structures observed in the fast wind as context from telescopes in earth orbit. Bi-directional remnants of spicules, macrospicules, X-ray jets, electron streaming could identify position on and H-alpha surges and hypothesize that the fast closed/open field lines. Note that for fast particles, wind results from the superposition of transient the aberration is essentially due to db/B (max esti- reconnection-generated jets. If this hypothesis is mated in the range 0.2-0.3). In coronal holes, fila- correct, then the heating of the corona leading to its mentary structures such as coronal plumes are ob- time-dependent acceleration to form an ensemble served out to 30 Rs, the range of speeds between 300 of outward-going jets could be accompanied by the km/s (perihelion) implies speeds across plumes in annihilation of oppositely-directed magnetic flux the range 100-200 km/s. With an expected size less 3 5 bundles clustered near the magnetic network, in from 10 - 10 km at 8 Rs, crossing time of an indi- turn leading to transient hard X-ray and gamma-ray vidual plume should last between 5s to 1 hour. Dur- bursts, along with neutron production in the 1 to 10 ing that time one must ensure a sufficient number of MeV energy range, which could be detected by plasma, particle and velocity measurements. Radio PHOIBOS. measurements at the plasma frequency appear also essential to give a separate and independent measure For the slow solar wind, evidence in favor of an of density, speed and temperature of the core of the intermittent origin is even more abundant. As men- electron distribution function. tioned above, blobs of plasma appear to be lost by helmet streamer structures overlying active regions Measurement Requirements. and various mechanisms have been proposed for this process. At solar maximum, an important and

- 12 - • Full distribution function measurements (density the intensities by orders of magnitude, and leads to temperature and composition of solar wind as a mixing of particles from different acceleration sites. proxy for individual flow tubes) PHOIBOS measurements, made at distances as close • In situ magnetic field and plasma velocity at to the Sun as 4 RS, will not suffer from transport high cadence in inner heliospheric regions (below effects because the Probe will sample energetic par- 20 Rs), continuous, lower cadence below 0.3 AU ticles close to their acceleration sites on the Sun and • Density, temperature and composition of solar will explore, in situ, acceleration sites in the corona wind and inner heliosphere. In particular, recent results • Electron distribution function and strahl (bidi- from ACE, SOHO, and WIND point to the increas- rectional streaming as evidence of closed magnetic ing importance of the high corona (2RS < r < 20RS) field line topology and correlations with composi- as an acceleration site for energetic ions and elec- tion and wind speed and magnetic field) and high trons—a region that PHOIBOS will sample directly. energy tails of proton helium distribution functions These measurements will address key questions im- at high cadence. Time-dependent neutron and portant for understanding solar energetic particle gamma-ray energy spectra. acceleration and transport. • Energetic electrons and ions • Photospheric magnetic field measurements at Simultaneous solar observations from 1 AU it will be high latitude and line of sight velocity fields possible to trace events observed by PHOIBOS to the flare site, to measure the flare properties, and to • All sky coronograph measurements of coronal obtain the underlying magnetic field configuration. structure above 10-20 R . s In addition to composition measurements, PHOIBOS 2.3) What mechanisms accelerate, store and will measure near-relativistic (V > 0.1 c) electrons transport energetic charged particles? from these events within a fraction of a minute of their release. These electrons are particularly impor- tant for untangling acceleration processes because Solar energetic particle events (SEPs) come in two their acceleration sites can be senses remotely by distinct categories. Gradual events are accelerated microwave radio emission or hard x-rays. Analo- by CME-driven shocks and are characterized by gously for energetic ions, PHOIBOS may also ob- roughly coronal abundances and charge states. serve gamma rays and neutrons from these Impulsive events are generally much smaller events events, providing information on the accelerated associated with impulsive x-ray flares and are char- 3 particle components on closed field lines in the solar acterized by enrichments in He, heavy ions such atmosphere. as Fe, and electrons, with charge states characteris- tic of temperatures ranging from ~5 to 10 MK. This Although the occurrence rate of SEP events is paradigm distinguishes between two separate ac- greatly reduced at solar minimum, strong evidence celeration processes and acceleration sites, both suggests that particle acceleration occurs continu- driven by eruptive events on the Sun: a) CME- ously on the Sun or in the inner heliosphere,. All driven shock acceleration starting in the high co- solar wind species that have been measured (H+, He+, rona and continuing into interplanetary space and and He++) exhibit suprathermal tails that extend up b) acceleration at the flare site, presumably driven from several times the solar wind speed (~10 by magnetic reconnection. Both processes are keV/nucleon). These tails are more prominent in the known to operate in larger SEP events, and studies ecliptic than over the poles, and they are continu- at 1 AU during solar cycle 23 present a complex ously present, even in the absence of solar activity or picture of events that often exhibit characteristics interplanetary shocks (e.g., Gloeckler et al. 2000). of both gradual and impulsive SEP events (e.g., The fact that even interstellar pickup He+ exhibits a Cohen et al., 1999; Cane et al., 2003; Tylka et al., suprathermal tail suggests that the acceleration oc- 2004). In addition to such transient energetic curs in the inner heliosphere (e.g., Ruffolo et al., events, observations at 1 AU show a continual TBD). However, evidence also indicates that 3He is outflow of intermediate-energy particles from the continuously accelerated at the Sun, even during the Sun extending from suprathermal energies to >10 quietest periods, suggesting that more or less con- MeV/nucleon. The mechanisms responsible for the tinuous acceleration may be occurring in microflares acceleration of these particles are not known. such as those reported by RHESSI (Krucker et al. 2002). The small-scale, randomly occurring “com- Distinguishing the various acceleration processes ponent” reconnection that typifies microflares may occurring at the Sun on the basis of data acquired at be an indicator of a scale-invariant dissipation proc- 1 AU is difficult. Transport through the interplane- ess that not only heats coronal plasma, but also pro- tary medium washes out the time structure, reduces

- 13 - duces a stochastic component of the electric field the Sun, the density and energy spectrum of su- that contributes to particle acceleration. Hard x-ray, prathermal seed particles, and the spectrum of mag- gamma-ray, and neutron observations by PHOI- netic turbulence directly. It will thus be able to ascer- BOS can also reveal the occurrence of sporadic tain the presence of shocks and discontinuities and and/or continuous particle acceleration on the Sun. determine their role in particle acceleration. Solar neutron observations on PHOIBOS are of special interest because low-energy neutrons that The probability that PHOIBOS will encounter parti- do not survive to 1 AU can only be observed close cle intensity levels characteristic of large SEP events to the Sun. (~1 MeV (10 MeV) neutron intensities at 1 AU (e.g., >100 particles/cm2sr-s with E >10 10 6 at 5 RS are ~1.5 x 10 (3.7 x 10 ) times greater MeV) is about 80% during solar maximum condi- than at 1 AU.) Neutron observations close to the tions (Feynman et al., 2000). It is much less likely, Sun may reveal evidence of small nanoflares, however, (~10-20% probability) that the PHOIBOS which have been suggested as a principal source of flyby will take place while a CME-driven shock is energy for heating the corona. accelerating >10 MeV particles inside 100 RS. None- theless, PHOIBOS measurements of the ambient To forecast large SEP events reliably, it is neces- conditions that exist prior to such events will be of sary to determine why some CMEs accelerate par- enormous value to our efforts to understand SEP ticles more efficiently than others. The suggested acceleration and transport. possibilities include: (1) the presence or absence of a pre-existing population of suprathermal ions, left Ulysses measurements have shown that solar ener- over either from a previous gradual event (e.g., getic particles (SEPs) can reach high latitudes Kahler 2001) or from small impulsive flares (Ma- (McKibben et al., 2001). Three explanations for son et al., 1999); (2) the presence or absence of these observations have been proposed: (a) the CME successive, interacting CMEs (Gopalswamy et al., shocks accelerating the particles extended to high 2002); (3) pre-conditioning and production of seed- latitudes and crossed the interplanetary magnetic particles by a previous CME (Kahler, 2001); (4) field lines connecting to Ulysses; (b) significant par- improved injection efficiency and acceleration rate ticle cross-field diffusion took place; and (c) mag- at quasi-perpendicular (as opposed to quasi- netic field lines connecting high latitudes with low parallel) shocks (Tylka et al., 2004); (5) variable latitude active regions existed in the solar corona, contributions from flare and shock-accelerated allowing particles to reach high latitudes close to the particles (Cane et al., 2003), including acceleration Sun. On the basis of a comparison of onset times at of associated flare particles by the shock (Li and Ulysses with onset times in the ecliptic for events Zank, 2004; Cliver et al., 2004); and (6) production with the same solar origin, Dalla et al. (2002) con- of SEPs in polar plumes, where shock formation clude that high-latitude events are not compatible may be easier (Kahler and Reames, 2003). with direct scatter-free propagation along a magnetic field line, but rather the large path lengths and late Timing studies have shown that gradual SEP release times suggest that propagation to high lati- events are first accelerated at distances between ~3 tudes requires scattering. By approaching the Sun and 12 RS (Mewaldt et al., 2003). It has therefore along a polar trajectory PHOIBOS will encounter been suggested that SEPs originate beyond ~3 RS energetic particles at all latitudes and determine how because there is a peak in the Alfvén velocity at ~3 scattering properties from the corona into the solar RS, such that it is only beyond this radius that wind vary with magnetic field and turbulence inten- shocks can be easily formed and sustained for typi- sities. These measurements will also identify large- cal CME speeds (e.g., Gopalswamy et al., 2001). In scale deviations from the Parker spiral configuration MHD simulations of SEP events driven by coronal (Objective 2) and determine their role in energetic shocks (e.g., Zank et al., 2000; Sokolov et al., particle scattering. 2004) it is necessary to assume or model a variety of conditions in the region where gradual SEP Energetic electrons are observed in both impulsive events originate, including the magnetic field and and gradual SEP events. Because of the electrons’ density profiles, the solar wind and Alfvén speeds, near-relativistic velocities, the onset times of elec- the density of seed particles, and turbulence levels tron events at 1 AU are often used to deduce SEP that determine the particle diffusion coefficient. It release times near the Sun for comparison with their remains a mystery, why, for a given CME speed, associated electromagnetic signatures. Surprisingly, the peak intensity of >10 MeV protons can vary by the deduced release times almost always appear to be a factor of ~104 (Kahler, 2001). PHOIBOS will delayed by ~10 minutes with respect to electromag- measure the solar wind and magnetic field close to netic signatures such as soft x-ray and optical emis-

- 14 - sions from flares and associated radio emissions interplanetary magnetic field. The degree of deflec- (e.g., Krucker and Lin, 2000; Haggerty and Roelof, tion depends on the surface charge, which has not yet 2002). This discrepancy has resulted in consider- been directly measured for dust particles in space, able debate concerning its cause—whether storage and on the exact magnetic field parameters and their and subsequent release of the electrons, longitudi- variation in time (Mann et al., 2000). In addition, nal propagation of the acceleration mechanism dust dynamics is likely to be influenced by events from the flare site to the injection site, or radial such as coronal mass ejections, which may even lead transport of the acceleration mechanism in the form to dust destruction (Misconi, 1993). of a CME-driven shock (Haggerty and Roelof, 2002). Close to the Sun propagation delays will be The interaction of dust in the inner heliosphere and minimized, and energetic electron measurements the solar wind plasma influences not only the dust combined with interplanetary magnetic field obser- population but the local plasma and gas environment vations will reveal where and how particles are as well. Notably, dust grains in the inner heliosphere released from the Sun and/or accelerated in inter- are important as a source of pickup ions, protons as planetary space. well as heavier species, which differ from the solar wind in their charge state and velocity distribution. Measurement Requirements These “inner source” pickup ions,. discovered with Ulysses, have provided limited knowledge concern- • High-energy ions and electrons ing the composition of the gas released from dust • In situ vector magnetic field and constraints on the spatial distribution and fluxes • Remote sensing of active regions, flares, solar of dust grains. One of the surprising results has been radio bursts, and CMEs. the detection of noble gases and light elements in the • Basic plasma (proton, alpha particles) and mag- inner source pickup ions having a composition re- netic field measurements, and their gradients markably similar to that of the slow solar wind. • Major and minor ion distribution functions ex- Molecular ions in the mass range up to ~40 amu tending to high-energy tails. have also been detected. These measurements imply that recycling of solar wind particles through adsorp- • Composition and spectra of ions extending from tion and desorption constitutes an important mecha- energies through ~100 MeV/nuc, including 3He nism for the origin of the inner source pickup ions. • Plasma wave spectroscopy measurements (elec- However, the fluxes of dust required to account for tron density, temperature, velocity). the amounts of observed pickup ions exceeds by • Magnetic field and plasma fluctuation spectra, orders of magnitude the fluxes deduced from zodia- • Correlation with underlying magnetic structure cal light observations. Further progress in resolving (imaging) the origin of inner source pickup ions will require in- situ measurements close to the Sun as well as better 2.4) Explore dusty plasma phenomena and their models of dust microphysics. Inner source pickup influence on the solar wind and energetic parti- ions are also potential candidates for subsequent cle formation acceleration and may contribute to the anomalous cosmic ray population (Cummings et al., 2002). The origin of dust in the inner solar system is not well understood. The ultimate sources of the dust Despite some valuable observations (e.g., from He- population are thought to be the release of dust lios and Ulysses), many basic questions require de- from comets and asteroids and the breakup of me- tailed measurement of the near-Sun dust population. teoroids. Subsequent dust-dust collisions lower the What, are the mass distributions and fluxes of dust average mass of the dust particles. Dust orbital particles as a function of distance from the Sun? motion combines with Poynting-Robertson decel- How are dust fluxes correlated with fluxes and ve- eration to increase the dust number densities to- locity distributions of pickup ions? What are the wards the Sun (Burns et al., 1979). Inward from 1 major elemental compositions and bulk density of AU, the fragmentation of cometary meteoroids the dust and how do they vary with distance from the locally is believed to produce a majority of dust Sun? In-situ observations with PHOIBOS will be particles (Grün et al., 1985; Ishimoto, 2000; Mann crucial in resolving many of the present uncertainties et al., 2004). Dust particles attain electric surface regarding dust origin, its composition, and spatial charge through photo-ionization, electron emission, distribution. Since dust is a common component of and interaction with the solar wind. While larger interstellar material as well as most likely of other (>1 micron) particles move primarily in Keplerian stellar systems, PHOIBOS results will have a direct orbits, smaller charged grains are deflected by the bearing on certain astrophysical problems, with the

- 15 - near-Sun dust cloud serving as an analogue for ated from the dust grains and elucidate the mecha- circumstellar dust clouds, for example. nisms by which material is released from the dust.

PHOIBOS will characterize the near-Sun dust envi- Measurement Requirements ronment by determining how the mass distribution of dust and impact directions vary along the space- • Spatial variation of dust flux as a function of radial craft trajectory and how the observed impact sig- distance and latitude from 4 RS to 5 AU nals vary with the mass and impact parameters of • Distribution functions and composition of inner the dust particles. PHOIBOS dust measurements source pickup ions will likely require substantial revision of the para- • Solar wind bulk parameters digm of a homogeneously distributed dust cloud • Solar wind ion composition that is stable in space and time. • Plasma wave measurements • Energetic particle spectra and composition In addition to its interaction with the quasi- • Magnetic field orientation and strength. stationary wind, the near-Sun dust population also interacts with and is influenced by transient events 3) Mission Profile such as CMEs (Ragot and Kahler, 2002). Colli- sional evaporation, particularly in cometary mete- oroid trails, is expected to influence the solar wind As described in the executive summary, the present parameters measured locally. For example, a recent proposal is interesting alternative scenario to the study shows that dust collisions in the inner solar NASA STDT/2005 study in which: (i) a classical system can produce some of the heavy species in solar panels technology combined with heat shield amounts comparable to the observed inner source thermal generators are used as power systems instead fluxes (Mann and Czechowski, 2005). The material of RTGs and (ii) a new original trajectory combined released in such collisions may be responsible for to the use of a plasmic electric propulsion system are the enhancements of the interplanetary field meas- implemented in order to reach the final scientific ured by Ulysses in association with meteoroid trails orbit instead of the classical Jupiter low altitude (Jones et al., 2003. These enhancements—which swing-by with a very high C3 for Earth escape. The last for minutes to hours, are clustered in space, PHOIBOS mission profile allows therefore more and occur more frequently in the inner solar sys- flexibility in the fine adjustment of the final orbit. It tem—may be the result of mass loading of the solar may avoid the possible drawbacks of a Jupiter wind plasma induced by collisional vaporization in swing-by such as the high departure V∞, the risky the dust trails (Mann and Czechowski 2005). It is low altitude Jupiter swing-by at 5.5 AU with an im- still an open question how noble gases observed in portant radiation level and no consequent trajectory the inner source are produced, with the solar wind changing capability. The selected science orbit, pre- surface interactions being a distinct possibility. sented below, is therefore the main mission require- ment and drives all the mission profile. Dust impacting the spacecraft will influence the plasma environment of the spacecraft and may bias Launcher and Orbit requirements plasma and field measurements. Signals due to impact-generated ion cloudlets have been observed The PHOIBOS spacecraft will use an Ariane AR5 by plasma experiments on several spacecraft in the ECA launcher and a plasmic propulsion stage in vicinity of planetary rings (Gurnett et al., 1983; order to reach it’s final scientific orbit described in Meyer-Vernet et al., 1986), in the interplanetary Table 3.1. The required escape conditions are a medium (Gurnett et al., 1997), and during encoun- modulus of the hyperbolic excess velocity v∞ = ters with the comets Giacobini-Zinner, Halley, and 3.0km/s and a declination of the hyperbolic excess P/Borrelly (Neubauer et al., 1990; Tsurutani et al., velocity δ∞ = - 6deg. The launcher performance for 2003). these conditions is 5000kg offering 25% of margin wrt the need. Dust fluxes are expected to be especially high and time-variable near Sun. PHOIBOS will measure Table 3.1 : PHOIBOS final orbit characteristics both the dust fluxes and pickup ion densities and perihelion radius 4 Rs composition as a function of radial distance and aphelion radius 3.5AU latitude with sufficient resolution, sensitivity, and sidereal period 853 days dynamic range to characterize the species gener- (2.33 years) eccentricity e = 0.9886

- 16 - inclination with respect 60deg to the ecliptic plane

- 17 -

Figure 3.1 : projection of the PHOIBOS trajectory on the ecliptic plane. The plasma thrusting phases are indicated in green.

Figure 3.2 : Sun-Earth-S/C angle as a function of time for the first two solar encounters.

possible launch dates are therefore May 2017 or Mission profile and cruise scenario October 2015. The final launch date should also be The overall mission profile is summarized on Figure chosen so that the first flyby of the Sun occurs dur- 3.1. For a launch in December 2018, the acquisition ing solar minimum of activity which is approxi- of the final orbit is planned in June 2027, the first mately the case for a December 2018 launch. flyby of the Sun at 4 Rs will take place on 29 Sep- tember 2027 and the second one on 29 January In order to reach the final orbit, a total Delta-V of 2030. The launch opportunity for such a scenario is 12.25km/s is necessary. To obtain such a Delta-V, a window every 584 days (1.6 years). The earlier two Earth swing-by (March 2025 and August 2026),

- 18 - one swing-by (September 2025) and plasmic be necessary therefore to use a large onboard mem- propulsion are necessary. The envisaged plasmic ory to store as much data as possible during the engines are PPS5000 with one or two engines work- solar passes and process and transmit them after ing in parallel depending on the available power perihelion (specific impulse, Isp = 1900s, thrust magnitude, 185mN < F < 280mN, electrical power, 3kW < P < Onboard communication system 4.5kW). The total duration of the cruise phase will be 8.5 years (3100 days). The complete on board communication system is based on the Bepi Colombo one. Its features can be The Sun closest approaches during the cruise phase estimated to 25 kg in mass and 90 W in consump- will be in August 2025 (0.56AU) and December tion. 2029 (0.26AU), which is equivalent to the ESA Solar Orbiter mission. The two most efficient thrust- 4) Payload description ing phases in term of Delta-V will occur at helio- centric distances between 1.8 and 3.7 AU. These As we already indicated in the executive summary phases are indicated on Figure d.1 as aphelion of the proposal, the overall philosophy of the cur- thrusts 1 & 2. While the main thrusting axis during rent proposal in terms of scientific objectives and the “aphelion thrust 1” period remains in the eclip- necessary payload in order to achieve them has tic, for “aphelion thrust 2” the probe will be oriented greatly been inspired by previous Solar Probe pro- in such a way, that thrusting will allow not only to posals and especially by the last NASA STDT/2005 reach the necessary Delta-V in order to lower the report. perihelion, but also to increase the inclination of the final orbit up to 60 deg. from the ecliptic plane. The 4.1) Overview of all proposed payload elements total Xenon mass for the plasmic engines required To meet the PHOIBOS science objectives, we rec- for the mission is 1949kg. Therefore the final space- ommend a payload comprising both comprehensive craft composite dry mass will be around 2050kg. in-situ and relevant remote-sensing instruments.

However since we want to emphasis the in-situ Alternative launchers measurements, the in-situ payload should have the

priority in terms of resources and accommodation A slight variation to proposed scenario with only trade-offs. The complementary remote sensing ob- one Earth swing-by and a total duration for the servations may be provided by Solar Orbiter, de- cruise phase of 7.5 years could be envisaged with pending upon the schedules of the two missions. the use of the new generation of Ariane 5 launcher

(Ariane 5 ECB). Finally, one should note also that PHOIBOS in-situ instrumentation consists of a Fast the use of the PROTON M launcher, with its Ion Analyzer, a Fast Electron Analyzers, an Ion BREEZE M upper stage fulfils also, albeit without Composition Analyzer, an Energetic Particle In- performance margin, the mission profile that we strument, a Magnetometer, a Radio and Plasma have described. Wave Instrument, a Neutron/Gamma Ray Spec-

trometer, and a Coronal Dust Detector. The remote- Ground segment requirements sensing instrumentation comprises a Hemispheric

Imager for white-light imaging of coronal structures Since the most valuable scientific measurements are and a Polar Source Region Imager for EUV and performed during the Sun’s closest approaches, the magnetic imaging of the photosphere. The payload tracking and command capabilities are mainly is serviced by a common data processing unit driven by the ability to download in real time as (CDPU) and low voltage power supply (LVPS). most of the data as possible Figure 3.2 displays the Table 4.1 summarizes the contribution of the vari- Sun-Earth-S/C configurations during the first two ous instruments to the PHOIBOS scientific objec- solar passes. X-band/Ka-band downlink allows (i) tives. The specifications for each instrument and to gain 11.6 dB of S/N ratio wrt the coronal scintil- their rationale are discussed in the sections that fol- lation and (ii) a beam of 0.2° allowing a Earth an- low. The mass, power and data rate allocations for tenna tracking at 0.2° from the Sun disc. the baseline payload are shown in Table 4.2. They

are based on those of past or existing instrumenta- A BepiColombo configuration allows a telemetry of tion or components. 45 kbits/s at 1 AU. However, taken into account the

35m Earth antenna temperature noise when turned toward the Sun, a flow of 2 kbits/s can be expected for the TM budget at the most critical point. It will - 19 - Table 4.1: Science Objectives and Contributing Instruments

G A

I C HI CD EPI Scientific Objectives F FEA NGS PSRI MA RPWI

Explore the fundamental processes underlying coronal heating and solar R S R R S S S wind acceleration. Determine magnetic field structure and dynamics in the source regions of the R S S R S R R fast and slow solar wind. Determine what mechanisms store, R R R R S S S accelerate and transport energetic particles. Explore dust and dusty plasma phenomena in the vicinity of the Sun and their influence R S S S R S on the solar wind and energetic particle formation. FIA: Fast Ion Analyzer FEA: Fast Electron Analyzer NGS : Coronal Dust Detector ICA: Ion Composition Analyzer EPI: Energetic Particle Instrument PSRI : Polar Source Region Imager MAG: DC Magnetometer HI: Hemispheric Imager

RPWI: Radio and Plasma Waves Instrument R : required NGS : Neutron/Gamma-Ray Spectrometer S : supporting

Table 4.2 Instrument Resource Requirements1

Instrument Mass Power Peak Data (kg) (W) Rate (kbps) Fast Ion Analyzer (FIA) 2.5 3.7 10 Fast Electron Analyzer (FEA) 5.0 7.2 20 Ion Composition Analyzer (ICA) 6.0 6.0 10 Magnetometer (MAG) 1.5 2.5 1.1 Radio and Plasma Wave Instrument (RPWI) 7.0 5.0 4 Energetic Particle Instrument, Low Energy (EPI-Lo) 1.5 2.3 5 Energetic Particle Instrument, High Energy (EPI-Hi) 2.5 1.7 3 Neutron/Gamma Ray Spectrometer 2.5 3.0 0.5 Coronal Dust Detector 1.5 3.8 0.1 Hemispheric Imager (HI) 1.5 4.0 70 Polar Source Region Imager (PSRI) 3.5 4.0 70 Common DPU/LVPS 10 14 N/A Total 45.0 57.2 123.7

1. Source is NASA STDT 2005 Report.

- 20 - tion needs to be ~5° around the solar wind beam and 4.2 Baseline Payload ~30° over the remainder of its FOV.

4.2.1 Fast Plasma Instrumentation. Fast Electron Analyzer (FEA). The FEAs should There are several basic requirements for the meas- be capable of measuring two- and three-dimensional urement of the coronal thermal plasma. The ion electron distribution functions over the energy range instrumentation should be able to distinguish alpha from ~1 eV to 5 keV. This energy range covers particles from protons under all conditions. The from the lowest energy photoelectrons, through the field of view (FOV) coverage for the distribution thermal core population and well up into the supra- functions should be as complete as possible. The thermal halo population. The FEAs’ 3D temporal basic moments of the distributions, density, velocity resolution of 3 s (0.1 s for 2D distribution functions and temperature should be obtained fast enough and of energy and one angle) is matched to the FIA to accurately enough to enable Alfénic and MHD tur- help resolve plasma conditions and structures on the bulence to be analysed same scales. The energy resolution (∆E/E) should be approximately 10%, which does a good job of The PHOIBOS Fast Plasma Instrumentation will resolving the hot electron distributions. Like the consist of a single Fast Ion Analyzer (FIA) and a FIA, the FEA requires a sensitivity and dynamic pair of Fast Electron Analyzers (FEAs). The FIA range adequate to measure the 2D distributions in and one of the FEAs are mounted, together with the 0.1 s at 20 RS without saturating the detectors all Ion Composition Analyzer (ICA), on a movable arm the way into perihelion. Together the FEAs need to on the ram side of the spacecraft; the arm is gradu- observe as much of 4π steradians as possible; all- ally retracted as the spacecraft approaches the Sun. sky imagers and deflecting top-hat analyzers are This arrangement provides viewing to near 5° (i.e., both appropriate approaches for achieving the includes attitude control margins and finite size of needed FOVs. To resolve possibly very narrow halo charged particle entrance apertures) inside of the electron beams (the strahl), the FEAs need angular edge of the heat shield umbra. The second FEA is resolutions that approach 3° in at least one dimen- mounted on the anti-ram side of the spacecraft sion at higher energies around the magnetic field body, pointing 180° away from the first. While the direction (this information is supplied real-time mission-unique aspects of PHOIBOS will require from the magnetometer via the payload DPU), while new designs for the FIA and FEA instruments, the ~30° angular resolution is adequate to measure the basic designs and subsystems can be drawn from a remainder of the halo population and the core and wide variety of previous heritage missions such as photoelectron populations at lower energies. Ulysses, ACE, Helios, Wind and Stereo. 4.2.2 Ion Composition Analyzer (ICA). Fast Ion Analyzer (FIA). The FIA should be capa- The ICA is mounted, together with the FIA and one ble of measuring two- and three-dimensional distri- FEA, on the movable ram-looking arm referred to bution functions for protons and alphas over the above. The ICA should be capable of measuring energy/charge range of 50 eV/q to 20 keV/q. This two- and three-dimensional distribution functions of energy range covers the lowest and highest expected He and heavy ions in the solar wind, over an energy speeds for 100 km/s protons and 1400 km/s alpha range from ~100 eV/q to ~60 keV/q and a mass particles, respectively. The FIA’s 3D temporal reso- range from 2 to > ~60 amu. The required energy lution of 3 seconds and 0.1 second for 2D distribu- range covers all major solar wind species that will tion functions allows identification of boundaries in be observed during the solar encounter. ICA’s 3D the solar wind down to ~1000 km near perihelion temporal resolution of 10 s (at 20 RS) permits tem- and wave modes (e.g., the gyrofrequency is ~30 Hz poral and spatial effects to be distinguished and over the poles). The energy resolution (∆E/E) allows comprehensive assessment of the non- should be approximately 5%, which does a good job thermal properties of the distribution functions that of resolving the supersonic solar wind beam out to are generally expected from various solar wind ac- beyond 1 AU. The sensitivity and dynamic range celeration and heating mechanisms. Furthermore, need to be adequate to measure 2D (energy and one with the required mass range the ICA will measure angle) ion distributions in 0.1 s at 20 RS without species with low ionic charge states (i.e., He+) and saturating the detectors all the way into perihelion. high masses (i.e., SiO2), such as those produced The FIA’s field of view (FOV) needs to observe as from neutral sources in the inner heliosphere or much of the ram side of the viewing space as possi- created by the solar wind’s interaction with dust ble. To resolve the ion distributions everywhere near the Sun (e.g., inner source pickup ions). The from 0.3 AU into perihelion, FIA’s angular resolu- energy resolution (∆E/E) should be 4–5%, sufficient

- 21 - to resolve the supersonic solar wind beam out to EPI High-Energy Instrument (EPI-Hi). The EPI beyond 1 AU. The sensitivity should be sufficient to high-energy instrument (EPI-Hi) is required to measure He/O ratios every 10 s at 20 RS which can measure the composition and energy spectra of en- be achieved scaling from 1 AU observations of solar ergetic nuclei with 1 ≤ Z ≤ 26 from ~1 to 100 wind composition and charge states. The dynamic MeV/nucleon, as well as energetic electrons from range should be ~104. The ICA FOV needs to ob- ~0.3 to 3 MeV. The source of the energetic ions to serve as much of the ram side of the viewing space be observed over the course of the PHOIBOS mis- as possible due to the large amount of variability sion range from quiet-time intensities of cosmic expected due to turbulence or waves in the outer rays, to low-energy ions accelerated in CIRs and corona. This can be achieved, for example, with a transient interplanetary shocks, to ions accelerated top-hat and swept FOV, or with an instrument with in small, impulsive events associated with solar large instantaneous FOV as done on MESSENGER, flares, to solar energetic particles accelerated in provided that the edge of the FOV extends to close large gradual events. As a minimum, the charge to the heat shield. To resolve the ion distributions resolution should be sufficient to measure differen- everywhere from 1 AU to perihelion, ICA’s angular tial intensities of H, He, C, N, O, Ne, Mg, Si, and resolution needs to be ~10° around the solar wind Fe, although minor species are also of interest. It beam and ~20° over the remainder of its FOV. would also be very useful to include nuclei with 30 ≤ Z ≤ 83 that are found to be enhanced in some SEP 4.2.3 Energetic Particle Instrument. events associated with impulsive solar flares. It is 3 4 The PHOIBOS Energetic Particle Instrumentation required that He and He be separately identified 3 4 (EPI) consists of a low-energy sensor (EPI-Lo) and whenever the He/ He ratio exceeds 1%. Assuming a high-energy sensor (EPI-Hi). Both packages are to that onboard particle identification is used to sort be mounted on the spacecraft body, where they species into a matrix of species versus energy bins, view particles incident from both the sunward and the energy resolution of these bins should be no anti-sunward hemispheres. worse than six intervals per decade. Near the Sun it can be expected that energetic ions may be highly EPI Low-Energy Instrument (EPI-Lo). The EPI anisotropic and beamed along the interplanetary low-energy instrument is required to measure the magnetic field, which is expected to be on average composition and pitch-angle distributions of ener- radial at closest approach, but could be highly vari- getic particles. The composition includes hydrogen able. It is therefore desirable for the EPI-Hi instru- to iron as well as energetic electrons. As a minimum ment to sample as much of 4π steradians as possi- the detector should be able to make the ion meas- ble, including, in particular, the forward hemi- urements from ~20 keV/nucleon to ~1MeV/nucleon sphere. As a minimum EPI-Hi should be able to and the electron measurements from ~25 keV to ~1 observe particles with pitch angles ranging from 30° MeV. Composition measurements should discrimi- to 120° with respect to the spacecraft Zaxis with an nate protons, 3He, 4He, C, O, Ne, Mg and Si, and angular resolution no worse than 30°. EPI-Hi should Fe. The measurements should have sufficient angu- have sufficient directional information to be able to lar spread and resolution to enable pitch-angle determine the magnitude and direction of 3D anisot- measurements of the differential particle fluxes for a ropies. (nominal) radial magnetic field. A “slice” field of view of ~10° wide and >120° and at least 5 angular Although not well known, it is expected that the bins would suffice; at least 120° coverage and an intensity of SEP events will scale with distance angular resolution of no worse than 30° are re- from the Sun (R) approximately as R–3 (cf. Reames quired. The wider opening should be aligned with and Ng, 1998, and references therein). To observe the spacecraft spin axis with the field of view just particle populations that range from quiet-time lev- clearing the thermal protection system. Larger solid- els near 1 AU to solar energetic particle (SEP) angle coverage and better species resolution are, of events near the Sun requires a dynamic range of course, preferred. The sensitivity should be at least ~107. The peak intensity of a typical impulsive ~1 (cm2 ster s keV)–1. Timing resolution should be event at 1 AU is ~1 to 10 protons/cm2-sr-s >1 MeV. – –3 no worse than 1 s for e , 5 s for protons, and 30 s for Scaling this to 4 RS by R suggests that intensities heavier nuclei. The capabilities described here can up to ~106 protons/cm2sr-s >1 MeV should be be achieved with energetic particle instruments of measurable. Particle intensities should be measured the type currently being flown on MESSENGER with a timing resolution that is no worse than 1 s for and STEREO. electrons, 5 s for H, and 30 s for Z = 2 nuclei. There is considerable heritage for energetic particle in- struments in the 1 to 100 MeV/nucleon energy

- 22 - range. Instrument designs that could be adapted to With some adjustment to accommodate the upper meet these requirements (assuming modern, low- range, this requirement could be met with magne- power, low mass electronics) have flown on Helios, tometers commonly flown on magnetospheric mis- Voyager, ISEE-3, Ulysses, Wind, ACE, and STE- sions today. REO. 4.2.5 Radio and Plasma Wave Instrument (RPWI). 4.2.4 DC Magnetometer. The RPWI sensors consist of a 3-axis search coil for The PHOIBOS direct current Magnetometer (MAG) detecting magnetic field fluctuations and a 3- will provide context and definition of local mag- element electric field antenna system. The search netic structure and low frequency (<10 Hz) mag- coil sensor is mounted on the aft spacecraft boom, netic fluctuations. MAG consists of one or more 3- with the separation from the DC magnetometer and axis sensors mounted close to the end of a deploy- other instruments to minimize contamination of the able, non-retractable axial boom extending from the search coil data to be determined. The electric field bottom deck of the spacecraft. (Owing to the size of antenna system should be designed is such a way to the Thermal Protection System (TPS), MAG sen- accommodate, if possible, both DC electric field sors can not be placed sufficiently far from the and high frequency Quasi-Thermal Noise (QTN) spacecraft body for a dual magnetometer configura- measurements (see below). The antenna system is tion to be practical in removing spacecraft fields. A mounted on the base of the spacecraft, with the second MAG sensor could be used to provide low- three antenna elements separated by ~120°. Each power and low-mass redundancy.) The MAG sensor element is about 1.75 m long. The antenna inclina- will be located close to the search coil component of tion to the spacecraft axis is varied with distance the Plasma Waves Instrument (PWI), making it from the Sun, so as to maintain permanently some necessary for both to work together to provide a portion of it in sunlight, while minimizing heat in- suitable measurement environment. Close collabo- put into the spacecraft. The portion of antenna in ration between the two teams is critical to contain- sunlight needs to be the same on each element in ing cross-contamination of the instruments and pro- order to enable low frequency (< ~3 kHz) plasma viding for the success of the mission. Signatures of waves to be sampled. Plasma wave instruments with plasma processes at the proton inertial scale, which the necessary capabilities have been implemented result in the conversion of magnetic energy into on numerous missions, including Ulysses, Wind, heat, fall within the frequency range of the PWI, Cluster, Polar, FAST, Geotail, Cassini and Stereo. and only two suitably configured instruments will Similar antenna concepts have been used on several be able to provide the needed plasma diagnostics. of these missions; however, they were not designed to work in the thermal environment expected for MAG Performance. Photospheric structures with PHOIBOS. To be accommodated safely on the scale sizes of tens of km will have scale sizes of spacecraft, the PWI antenna will need to be made hundreds of km if they are coherent out to the orbit from a refractory material that will operate at tem- of PHOIBOS. A sample rate of 20 Hz gives a spa- peratures up to 1400°C. tial resolution of approximately 30 km over the Sun’s pole, which will provide minimal resolution Search Coil Magnetic Field Measurements. The of the magnetic structures. A burst or snapshot PWI magnetic field experiment should operate in mode of higher time resolution is used for compari- the frequency range ~1 Hz to 80 kHz, allowing son with the PWI. Data telemetry compression will overlap with the DC magnetometer at low frequen- permit adequate retention of measurement resolu- cies and to measure fluctuations beyond the ion tion. Total telemetry dedication of 960 bps will cyclotron frequency at high frequencies (The sensi- permit adequate download of continuous 20-Hz tivities of the search coil and DC magnetometer are vector measurement plus snapshot buffer. expected to be equivalent at approximately 10 Hz). Extrapolation of Helios data acquired at distances The strawman instrument samples in frequency ≥0.3 AU yields an average interplanetary magnetic space at 40 channels per decade, with cross-spectral field (IMF) of approximately 260 nT at 20 Rs, the power between the field components at 20 channels distance at which the primary mission begins. Vari- per decade. Bursts of waveform data are also col- ous measurements and theories suggest that, within lected at a cadence of up to 60 s to allow detailed some regions and structures, the magnetic field study of small-scale processes in the near-Sun might be as high as 1 to 6 G at 4 Rs. MAG should plasma. be capable of switching sensitivity ranges. At least four ranges are needed, with the most sensitive be- Electric Field Measurements. The RPWI electric ing |B| < 0.1 nT and the high-field range |B| < 8.2 G. field experiment should measure fluctuations in the

- 23 - electric field from close to DC to above the plasma 4.2.6 Neutron/Gamma-Ray Spectrometer (NGS). frequency (1 Hz to 30 MHz was chosen for the The NGS detector should be capable of detecting strawman instrument) so as to return information on and positively identifying neutrons and γ-rays from low-frequency wave, turbulence, small scale struc- the Sun having energies that range up to 10 MeV. tures and processes at and below the ion inertial The neutron component should be capable of intrin- scale. RPWI will be also designed to diagnose accu- sic energy resolution sufficient to separate neutrons rately the electron plasma parameters (density and having energies below and above 1 MeV, and better temperature) using the technique of the quasi- than 50% energy resolution for neutron energies thermal noise (QTN) spectroscopy. QTN requires between 1 and 10 MeV. This last requirement is sampling the electric field fluctuations from low needed to separate quasi-steady-state neutron emis- frequency to above the plasma frequency. The sion from transient neutron emission. The NGS will strawman instrument has a sampling density of 40 measure the products of the acceleration of protons samples per decade and a temporal sampling period (via neutrons and γ-rays) and electrons (via γ-rays) of 0.1 s to allow rapid sampling of plasma parame- as they interact with the dense low chromosphere ters local to the spacecraft. A sensitivity of 2 × 10–17 2 and photosphere. If microflares or nanoflares play a V/m /Hz at 10 MHz provides adequate signal to significant role in coronal heating, these signatures noise for QTN measurements. The strawman in- of particle acceleration will be present. Their spec- strument returns 3-axis measurements sampled at 40 trum and time variation provide information on the samples per decade, and as with the magnetic field, acceleration process(es). cross spectra between components are returned. In the low frequency regime (<10 kHz), cross spectra Upward-propagating protons and electrons may be between E and B are measured to facilitate identifi- directly detected by PHOIBOS, although the prob- cation of wave modes. Waveform data that allow ability of crossing the appropriate field lines at the the study of small-scale phenomena are returned as critical time may be small. The neutron and γ -ray burst mode data with a 60-s cadence. detection suffers no such restriction. Furthermore, PHOIBOS’s close passage to the Sun provides tre- Electro-Magnetic Cleanliness (EMC). The EMC mendous advantage for detection of low energy requirements are driven by the RPWI and MAG neutrons because of their short lifetimes, as well as instruments, carrying an electric antenna, search for spectroscopy of faint γ-ray bursts. These obser- coils and a magnetometer. As for magnetic issues, vations will, for the first time, provide solid statisti- with expected field of ~250 nT, a DC cleanliness cal knowledge of frequency of energetic accelera- requirement at the magnetometer of 10 nT would be tion in small solar flares. reasonable, with a low frequency AC requirement of

1 nT. With enough resources, a twin magnetometer A detection of a burst of -rays would help refine configuration could also be used to detect and γ the energy spectrum of transient neutrons through eliminate spacecraft fields, reducing the magnetic use of the measured time of flight between neutron cleanliness requirements as well as providing re- arrival times and the time of the gamma burst. The dundancy. As for electrical issues, several inexpen- detection sensitivity of the NGS should be sufficient sive measures can be taken to ensure that the to measure neutrons produced by flares that release PHOIBOS spacecraft is clean from the point of greater than 1024 ergs. view of both conducted and radiated electromag- netic interference. The sensitivity of the RPWI instrument will be approximately 2 × 10–17 V/m2/Hz A broad-band analysis of the γ-ray spectrum can at 10 MHz. In addition the waveform analyser is provide a measure of the electron and ion compo- sensitive to impulsive interference of duration as nents that will complement the detection of neu- trons. The neutron measurements are most sensitive short as a fraction of a µs. It is most important that to the lowest-energy heavy-ion interactions, while these requirements be considered in the design from day one, with the cooperation of all concerned - the ion-induced gammas sample higher energies that may be present in the ion population. spacecraft, payload and AIV. Although these re- Bremsstrahlung from accelerated electrons will quirements may appear to require an excessively manifest themselves in a continuum spectrum that is “clean” spacecraft, they are not difficult to achieve distinguishable from that of the ion-induced gam- if good EMC practices are incorporated in the mas. spacecraft design.

NGS Performance. The NGS is mounted behind the hydrazine tank. The presence of hydrazine onboard can be used as a separate detection channel via

- 24 - moderated neutrons. To adequately resolve the on- provide a second look direction. The allocated set and duration of a γ-ray burst requires a sample power is adequate to two alternately operated sen- period of 4 s in each of 64 energy bins, encompass- sors. Measurements from two sensors on different ing an energy band of 0.1 to 10 MeV. The neutron spacecraft locations can be used to distinguish be- channel, because of the moderating influence of the tween particles in prograde and retrograde motion hydrazine, will be detecting degraded neutrons with as well as between particles in near-ecliptic and out timing but less spectroscopic information. The neu- of ecliptic orbits. If resources permit, an alternative tron channel will require a sample period of 16 s to two sensors would be a single detector with time over a neutron energy range of 0.05 to 20 MeV, also of-flight (TOF) capability. This would enable TOF with 64-channel spectra. The detector must be able measurement of the impact-produced ions, yielding to distinguish statistically between fast neutrons and mass spectra and allowing the elemental composi- gammas and should possess an unambiguous neu- tion of the dust to be derived. tron-detection channel. A spectral resolution better than 50% will allow broad-band analysis of the The RPWI instrument will also be sensitive to dust gammas and neutrons, sufficient to resolve the impacts via corresponding plasma cloud and pickup bremsstrahlung, nuclear, and neutron components. signal on the electric field antenna. This is based on Both neutron and gamma spectrometer functions dust impacts observed by similar instrumentation of must efficiently reject charged particles. the Voyager or Cassini spacecraft at Saturn (Gurnett et al., 1983; Meyer-Vernet et al., 1986; Moncuquet 4.2.7 Coronal Dust Detector (CD). et al., 2007). The CD should be compact and lightweight and must be able to cope with the near-Sun thermal and 4.2.8 Hemispheric Imager (HI). particle environment. The CD assumed for this HI is a broadband, very-wide-angle, white-light study is an impact ionization detector based on the coronagraph with ~160° FOV to image the local Dust Counter (Igenbergs et al., 1998). Such coronal environment and provide tomographic im- devices measure ions and electrons produced by the aging of coronal structures (e.g., polar plumes) as impact of dust particles on the detector’s target area PHOIBOS flies through the corona. HI will also be to derive particle mass and have been successfully able to observe coronal mass ejections (CMEs) and flown on Ulysses, Hiten, Galileo, and Nozomi. The other dynamic structures as they evolve. Coronal CD is mounted on the +X (ram) face of the PHOI- tomography is a fundamentally new approach to BOS spacecraft, where it will be exposed to the coronal imaging (similar to a medical CAT scan) maximum dust flux. For an aperture area of 140 and is possible only because the imaging is per- cm2, the dust model described in Appendix B pre- formed from a moving platform close to the Sun, dicts that 2 × 104 particles of masses larger than 10- flying through coronal structures and imaging them 17 g and up to 2 × 106 particles of masses larger than as it flies by and through them. HI observations of 10-19 g would be detected at the high-impact veloci- the 3D coronal density structure are required to ties that PHOIBOS will experience. Independent resolve ambiguities in the interpretation of spatial pointing and special pointing accuracy are not re- and temporal changes seen in the in-situ measure- quired. The CD will be operated continuously (ex- ments. HI heritage stems from the all-sky corona- cept when in direct sunlight). Only modest teleme- graph on SMEI, the HI wide-angle coronagraph on try allocation is required. The CD should have an STEREO, and instrument prototypes developed as external cover to be removed after launch. No spe- part of the 1995 PHOIBOS Instrument Develop- cial cleanliness is required, but purging with N2 ment Program (Buffington et al., 1998). The HI’s should be considered. Issues to be addressed for FOV and resolution derive from the need to provide further development of the CD are the high voltage context for the in-situ instruments and to be able to parts and the influence of the radiation environment reconstruct the 3D density structure of the corona and outgassing from the heat shield on the meas- tomographically. The 160° FOV is sufficiently large urements. Measurement of particle mass is standard to view the corona from near the solar limb to be- for impact ionization detectors but has not yet been yond the zenith. A wide-angle view is particularly demonstrated for the high impact speeds that the important for imaging faint coronal features, be- PHOIBOS CD will experience. cause the coronal intensity contrast is greatest along flux tubes and other magnetic structures near the Although a single sensor has been assumed in the zenith. The spatial resolution required to image payload design, we have conservatively included small-scale coronal structures is of order 1°. The enough mass to accommodate a second sensor temporal cadence required to provide continuous mounted at a different location on the spacecraft to

- 25 - observations and sufficient data for 3D tomographic channel FOV requirements are identical to those of reconstruction is ~90 s at perihelion. the EUV channel. The magnetograph channel spa- tial resolution is driven by the need to spatially re- 4.2.9 Polar Source Region Imager (PSRI). solve small-scale mixed polarity structures (~4 PSRI uses an imaging periscope to view the Sun’s Mm). Because of the differential nature of magnetic poles above 60° latitude at distances beyond 20 RS. flux measurements, the signal-to-noise ratio must be The PSRI consists of two channels, a magnetograph at least 100 in each pixel to achieve quantitative channel to image the polar magnetic fields and an measurements of the magnetic field. EUV imaging channel to identify small-scale hot coronal plasma structures. These two channels will 4.2.10 Common Data Processing Unit (CDPU). make it possible to relate the magnetic field struc- The CDPU integrates the data processing and low ture to the heating of coronal structures at the poles voltage power conversion for all of the payload and to establish the linkage between these source science instruments into a fully redundant system region(s) and the plasma flows measured at the that eliminates replication, increases redundancy, spacecraft. PSRI magnetograph channel heritage and reduces overall payload resources. The CDPU stems from tunable etalon magnetographs flown on provides a unified interface to the payload for the the Flare Balloon and instrument designs spacecraft. The spacecraft selects which side of the developed as part of the 1995 PHOIBOS Instrument CDPU will be powered, leaving the redundant side Development Program (Title et al., 1999). PSRI off as a cold spare. The payload CDPU communi- EUV channel heritage stems from SOHO/ EIT, cates with the spacecraft over a MIL-STD-1553 bus, TRACE, STEREO, GOES/SXI, and various rocket accepting commands and producing CCSDS pack- programs. The imaging periscope uses two chan- ets ready for final processing by the spacecraft for nels, each 1 inch in diameter, to view the solar sur- telemetry to the ground. face at the Sun’s poles. The periscope will be ex- tended for 10 seconds (and then retracted) every 10 The current heritage and Technology Readiness min while PHOIBOS is beyond 20 RS and above Level (TRL) and critical issues for the payload are 50° heliolatitude. This operational sequence is based discussed in section 7. The proposed payload pro- on a detailed thermal and mechanical analysis by curement approach is discussed in section 8. the PHOIBOS Engineering Team. During perihelion passage, inside 20 RS, the periscope will be stowed 5) Spacecraft description behind the heat shield. The spacecraft is constituted by the Phoibos Trans- The FOV for the PSRI EUV channel is 3°. This fer Module (PTM) and by the Phoibos Solar Probe requirement is driven by the need to view the entire (PSP), both integrated into the Phoibos Composite polar region below the spacecraft from 20–65 RS. Spacecraft (PCS) at launch and during transfer. The The spatial resolution requirement is driven primar- mission will commence in December 2018 with the ily by the need to resolve small-scale EUV struc- launch of the PCS on an Ariane 5. After a long in- tures at the coronal base of polar plumes. This spa- terplanetary cruise phase, during which the PCS is tial resolution can easily be achieved with the cur- powered by the transfer module (PTM), the probe rent EUV imaging channel baseline pixel size of 10 (PSP) will be delivered to its final solar high ellipti- arcsec (two pixel resolution = 20 arcsec), corre- cal orbit in June 2027. sponding to a 2-pixel resolution of ~2 Mm at 20 RS (comparable to a spatial resolution of 2.5 arcsec at 1 5.1) Spacecraft architecture AU). This single-pixel angular resolution is derived from the assumption of using a 1024 × 1024 format Composite spacecraft architecture detector to image the full 3° FOV. A 1-s exposure in The Phoibos Composite Spacecraft (PCS) is com- the EUV yields a single-pixel signal-to-noise ratio posed of the Solar probe, mounted on top of the of ~10 for quiet Sun observations and ~30 for bright transfer module, featuring four solar arrays of structures, which is adequate for the science re- Rosetta size, mounted by pair on two Solar Array quirements. Drive Mechanisms (SADM), and 5 or 6 plasmic engines (see discussion below). Figure 5.1 shows The PSRI magnetograph channel is designed both to the PCS in launch configuration. probe the overall magnetic structure of the pole and to compare the presence of mixed polarity magnetic structures to the solar wind conditions associated with polar plume footpoints. The magnetograph

- 26 -

Figure 5.1 : Phoibos Composite Spacecraft in launch configuration

After the launch phase and the early operations, the solar arrays are deployed and orientated so that they are perpendicular to the orbit, facing the Sun. The spacecraft body lies therefore in the orbit plane and can rotate freely thanks to the SADM around the orbit normal direction to orient the plasmic engines during propulsive arcs. Figure 5.2 shows the PCS in Figure 5.3 : Separation of the Phoibos Solar Probe from the PCS. deployed configuration during the cruise phase. The control of the PCS is ensured by the Probe data management system and AOCS sensors, but using Phoibos solar probe architecture dedicated actuators (wheels and hydrazine system). The Phoibos Solar Probe (PSP) spacecraft is com- The solar panels are orientated with null incidence posed of a conical Heat Shield and of an orbiter with respect to Sun for the approach at 0.26 AU including the science payloads and the platform. during the cruise (after the second propulsive arc), The PSP is separated from the transfer module as and a dedicated protection system (small heat shield soon as the science target orbit is reached. on the side of each panel) will be accommodated. There is no propulsion required below 0.85 AU. The probe includes 3 solar panels of 1.36 m² each that can be folded behind the heat shield during perihelion passage, using the one axis orientation mechanism on each panel. The heat shield The heat shield is based on the strong Solar Orbiter heritage, and from the NASA STDT/2005 study. Promising related tests are also currently being car- ried out at the Odeillo Solar furnace facility in France. These tests are intended to study C/C com- posites behaviours under high temperature, ion and UV irradiations (Paulmier et al., 2001; Eck, 2007).

The heat shield has a conical shape with a 15 deg half-cone angle and is built with C/C structure. De- pending on the acceptable conduction in the struts Figure 5.2 : Phoibos Composite Spacecraft in deployed supporting the shield, basically two options are pos- configuration during cruise sible regarding the coating of the shield. A white After the final orbit acquisition, the PSP and the coating will result in an α/ε around 0.6 and a cooler PTM are separated and the Phoibos Solar Probe shield. Bare C/C will result in an α/ε around 1 and a continues its journey on a ballistic orbit towards the hotter shield. For instance, the thermal load received Sun. by the heat shield at perihelion being around 21 MW, the heat shield external layer temperature will

- 27 - be around 1800°C in the case of white coating. Note white coating can melt leading to very important however that two problems may occur with the damage of the structure. white coating. Firstly the value of α/ε around 0.6 should be measured at higher temperatures is order The heat shield is separated in two parts: (i) the to check whether it is constant or no. Secondly, the conical part and (ii) a secondary flat heat shield that problem with the white coating is that possible closes the cone at its base, using also Solar Orbiter cracks can occur and also delamination due to the technology. The total thermal input to the bus from different coefficients of thermal expansion of the the heat shield is around 50 W, and the spacecraft materials. Moreover if the temperature is higher, the platform is therefore kept at a temperature of 40°C.

Figure 5.4 : Power management beyond 0.2 AU.

Figure 5.5 : Alternative solution for the power management beyond 0.2 AU.

kW equivalent power at 1 AU. MPPT (Maximum 5.2) Key factors for power management Power Point Tracking) are used to limit currents, The power management is a key aspect of the Phoi- and the solar panels are coupled for one SADM. bos mission for both the transfer module and for the Power in the Probe configuration at Aphelion solar probe. In the composite configuration, all the power is ensured by the 130 m² surface of solar In the probe configuration, the power management panels (4 Rosetta panels), providing the required 33 is ensured by the three deployable 1.36 m² solar

- 28 - panels. However, these panels only provide 80 W at Power 55 kg the aphelion of the science orbit. The probe is there- Thermal protection system 134 kg fore put in a very low state of activity during the thermal control 16 kg major part of the science orbit, with a low spin sta- DH 20 kg bilization around the Sun direction, and the battery Propulsion 26 kg is used at regular interval to perform spacecraft mechanical 108 kg control actions & communications. The battery is harness 30 kg charged between two actions using the 80 W power, first total dry 516 kg together with spacecraft thermal control. System margin (30%) 154 kg TOTAL dry 670 kg Power in the Probe configuration at Perihelion propellant 40 kg PSP total mass at launch 710 kg From the Solar Orbiter and Bepi Colombo devel- opments, it appears that the three panels of the probe can only be used up to 250°C, tilted at ~70°, Phoibos Transfer Module mass budget for heliocentric distances down to 0.2 AU. Below For the transfer module, the main driver of the com- this distance, the panels will therefore be stowed posite mass is the power sub-system. For classical behind the shield and a new power system shall technology such as the Rosetta solar panels, the 130 relay this standard one. The required power can be m² is estimated to 460 kg. Adding the MPPT, generated by thermo-electrical elements and con- SADM, Power harness, battery and regulation, a verters, accommodated at the secondary heat shield total mass of 640 kg can be found. Relying on po- level inside the cone, to take maximum benefit of tential future technologies such as Milard technol- the heat generated by the heat shield (Figure 5.4). A ogy or deployable structure technology, both at ESA plate is used as hot source and the secondary heat R&T level, a less efficient solar panel (330 m² need) shield is used as cold source with a calibrated radia- but lighter one could be set-up, with a total of 345 tor. These thermo elements have to be selected kg for the power sub-system. The Power sub-system pending on the temperature range experienced by is therefore estimated somewhere between 345 kg the plate. The provided power has been estimated to and 640 kg. more than 200 W below 0.1 AU, but the zone be- tween 0.2 AU and 0.1 AU is still a critical part of The needs for the plasmic propulsion are the follow- the mission: this power sub-system of the probe, ing: Far from the Sun, two engines in series are working below 0.2 AU is therefore one of the key needed for time life reasons. Closer to the Sun, two driver of the probe design. An alternative to this engines in parallel are used to optimize the trajec- system would be to deploy small mirrors behind the tory taking advantage of the high power capability. shield, reflecting part of the solar light at 4 Rs to- At intermediate distances from the Sun, one engine wards the folded solar panels (Figure 5.5). With plus very partially a second one in series are needed very high emissive coating and very low absorption to guarantee the duration of the thrusting. Therefore (that can be maintained without ageing effect since for reliability reasons, two cases are considered. they would only be in Sun below 0.2 AU), a small They are displayed in Figure 5.6. mirror would reach ~1200K, below its fusion tem- perature.

5.3) Spacecraft mass budgets Phoibos Solar Probe mass budget The Phoibos Solar Probe (PSP) mass budget has been estimated using the outputs of the NASA STDT/2005 study. It takes into account modifica- tions mainly on the power sub-system which no more uses RTG. Table 5.1 shows this mass budget, which has been estimated to 710 kg, including a 30% system margin and a 50 kg payload. Figure 5.6 : Minimal (top) and high (bottom) configura- tions for the plasmic propulsion module. Table 5.1 : Mass budget for the Phoibos Solar Probe. payload 50 kg The minimal configuration with no additional re- payload accommodation 18 kg dundancy consists of five PPS5000 family engines, Telecoms 21 kg four of them being mounted by pair on two Thrust GNC 38 kg Orientation Mechanisms (TOM) and one being

- 29 - fixed. The high configuration with additional re- The Science operations architecture and share of dundancies consists of six PPS5000 mounted by responsibilities need to be discussed after the agree- pair on three TOMs. The total propulsion system ment between ESA and NASA. includes also Xenon tanks which represents 8.5% of the Xenon mass and a small hydrazine system for Archive approach: Instrument teams will be coor- wheels off-loading and swing-by fine adjustments if dinated by a European and American PI who shall needed. The PTM mass budget is given in Table 5.2 be responsible for providing the Science data (Level where the heavy and light versions of both the 2 products) from the engineering data (Level 1 power sub-systems and the plasmic propulsion products), made from the raw data (Level 0 prod- modules are summarized. It can be noticed that the ucts). The production of science data will be funded structure mass is taken at 20% of the total dry mass by National Agencies. The data should be stored in before system margins. a long-term archive data center at NASA and ESA (ESAC). Table 5.2: Mass budget for the Phoibos Transfer Mod- ule. Proprietary data policy: There shall not be any Heavy Light proprietary data rights for the instrument teams. version version The Level 2 data products should be freely available Power system 640 kg 345 kg to the science community as soon as made available Propulsion 380 kg 330 kg by the instrument teams. Such data policy is now Attitude Control system 30 kg 30 kg common amongst most of the solar and heliospheric Structure (20%) 260 kg 175 kg missions. Sub-total 1310 kg 880 kg System Margin (30%) 390 kg 260 kg 7) TRL & Key technology areas Total dry mass 1700 kg 1140 kg 7.1) Spacecraft TRL & Technology areas As a result, Table 5.3 summarizes to total mass budget for the Phoibos Composite Spacecraft. The following critical technologies are all at R&D stage (TRL 2-4): the advance option of power sub- Table 5.3 : Total Mass budget for the Phoibos system for the Transfer module (solar arrays) and Composite Spacecraft the power subsystem for the Solar probe close to the Heavy Light Sun (thermo-elements). version version Phoibos Solar Probe 710 kg 710 kg The conical heat shield is of prime importance for Phoibos Transfer Module 1700 kg 1140 kg the mission. On the European side there is heritage Total end of cruise 2410 kg 1850 kg from the Solar Orbiter studies (TRL 5) and relevant Xenon for cruise 2291 kg 1756 kg studies are carried out at the French Odeillo Solar Total at launch 4701 kg 3606 kg Furnace facilities (Paulmier et al., 2001; Eck, 2007). On the US side, extensives studies have been car- The "total end of cruise" values have to be com- ried out by both JPL & APL (NASA STDT/2005). pared with the 2051kg from the flight dynamics assumptions. The "total at launch” values have to be Concerning the propulsion module, a PPS5000 pro- compared with the 4000 kg from the flight dynam- totype (called X1000) has passed 1000 functioning ics assumptions. Table 5.3 shows the two bounds of hours on a SNECMA bench. The other parts of the the area of solutions. Even if the heavier solution is spacecraft can reuse standard technology. not fully compliant with the flight dynamics as- sumptions, this table shows that intermediate solu- 7.2) Payload TRL & Technology areas tions can be found to validate the mission feasibil- ity. For most of the instruments, there is a concept that has already been flown in a space environment The Technology Readiness Levels for the spacecraft (Ulysses, SOHO, Wind, ACE, Cluster, Stereo). At a sub-stems are discussed in section 7. conceptual level, the payload is TRL 9.

6) Science Operations and Archiving However, because of the important thermal con- straints, the limited space for accommodation be- hind the shield and the observational configuration at perihelion a whole Technology Development

- 30 - Plan (TDP) has to be set-up from the beginning of star” series of public lectures, informal science pro- the pre-implementation phase in order to optimize grams for science centers and museum visitors. This the quality of both the in-situ and remote-sensing educational effort will focus on magnetism on the observations. For instance this TDP should explore sun and its connection to the corona, the solar wind the actual possibilities for Nadir viewing for both and the influence on the Earth's magnetic field, plasma and energetic particle measurements (at least leading to the questions PHOIBOS scientists hope crude approach using possibly high temperature to answer. An outreach web site will be developed detectors. It should also explore the possibility for with access to the PHOIBOS orbit and science, the RPWI electric field antennas to be in sunlight remote sensing images from current spacecraft and and accommodate both DC and high frequency ground based solar telescopes, providing also for the measurements. Finally miniaturization of electron- active participation of amateur solar astronomers ics and sensor heads is also a desired feature. from around the world. The solar encounter itself will be treated as a climactic event, analogous to 8) International partnership planetary conjunctions or comet approaches to the sun. Indeed, PHOIBOS is a man-made sun-grazing 8.1) International partnership comet: and we plan to develop school challenges at primary and secondary school levels for a logo and The international partnership and sharing of respon- story-line leading up to the first perihelion pass. sibilities will be discussed between ESA and NASA. In section 8.3 the overall mission cost is Acknowledgements : estimated to be around 900 Meuros. We propose The overall mission scenario presented in this pro- therefore to implement PHOIBOS as an L mission posal is the outcome of a specific CNES/PASO with a cost sharing that could range between 1/2 to study1 conducted by Régis Bertrand for the mission 2/3 for ESA & member states and consequently profile and trajectory, Emmanuel Hinglais for the between 1/2 to 1/3 for NASA. In such a situation, system analysis and Jean-Yves Prado for the overall the PHOIBOS launch could occur in December concept. This study has been performed in the frame 2018, as discussed in section 3, or every 1.6 years of the CNES support to the French scientific con- later following the mission launch window. tributors to the ESA “Cosmic Vision” call. The spacecraft accomodation has been provided by As an alternative option, in the case there were an EADS/Astrium. We thank Ivan Juiz for the realiza- opportunity for an earlier launch and an ESA/NASA tion of the cover figure. agreement, PHOIBOS could be envisage as an M mission with a cost sharing of 1/3 for ESA & mem- 1DCT/PO/PA/2007-0011299, Solution technique ber states and 2/3 for NASA. The launch could oc- pour la proposition Phoibos à Cosmic vision, E. cur in this case in October 2015 or May 2017. Hinglais (in english), 2007.

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