Ultraviolet and Extreme Ultraviolet Spectroscopy of the Solar Corona at the Naval Research Laboratory

F222 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article

Ultraviolet and extreme ultraviolet spectroscopy of the solar corona at the Naval Research Laboratory

1, 1 1 2 1 1 J. D. MOSES, * Y.-K. KO, J. M. LAMING, E. A. PROVORNIKOVA, L. STRACHAN, AND S. TUN BELTRAN 1Space Science Division, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington DC 20375, USA 2University Corporation for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307, USA *Corresponding author: [email protected]

Received 8 June 2015; revised 3 August 2015; accepted 17 August 2015; posted 18 August 2015 (Doc. ID 242463); published 17 September 2015

We review the history of ultraviolet and extreme ultraviolet spectroscopy with a specific focus on such activities at the Naval Research Laboratory and on studies of the extended solar corona and solar-wind source regions. We describe the problem of forecasting solar energetic particle events and discuss an observational technique designed to solve this problem by detecting supra-thermal seed particles as extended wings on spectral lines. Such seed particles are believed to be a necessary prerequisite for particle acceleration by heliospheric shock waves driven by a coronal mass ejection. OCIS codes: (300.2140) Emission; (300.6540) Spectroscopy, ultraviolet; (350.1270) Astronomy and astrophysics; (350.6090) Space optics.

http://dx.doi.org/10.1364/AO.54.00F222

1. INTRODUCTION with the launch of the Solar and Heliospheric Observatory [3] Many activities of the sun that affect the terrestrial environment (SoHO) in 1995, and the Solar Terrestrial Relations Observatory [4] (STEREO) in 2006. Instruments on SoHO and human society lie in the solar corona. Such activities in- – clude the solar wind, solar flares, and coronal mass ejections studied CME eruptions from about 1.5 30 solar radii (R⊙), (CMEs). Tousey and Friedman at the Naval Research while STEREO provided a 3D view of these events from Laboratory (NRL) made the initial discoveries of ultraviolet 1.5 R⊙ to the orbit of Earth. and x-ray emission from the solar corona shortly after This paper describes new instrumentation designs required to attack two specific problems arising out of the SoHO ob- World War 2 with the aid of captured German V2 rockets servations: the role of wave-particle interaction in the acceler- [1]. In 1958 Friedman demonstrated that solar x-ray emission ation of the solar wind and the initiation of solar energetic is extended beyond the visible solar disk and that the solar particle (SEP) events by shock waves driven by CMEs. corona is structured across the disk. This was achieved through Section 2 describes observations of the solar corona in a broader observation during a solar eclipse with a series of Nike-Asp context with the ultimate goal of extending the initial SoHO flights carrying nonimaging x-ray sensors. This use of the moon results on solar wind acceleration. Section 3 describes the as an optical element from a space platform was reprised in particular problem of particle injection into the CME shock 1964 with Freidman’s experiment to measure the angular size acceleration process while Section 4 describes the observables of the x-ray source in the Crab nebula. that should be associated with it. Finally, in Section 5 we dis- Koutchmy [2] gives an account of the early history of the cuss the instrumentation required both for a scientific valida- development of the technique of white-light coronagraphy by tion of our hypothesis and for an experiment designed to yield groups at NRL and other institutions where outside of solar real-time monitoring. eclipses, an artificial occulter is used to block out light from the solar disk allowing the outer corona to be studied. Coronal mass ejections were discovered from space in 1971. 2. SPECTROSCOPY OF THE CORONA AND NRL instruments on Skylab in 1973/74 revolutionized our SOLAR WIND SOURCES view of the solar upper atmosphere, and the SolWind The extended solar corona far off the solar limb is the site where Coronagraph observed the first Earth directed halo coronal the solar wind is accelerated and coronal plasma is nonther- mass ejection (CME) in 1979. Our view of not just the solar mally heated to millions of degrees. It is also the region where atmosphere but the extended corona was further transformed propagating CME shocks are formed (e.g., see [5]) and Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F223 subsequently produce SEPs [6,7]. Observations of the extended understanding how the fast solar wind is accelerated. corona are thus essential for understanding the formation and Cranmer et al. [10] present a summary of these and other physi- evolution of these activities. Especially valuable are observations cal insights that were gained from the UVCS observations. The by spectroscopic means. The very same technique has long most important finding from UVCS has been the discovery of been used to understand our universe. The power of spectros- broadened emission line profiles of O VI (the spectrum of five- copy is that it allows us to probe the local physical state of the times ionized oxygen). These broad line widths suggest that the corona and other remote locations in the universe where direct O5 ions are heated by transverse Alfvén waves, which are in in situ sampling is still not possible. resonance at the ion cyclotron frequency for the ion. While The solar corona presents dozens of ultraviolet and extreme there is no direct detection of the waves themselves, over ultraviolet (UV/EUV) emission lines that allow the plasma the years a number of authors (e.g., [11] and references therein) properties (e.g., densities, temperatures, outflow velocities, have developed models to show how these waves are generated and abundances) in the corona to be determined from remote by motions at the coronal base. Another key UVCS observation sensing diagnostics. In fact, NRL’s Space Science Division has a that supports the presence of Alfvén waves is the fact that the rich history of solar spectroscopy of the disk and limb (see oxygen ions in fast solar wind appear to flow out of coronal Doscheck et al., this issue). However, to make such measure- holes faster than the protons above ∼2.5 R⊙ [12]. This differ- ments at heights beyond about 1.5 R⊙ (from sun-center) ence in speed is preserved in the distant solar wind as measured, requires that the solar disk be occulted in order to keep the for example, by the Advanced Composition Explorer at 1 AU, intense brightness of the disk from overwhelming the faint which shows that, in general, ions have outflow speeds above UV and EUV emissions in the extended corona. the protons by an amount on the order of the local Alfvén The observational problem is shown in Fig. 1. It shows coro- speed [13]. nal-hole intensities, relative to the disk, for a few of the bright- In addition to the investigations of the fast solar wind, more est emission lines observable by the ultraviolet coronagraph recent work involves understanding the sources of the slow- spectrometer (UVCS) on SoHO [8] (plus He II 30.4 nm, speed wind. The energy requirements are comparable for the which is not observed with UVCS). The He II intensity for slow wind but the large variability in many parameters, including all heights and the intensities of the weaker lines (Mg X and wind speed [14], density, elemental abundances, and the Si XII) above 2.5 R⊙ are extrapolated based on the density first ionization potential (FIP) effect (see below), makes this variation with height. (Intensities for coronal streamer and problem difficult. Theoretical ideas involving the magnetic field CMEs can be 3–10 times higher.) As can be seen in the figure, foot-point interchange reconnection [15], expansion factor radiation from the disk is orders of magnitude greater than the (e.g., [16]), and unique field-line topologies (e.g., S-Web, [17]) emission lines we wish to observe. The solution is to use a com- have been proposed to explain various properties of the slow bined spectrograph and coronagraph [9] to make the required wind. A surprising outcome of this work is that it is now becom- observations of emission lines far off-limb in the extended ing clear that the wind speed alone is no longer sufficient to corona. As described in Section 5, the coronagraph reduces identify slow solar wind from fast wind. Elemental and charge the disk radiation to acceptable levels with occultation, while state abundances may be a better discriminant for determining the spectrograph is used to reject off-band radiation from both the type of solar wind and its coronal source regions [18]. Off- nearby lines and scattered disk radiation within the instrument. limb coronal abundance determinations (e.g., [19–22]) have The selection of optical coatings also plays a role in the rejection proved useful in identifying sources of slow wind in streamers of off-band radiation. and other localized regions of the corona. Elemental abundance The UVCS mission on the SoHO was the first to use measurements on coronal loops have provided constraints on these techniques to make groundbreaking advances in our ion-neutral fractionation processes occurring at the loop chromospheric foot points that point to the important role of magnetohydrodynamics (MHD) waves [23]. This leads to the FIP effect, where elements with low FIP that are ionized in the chromosphere (e.g., Fe, Si, and Mg) are enhanced in abundance in the corona with respect to high FIP elements (e.g., O, Ne, and Ar) that remain neutral. New observations with a dedicated coronal spectroscopy mission are needed to determine the properties of these waves and to understand their role in FIP fractionation and the solar-wind acceleration processes. Doppler shift and linewidth measurements have contributed to our understanding of the 3D dynamic structure of CMEs and the CME shock properties, both of which can only be obtained unambiguously from spectroscopic means. Doppler shifts, when combined with the plane of sky velocities, have revealed helical motions of CME plasmas, which provide infor- Fig. 1. Coronal hole intensities, relative to the disk, for a few bright mation on the 3D magnetic-field structure. Spectroscopic de- UV/EUV lines. The relative intensities of all lines, except He II, are terminations of ion temperatures from linewidths and charge based on UVCS observations. states have also provided valuable tools for understanding F224 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article

CMEs. Studies of CME energy budgets [24] show that addi- This indicates that there are other important physical factors tional heating beyond the kinetic energy of the mass is required affecting this variability beside CME shock speed. to explain the observed spectral lines. These CME studies have Several recent observations of SEPs supported by theoretical relevance beyond providing a better understanding of physical studies presented clear evidence for existence of supra-thermal processes in the solar atmosphere. They are also important be- seed particles in the corona as a prerequisite for intense SEP cause CMEs are a major driver of space weather [25–27], which events. Kahler et al. [29] considered various factors other than has the potential of causing severe damage to space-based CME speed that may be important for production of SEPs such systems, power grids, and oil pipelines. as CME width and location, its origin in the corona, and am- While SoHO UVCS provided critical clues about the proc- bient SEP intensity. They analyzed 17 fast CME events in a esses that take place in the corona, it was limited by the effective relatively narrow speed range of 650–850 km/s with associated area of its light gathering optics. As an example of the limita- SEP intensities spanning more than four orders of magnitude. tions, significant nonthermal heating above ∼1.5 R⊙ in coro- They concluded that the background intensity of the SEPs nal holes could only be determined for two heavy ions, O5 prior to the CME eruption is the most significant factor for 9 and Mg . In order to study the charge-to-mass dependence the CME-associated SEP intensity. Other factors were shown of the nonthermal heating, the measurements of line profiles not to be important. Gopalswamy et al. [30] found that the for many more ions are desired. For another example, high-intensity SEP events occurred whenever a CME is pre- UVCS was not able to determine if there exists a supra-thermal ceded by another wide CME from the same active region. population of protons or other ions in the corona. The presence Their interpretation was that a preceding CME disturbs the of sufficient quantity of these particles could provide the nec- interplanetary medium and may produce seed particles for essary seed particles for shock acceleration in the corona. (See the acceleration at the following CME shock. Cliver [31] con- [28] and Sections 3 and 4 for more details.) Direct measure- sidered two exceptional ground-level enhancement (GLE) ment of the usually faint CME shock front and its interaction events associated with relatively weak flares and slow CMEs with the ambient corona would also benefit from enhanced as opposed to GLEs normally produced in intense flares and light gathering power of a more advanced instrument. fast CMEs. He suggested that enhanced-background proton- intensity enabled the acceleration of protons to GeV energies 3. INITIATION OF SEP EVENTS in these weak solar events. SEPs are high-energy charged particles produced in eruptive Element abundance variations in SEPs have also been inter- processes such as CMEs and flares low in the solar corona. preted in terms of seed particles produced as supra-thermal ions Their energy reaches from a few keV to several GeV. SEPs in impulsive flares [32,33]. In large SEP events, at the highest streaming to Earth can damage satellites, disrupt radio commu- energies, the abundance ratios Fe/O and sometimes 3He∕4He nication, and global satellite positioning. Gradual SEPs are are elevated over their usual coronal values. This suggests that thought to be accelerated primarily by the CME-driven shocks close to the sun, where what ultimately become the highest en- (a schematic picture is shown in Fig. 2.), as there is a correlation ergy particles are injected into the acceleration process, these between peak proton intensity of an SEP event at 1 AU and particles have a different provenance than the rest of the associated CME speed. However, for any given CME speed, SEP distribution, i.e., they are likely to be supra-thermals from peak intensities may vary by three to four orders of magnitude. an impulsive flare, where such abundance anomalies are well- known. All the above observations suggest that to explain the SEP intensity variations, it is crucial to understand the characteristics of the background plasma where they are produced and, in particular, the processes of generation of supra-thermal particles in the corona. Theoretical studies point out the necessity of high-energy particles to initiate diffusive shock acceleration and support the impact of pre-existing supra-thermal particle distributions on the variability of shock accelerated SEPs. Injection of particles into the shock acceleration process likely occurs when the CME shock is within a few solar radii of the solar surface [32,34,35]. Close to the sun, CME driven shocks are often expected to be quasi-perpendicular in geometry, requiring a higher energy or harder spectrum of seed particles to initiate SEP acceleration than would be the case for quasi-parallel shocks. Zank et al. [36] showed that a particle has to have sufficient initial energy to be accelerated by the diffusive shock acceleration mechanism, and this energy threshold is higher at a quasi-perpendicular shock than at a quasi-parallel shock. Laming et al. [28] revisited this seeking a prescription in terms Fig. 2. Schematic picture representing acceleration of SEPs at a of the supra-thermal particle distribution function rather than CME shock. the energy. Their conclusion was that quasi-perpendicular Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F225

and energy of accelerated nonthermal particles. Simple analytical studies [37–39] suggest that ions can be accelerated by the first-order Fermi mechanism in the interaction with convergent magnetized flows associated with magnetic reconnection. In this process, ions with mean free paths larger than the thickness of the current sheet cross the reconnection layer and gain energy from each crossing similar to the process of diffusive shock acceleration. Other possible ion acceleration processes include Fermi acceleration of ions trapped in plasma islands as they pass through the fast termination shock in reconnection outflows [40] and acceleration of pickup ions, with charge-to-mass ratios above a certain critical value, by outflows in reconnection regions [41,42]. Reconnection could produce a suitably hard spectrum of seed particles, if sufficiently high-density compression can be achieved in the reconnection region. Further studies are needed to confirm this.

4. OBSERVATIONAL SIGNATURES OF SEP SUPRA-THERMAL SEED PARTICLES

Out to a distance of about 3.5 R⊙ heliocentric distance, protons and neutral hydrogen are effectively coupled by charge exchange reactions so that the proton distribution function may be revealed by spectroscopic observations of emission lines from neutral H [43,44]. The Lyman α transition (Lyα) is by far the strongest emission line available. It is mainly excited in the outer corona by absorption of Lyα photons emitted from the solar disk followed by re-radiation. Thus the prediction Fig. 3. Contours of upstream growth rate from Laming et al. [27]in κ − θ κ of the observed line profile requires a treatment of the scattering BN space. ( is the index of the kappa distribution function of the θ of Lyα by H atoms with a supra-thermal ion distribution and supra-thermal ions, and BN is the shock obliquity.) The region of growth is to the lower left of the dashed lines, which represent zero moving radially with respect to the sun as the solar wind begins growth. The top panel is for shock Alfvén Mach number M A 2 (at to accelerate. the distance 2R⊙), the bottom for M A 4 (at the distance 2.6 R⊙). Photon redistribution functions in the corona are given by At lower M A, a lower κ (i.e., harder supra-thermal distribution) is Cranmer [45] in the form of κ distributions. Integrating over required to grow waves, which lead to SEP production. initial photon direction (azimuthal and polar angles), frequency, and along the observer’s line of sight, each point on the line profile is given as a four-dimensional nested integral. Such calculations are carried out by Laming et al. [28] for a shocks require a harder spectrum of supra-thermals than do variety of off limb positions, with the κ distribution in each quasi-parallel to initiate diffusive shock acceleration. Their case chosen at the threshold for exciting waves ahead of a − calculations demonstrated that a stronger non-thermal tail of 2000 km s 1 shock with obliquity 45° motivated by a study particle distribution function is required for shocks closer to of the 20 January 2005 CME. the sun with low Alfvén Mach number M A compared to those Figure 4 shows the results of such calculations meant further out from the sun with higher Mach numbers (see to match the cases shown in Fig. 3. In each case, the thin Fig. 3). A simulation of proton shock acceleration [28] with histogram shows the effect of a Maxwellian H atom distribu- higher and lower densities of seed particles (one order of mag- tion. The thick dashed histogram shows the effect of a κ dis- nitude difference) showed that the resulting high-energy proton tribution (κ 2 and 4.5, respectively), while the solid line intensity can vary by several orders of magnitude. The results of shows a possibly more realistic case of 10% κ, 90% Zank et al. [36] and Laming et al. [28] support the inference of Maxwellian distributions. These simulations assume a 103 s in- Tylka and Lee [32]. tegration time with an instrument of effective area 1cm2, and A number of theoretical studies demonstrated that particles give photon counts in 0.1 Å bins in a spatial pixel of 1 arcmin2. can be accelerated in the process of magnetic reconnection, Under such observing conditions, the supra-thermal distribu- which ubiquitously occurs in the strongly magnetized solar tion appears to be distinguishable from a Maxwellian out to corona. Magnetic reconnection is the process of reconfiguration about 2.5 R⊙. The intensity decreases by about an order of and annihilation of the magnetic field over a small-scale vol- magnitude between 1.8 and 2.5 R⊙. due to the decreasing ume. In this process the energy stored in the magnetic field density of scattering H atoms and the dilution of the incident is converted to the kinetic and thermal energy of the plasma radiation. F226 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article

instrument, a systems approach is required. If nothing else, launch costs would prohibit a simple 100× scaling of the SoHO UVCS instrument. A solution to this problem requires an approach combining development of each optical element and detector, a novel coronagraphic design, spacecraft design, and mission design. A. Grating Scatter Progress in the development of technology for the production of low-scatter optics of all kinds and of gratings in particular has been extremely rapid over the 20 years since the SoHO instruments were developed [46]. While these technologies were primarily driven by requirements from outside of the field of helio- physics, the happy consequence is the availability of gratings with scattering that can be characterized as an order of magnitude lower effective surface roughness over that of the SoHO UVCS gratings. A review of the relative contributions to the instrument-induced noise indicates this improvement is essential for the detection of supra-thermal seed particles within a time- scale commensurate with their residence time in the corona. In Laming et al. [28] we identified the 1–3 Å band from line-center as the most sensitive range for determining depar- tures from Maxwellians. In this range the dominant contributions are the off-band scatter from the diffraction grating and the stray disk light from the coronagraph occulting system. These components are shown in Fig. 5 as dot-dashed and dot- ted lines, respectively. The grating scatter is shown assuming Fig. 4. Profiles of H I Lyα predicted at distance 2R⊙ (M A 2; σ M two different rms surface roughness models: 20 Å (red top) and at distance 2.6 R⊙ ( A 4; bottom). The thin histogram curve) similar to the SoHO/UVCS gratings and σ 5 Å (blue shows the effect of a Maxwellian. The thick dashed histogram shows the effect of a κ distribution (κ 2 and 4.5, respectively). The solid curve), which is more typical of modern gratings. The residual line shows 10% κ, 90% Maxwellian. stray disk light is representative of a coronagraph with −8 stray-light rejection of B∕B⊙ ≤ 10 (typical of UVCS). The disk-line profile shows extended wings from radiation transfer effects making characterization/validation of the coronagraph 5. NEW INSTRUMENTATION stray-light performance an important issue for data reduction. The comprehensive summary of Kohl et al. [8], written after Another contribution to the off-band profile is the broad almost a decade of SoHO operations, outlines the technical and electron-scattered Lyα profile (triple-dot, dashed line in scientific progress made with the SoHO UVCS investigation. A Fig. 5) produced by free electrons in the corona. This profile comparison of the SoHO UVCS capabilities with the require- can be modeled and subtracted from the measurement with the ments of the investigations described in Sections 2–4 above help of knowledge about the electron density. The narrow geo- leads to the identification of two technological areas in which coronal Lyα contribution (not shown) has little effect in the improvements are necessary to achieve the new science out- lined above. 1. At least a 10× improvement in grating scatter to achieve measurement of the line shape of diagnostic UV emissions under the conditions provided in the solar corona. 2. At least 100× increase in light-gathering power to improve the signal to noise ratio obtained with integration times and areas characteristic of solar coronal phenomena. The NRL SSD Solar and Heliospheric Physics Branch has undertaken a broad-based investigation of instrument technologies over the past 20 years directly applicable to improvements in these two areas. To address the first issue, achieving the requisite improvement in grating scatter, a specific technological development task is needed—primarily encompassing incre- mental improvements in optics fabrication, coating technology, Fig. 5. Coronal streamer observation at 1.8 R⊙ showing the differ- and contamination control. In contrast, to address the second ent components of the observed Lyα profile (black solid line). See the issue of increasing the light-gathering power of a space-based text for a description of the various components. κ is as defined in [45]. Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F227 wavelength region where the κ profile makes a departure from distance from the coronagraph objective in flight, while com- a pure Maxwellian. The expected resonantly scattered Lyα pressing down to a very small volume for launch [51]. profile is shown as the black dashed line. All curves are on Considered risky at the time it was first proposed, the success the same vertical scale which is in counts/bin. The bin size of the Nuclear Spectroscopic Telescope Array (NuSTAR) small (radial × transverse) is 0.17′ by 1.1′. Explorer mission (see e.g., [52]) demonstrates a 10 m extend- In order to reduce the grating scatter for an instrument able boom can reliably perform with greater precision than re- optimized for seed-particle detection, it is desirable to use a quired for a coronagraph. The flight dynamics of a boom- holographic grating, which has been shown by Fineschi et al. based coronagraph of ≥10 m demand a dedicated spacecraft [47] to have a factor of 4 lower scattering, compared to conven- with a specific attitude control system and thus limit the tionally ruled gratings for the UV. Fineschi et al. [48] compared opportunities for flight. scattered light measurements of the UVCS Lyα holographic Another approach to increasing the occulter to objective grating to an analytical model of the grating scatter. From this distance is to locate the occulter and coronagraph telescopes they computed an effective rms roughness (σ) and correlation on separate, formation-flying spacecraft. Analogous to the length (γ) for the grating surface and determined their values to NuSTAR use of an extendable boom, formation flight has been be 20 Å and 6.0 μm, respectively. The UVCS gratings were extensively studied for various forms of astronomical instru- manufactured in the mid-1990’s but more recent holographic mentation. Use of this approach for a solar coronagraph has gratings have been produced with better surface properties. Ion been proposed for the first two European Space Agency etched, holographic gratings for two recent NRL programs, the (ESA) M-class missions (e.g., [53]). Further, this approach is Joint Astrophysical Plasma Dynamics Experiment (JPEX) in development for the ESA Proba-3 formation-flight technol- sounding rocket and the Extreme-Ultraviolet Spectrograph ogy demonstration mission as ASPIICS (the French acronym (EIS) on Hinode, were each measured to have an rms surface meaning “Association de Satellites Pour l’Imagerie et roughness σ < 3 Å[49,50]. Using only a value of σ 5 Å for l’Interférométrie de la Couronne Solaire”). ASPIICS is not the surface roughness, the modeled grating scatter shows a the prime mission objective, but serves as a means to gauge dramatic improvement in the estimated scatter (see Fig. 5), the success of the formation flight technology. ASPIICS in greater than the factor of 4 difference between holographic and its final form is a visible light coronagraph observing the K- mechanically ruled gratings found in [42]. corona and prominences in the He I 587.5 nm (D3) line The residual radiation from the solar disk that is scattered by [54]. Although the ASPIICS aperture is not the maximum al- the occulting system and interior surfaces has a smaller effect on lowed by the 144 m separation between the occulter and ob- the coronal profile in the near wings. This stray-light back- jective, the separation results in negligible vignetting of the ground can be estimated before launch by laboratory measure- telescope aperture at very low coronal altitudes (1.08 R⊙ inner ments or calibrated in-flight by determining if there are field of view)–similar to an eclipse of the sun by the moon as detector counts in the cooler spectral lines that are not expected viewed from Earth (3.5 × 108 m separation between occulter to to be produced in the extended corona. objective). In principle, accurate measurements of the coronal line In addition to the added cost of implementing two space- shape can still be made, even with backgrounds higher than craft for a formation flight coronagraph, the lifetime of the mis- the coronal signal in the wings of the line. This is achieved sion is severely constrained by high propulsion consumption to by carefully performing the optical characterization of the achieve the correct formation in all but a few orbital configu- instrument and then subtracting out the background compo- rations. Wood and Breakwell [55] showed that continuous for- nents from the observed line profile. However, even after these mation flight in a low Earth orbit (LEO) would require the corrections, residual noise on these backgrounds will still occulter spacecraft propulsion system to supply sufficient remain. thrust to achieve a Δv 8km∕s∕year (compared to Earth B. Light Gathering Power escape velocity of 11 km/s). In order to achieve an extended Various possibilities for improving the light gathering through- formation flight mission, Proba-3 uses a highly elliptical orbit put are summarized in Table 1. One approach to achieving (60; 530 km × 600 km altitude, with an eccentricity of 0.81). increased light gathering power is to consider optical designs The ASPIICS formation flight is limited to the approximately with increased collecting area. However, simply scaling the 6 h apogee arc, thus providing a total duty cycle of ∼30%. With UVCS design to achieve larger aperture quickly results in an this strategy, the required Δv per orbit is less than 10 cm/s, impractical instrument size for spaceflight. The difficulty with which is manageable with a normal spacecraft propulsion sys- externally occulted solar coronagraphs is, in general, the requi- tem. For continuous observations, inner and outer Lagrangian site occulter to objective distance increases linearly with the point orbits have been considered (e.g., [53]). Moses and unvignetted aperture available for observations at any given Fineschi [56] explored several variations on the Wood and coronal elongation. Thus, direct scaling of UVCS to the re- Breakwell paradigm in LEO, including the use of a series of quired collecting area would result in an instrument greater disposable nanosat occulters and the use of alternative means than 10 m length. of supplying the requisite Δv such as a photonic laser thruster The resources to launch a monolithic spaceflight instrument [57] or a conductive tether [58] operating between corona- of >10 m length will not be available for quite some time. The graph and occulter. first solution to be considered for this problem was to employ An alternative to gaining light-gathering power by increas- an extendable boom to support the occulter at the correct ing the separation of occulter to objective can be achieved by F228 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article

dedicated to this specific measurement that could be designed to fit within the approximately 1m3 volume allotted for standard International Space Station attached payloads. In order to achieve this goal (and in contrast to the Beckers’ design), a linear occulter system was employed with the direction of the baffle aligned parallel to the orientation of the spectrometer entrance slits (as in UVCS). In order to ob- Fig. 6. Optical diagram of a nested-coronagraph design. For clarity, serve the range of coronal heights over which a shock is formed only four nested segments are shown in this illustration. Each of the in a point-and-stare mission operations approach, a spectro- four segments is roughly equivalent to UVCS. All direct solar-disk coronagraph design employing two slits at two coronal heights light entering the instrument through the forward external occulter is necessary, as described by Fineschi et al. [61]. Light is focused (left) is prevented from reaching the objective by the heat rejection on separate spectrometer entrance slits for the two observatio- mirrors (middle). The objective baffles (right) serve as internal occulters to prevent any diffracted light from reaching the spectrograph slit. nal heights. The dispersion of the spectrometer is adjusted so that the off-band radiation from the each slit can be baffled to prevent overlap of the two spectra that are imaged by the grating onto the detector with spatial separation in the direction following the path taken in increasing the light-gathering power of dispersion. A more detailed consideration of the corona- of grazing incidence x-ray telescopes, which are nesting multi- graphic portion of such an instrument is presented in Fig. 7. ple optical channels of the same focal length and the same focal Radiation from coronal heights of 1.8 and 3.0 R⊙ propagates plane. As in a nested-grazing incidence telescope, a nested through the slots in the A0 baffle aperture onto the primary coronagraph consists of a series of stacked confocal occulter- mirror, which focuses the light on two spectrograph slits in objective pairs in order to increase the net collecting area of the instrument. In contrast to nested-grazing incidence telescopes, one does not actually need to employ a physically separate annular optical segment for the objective elements of the individual nested channels of the class of nested coronagraphs considered in this paper. The difficult opto-mechanical challenge of multiple objectives can be circumvented in the case of a normal incidence instrument with a single objective and a segmented objective baffle. Figure 6 illustrates a design, which in cross-section is equivalent to four UVCS-like coronagraphs, with the same focal length but proportionally larger off-axis optical figures in order to maintain a common focal plane. As can be seen in the figure, the same result is achieved simply and accurately by masking a single off-axis mirror with an objective baffle with four appropriate apertures. After the initial nested coronagraph design work was developed at NRL [59], we discovered the concept was not entirely new due to investigation by Beckers and Argo [60] for the same application of H Lyα coronal spectroscopy. Although the stray- light performance of the Beckers design was successfully characterized in ground testing, and the performance of the nested-coronagraph approach was verified, the instrument was never used in a spaceflight environment for coronal observations. With the selection of UVCS for the SPARTAN program and then SoHO, development of the Beckers’ instrument stopped and this technique was largely forgotten. The design goal at NRL for a nested coronagraph was spe- cifically directed toward the objective of making a definitive test for the existence of supra-thermal seed particles considered essential in many acceleration models for the production of high-energy SEPs. As demonstrated above, this objective can be achieved with high-contrast spectroscopy of the resonantly scattered H Lyα line at 120 nm if the instrument can achieve Fig. 7. Schematic of a nested linear occulter coronagraph designed light-gathering power sufficient to collect 106 photons per spa- to obtain coronal spectra at 1.8 R⊙ and 3.0 R⊙. This combination of tial bin (e.g., 15 arcsec) per temporal bin (1000 s). The field of linear occulters (with light trap), internal occulters, and slits prevent view of the instrument must span the region of the corona any diffracted radiation from entering the spectrometer. Scatter from where a fast CME could first exceed the local Alfvén speed the mirror surface roughness and contamination is the only significant and become a shock: 1.8–3R⊙. We discovered an instrument source of stray light. Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F229

Table 1. Summary of Coronagraphic Design Approaches to Increase Light Gathering Power

Technique Pro Con Upscale SoHO Instrument properties are well understood Length of monolithic Instrument prohibitively UVCS expensive to launch Extendable Boom Optical and dynamic properties are well understood Dedicated spacecraft required External Occulter Formation Flight Extremely large occulter to aperture separations enable large 1. Cost of two spacecraft increases in aperture and low observational heights 2. Few orbits allow continuous observations 3. Rapid propellant depletion in most orbits Nested coronagraph Very compact approach to increasing total aperture with no stray 1. Inner FOV limits light penalty 2. Low f/number increases imaging aberrations Internally occulted Compact approach 1. Stray light limits outer FOV (UV better than visible) coronagraph 2. Stray light reduces dynamic range of spectroscopy—a challenge for seed particle observations. the focal plane. (Note the solar illumination comes from the It is interesting to note that although the number of baffle right side in Fig. 7 while it comes from the left in Fig. 6.) edges in the system is much greater than that of UVCS, the In Fig. 7 the path of coronal light from the two coronal ratio of stray light to signal from each channel (slot) is the same. heights is shown for three of the channels of the system, illus- Thus the total stray light to signal level ratio does not change trating for this particular design how the light from each for this new design. A modest over occultation is all that is channel does not overlap the light from the next in the plane required to achieve the necessary stray-light suppression −7 −8 of the objective mirror. Thus, each channel can be considered (<1 × 10 B∕B⊙ at 1.8 R⊙ and <1 × 10 B∕B⊙ at 3.0 R⊙. separately. Figures 7(b) and 7(c) show the path of the direct All of the above designs are externally occulted coronagraphs illumination, Fig. 7(b), and the scattered illumination, because this approach is usually necessary to achieve sufficient Fig. 7(c), through a single channel of the front baffle and pri- stray-light suppression of the disk intensity at the coronal mary mirror assemblies. Note that in Fig. 7(b) of this design, heights of interest to both solar wind and SEP investigations light from the two different coronal heights do not overlap in (i.e., ≥3R⊙). However, Kohl et al. [62], demonstrated the the plane of the objective. The direct radiation from the disk residual stray light of an internally occulted UV/EUV corona- and diffracted radiation from solar-illuminated edges is trapped graph at important lines useful for solar wind research can be by a set of mechanical baffles as shown in Figs. 7(b) and 7(c).In removed analytically at heights up to 3R⊙. The utility of this this example an extended occulter baffle, which absorbs the approach for the spectroscopically more demanding SEP seed- unwanted direct and scattered light, is used instead of the particle measurements is still under investigation. It is likely heat-rejection mirrors in the design presented in Fig. 6. that the best approach to achieve both objectives at the present Note in Fig. 7(c) that only two rays (illustrated in dark yellow) time is an externally occulting coronagraph with current state- of all possible light diffracted from the front of the occulter of-the-art gratings, mirrors, and other optical components. baffle at point α can be focused onto either of the two spectrograph slits. This is the only singly scattered disk light that could 6. SUMMARY potentially reach the slits. The objective baffles labeled (1) and UV spectroscopy of the extended corona has proven to be an (2) in Fig. 7(c) function as internal occulters and absorb those effective means for diagnosing the source regions of the solar two rays. The same is true of the two rays illustrated in dark wind and shows great promise for doing the same for solar en- blue generated from diffraction at the rear of the external ergetic particles. Measurements of line intensities, Doppler occulter assembly (point β), which are absorbed by the internal widths, charge states, and abundances can reveal details about occulters labeled (2) and (3). These rays include the only the plasma conditions of the bulk solar wind. These properties doubly scattered disk light and singly scattered light from lower can be compared directly with models of coronal heating and coronal heights that could potentially reach the slits. Note that acceleration. no direct disk ray can hit the rear baffle (point β), so the light Newer diagnostics for measuring the line shape in the far illuminating this edge is greatly reduced from that illuminating wings are being studied for investigating the properties of the front edge (point α). The mirror has a set of internal supra-thermal seed particles, which are believed to be necessary occulters for each stack pair. for SEP production. The theoretical work and advances in tech- The objective baffle edges diffract any light illuminating nologies for low-scatter gratings, high-efficiency coatings, and them but with even minimal over-occultation this illumination coronagraph designs have matured sufficiently for making the is small. Complementary baffles can be located in front of the required SEP measurements. grating to address the objective baffle diffraction but a ray trace indicates this is not necessary for the design illustrated. The Funding. NASA (NNG13WF95I); NRL CNR 6.1. remaining scatter from the objective mirror roughness and from particulate contamination are minimized with super-polished Acknowledgment. The authors acknowledge the contri- mirrors, which are now available, and by carefully avoiding bution to the development of these concepts by the C-SPEX contamination during instrument development. (Coronal Seed Particle Explorer) consortium (The C-SPEX F230 Vol. 54, No. 31 / November 1 2015 / Applied Optics Research Article investigation was proposed to the NASA 2011 Explorer 19. Y.-K. Ko, J. C. Raymond, T. H. Zurbuchen, P. Riley, J. M. Raines, and “ announcement of opportunity) and the HERSCHEL (Helium L. Strachan, Abundance variation at the vicinity of an active region and the coronal origin of the slow solar wind,” Astrophys. J. 646, Resonant Scattering in the Corona and Heliosphere) consor- 1275–1287 (2006). tium that developed the NASA Sounding Rocket Program 20. U. Feldman and E. Landi, “On the sources of fast and slow solar wind,” investigations 36.211DS and 36.307DS. J. Geophys. Res. 110, A07109 (2005). 21. M. Uzzo, Y.-K. Ko, J. C. Raymond, P. Wurz, and F. M. Ipavich, “Elemental abundances for the 1996 streamer belt,” Astrophys. J. REFERENCES 585, 1062–1072 (2003). “ 1. H. Friedman, The Astronomer’s Universe (Norton, 1998). 22. J. C. Raymond, Composition variations in the solar corona and solar ” – 2. S. Koutchmy, “Space-borne coronagraphy,” Space Sci. Rev. 47, wind, Space Sci. Rev. 87,55 66 (1999). “ 95–143 (1988). 23. J. M. Laming, The FIP and inverse FIP effects in solar and stellar ” 3. V. Domingo, B. Fleck, and A. I. Poland, “The SoHO mission,” Solar coronae, arXiv:1504.08325. “ Phys. 162,1–37 (1995). 24. N. A. Murphy, J. C. Raymond, and K. E. Korreck, Plasma heating 4. M. L. Kaiser, “The STEREO mission: an overview,” Adv. Space Res. during a coronal mass ejection observed by the solar and heliospheric ” 36, 1483–1488 (2005). observatory, Astrophys. J. 735, 17 (2011). “ 5. A. Vourlidas, S. T. Wu, A. H. Wang, P. Subramanian, and R. A. 25. D. V. Reames, Particle acceleration at the Sun and in the ” – Howard, “Direct detection of a coronal mass ejection-associated heliosphere, Space Sci. Rev. 90, 413 491 (1999). shock in large angle and spectrometric coronagraph experiment 26. G. Toth, I. V. Sokolov, T. I. Gambosi, D. R. Chesney, R. C. Clauer, D. white-light images,” Astrophys. J. 598, 1392–1402 (2003). L. de Zeeuw, K. C. Hansen, K. J. Kane, W. B. Manchester, R. C. 6. A. J. Tylka and M. A. Lee, “A model for spectral and compositional Oehmke, K. G. Powell, A. J. Ridley, I. I. Roussev, Q. F. Stout, O. variability at high energies in large, gradual solar particle events,” Volberg, R. A. Wolf, S. Sazykin, A. Chan, B. Yu, and J. Kóta, “ Astrophys. J. 646, 1319–1334 (2006). Space weather modeling framework: a new tool for the space science ” 7. D. V. Reames, “Solar release times of energetic particles in ground- community, J. Geophys. Res. 110, A12226 (2005). level events,” Astrophys. J. 693, 812–821 (2009). 27. W. D. Gonzalez, B. T. Tsurutani, and A. L. Clúa de Gonzalez, “ ” 8. J. L. Kohl, G. Noci, S. R. Cranmer, and J. C. Raymond, “Ultraviolet Interplanetary origin of geomagnetic storms, Space Sci. Rev. 88, – spectroscopy of the extended solar corona,” Astron. Astrophys. 529 562 (1999). Rev. 13,31–157 (2006). 28. J. M. Laming, J. D. Moses, Y.-K. Ko, C. K. Ng, C. E. Rakowski, and “ 9. J. L. Kohl, R. Esser, L. D. Gardner, S. Habbal, P. S. Daigneau, E. F. A. J. Tylka, On the remote detection of suprathermal ions in the solar ” Dennis, G. U. Nystrom, A. Panasyuk, J. C. Raymond, P. L. Smith, L. corona and their role as seeds for solar energetic particle production, Strachan, A. A. van Ballegooijen, G. Noci, S. Fineschi, M. Romoli, A. Astrophys. J. 770, 73 (2013). “ Ciaravella, A. Modigliani, M. C. E. Huber, E. Antonucci, C. Benna, S. 29. S. W. Kahler, J. T. Burkepile, and D. V. Reames, Coronal/interplan- Giordano, G. Tondello, P. Nicolosi, G. Naletto, C. Pernechele, D. etary factors contributing to the intensities of E > 20 MeV gradual so- ” Spadro, G. Poletto, S. Livi, O. von der Lühe, J. Geiss, J. G. lar energetic particle events, in Proceedings of the 26th International Timothy, G. Gloeckler, A. Allegra, G. Basile, R. Brusa, B. Wood, O. Cosmic Ray Conference, D. Kieda, M. Salamon, and B. Dingus, eds. H. W. Siegmund, W. Fowler, R. Fisher, and M. Jhabvala, “The ultra- (1999), Vol. 6, p. 248. violet coronagraph spectrometer for the solar and heliospheric 30. N. Gopalswamy, S. Yashiro, S. Krucker, G. Stenborg, and R. A. “ observatory,” Solar Phys. 162, 313–356 (1995). Howard, Intensity variation of large solar energetic particle events as- ” 10. S. R. Cranmer, M. Asgari-Targhi, M.-P. Miralles, J. C. Raymond, L. sociated with coronal mass ejections, J. Geophys. Res. 109, A12105 Strachan, H. Tian, and L. N. Woolsey, “The role of turbulence in coro- (2004). “ nal heating and solar wind expansion,” Philos. Trans. R. Soc. A 373, 31. E. W. Cliver, The unusual relativistic solar proton events of ” – 20140148 (2015). 1979 August 21 and 1981 May 10, Astrophys J. 639, 1206 1217 11. S. R. Cranmer and A. A. van Ballegooijen, “On the generation, (2006). “ propagation, and reflection of Alfvén waves from the solar 32. A. J. Tylka and M. A. Lee, Spectral and compositional characteristics ” photosphere to the distant heliosphere,” Astrophys. J. Suppl. 156, of gradual and impulsive solar energetic particle events, in Solar 265–293 (2005). Eruptions and Energetic Particles, N. Gopalswamy, R. Mewaldt, 12. S. R. Cranmer, J. L. Kohl, G. Noci, E. Antonucci, G. Tondello, M. C. E. and J. Torsti, eds., Geophysics Monograph Ser. 165 (American Huber, L. Strachan, A. V. Panasyuk, L. D. Gardner, M. Romoli, S. Geophysical Union, 2013), p. 263. “ Fineschi, D. Dobrzycka, J. C. Raymond, P. Nicolosi, O. H. W. 33. A. Sandroos and R. Vainio, Simulation results for heavy ion spectral ” Siegmund, D. Spadaro, C. Benna, A. Ciaravella, S. Giordano, S. variability in large gradual solar energetic particle events, Astrophys. – R. Habbal, M. Karovska, X. Li, R. Martin, J. G. Michels, A. J. Lett. 662, L127 L130 (2007). Modigliani, G. Naletto, R. H. O’Neal, C. Pernechele, G. Poletto, P. 34. J. W. Bieber, P. Evenson, W. Dröge, R. Pyle, D. Ruffolo, M. “ L. Smith, and R. M. Suleiman, “An empirical model of a polar coronal Rujiwarodom, P. Tooprakai, and T. Khumlumlert, Spaceship earth ” hole at solar minimum,” Astrophys. J. 511, 481–501 (1999). observations of the Easter 2001 solar particle event, Astrophys. J. – 13. L. Berger, R. F. Wimmer-Schweingruber, and G. Gloeckler, Lett. 601, L103 L106 (2004). “Systematic measurements of ion-proton differential streaming in 35. A. Saiz, D. Ruffolo, M. Rujiwarodom, J. W. Bieber, J. Clem, P. “ the solar wind,” Phys. Rev. Lett. 106, 151103 (2011). Evenson, R. Pyle, M. L. Duldig, and J. E. Humble, Relativistic particle 14. S. R. Cranmer, “Self-consistent models of the solar wind,” Space Sci. injection and interplanetary transport during the January 20, ” Rev. 172, 145–156 (2012). 2005 ground level enhancement, in Proceeding of the 29th 15. L. A. Fisk, “Acceleration of the solar wind as a result of the reconnec- International Cosmic Ray Conference, S. Sripathi Acharya and J. tion of open magnetic flux with coronal loops,” J. Geophys. Res. 108, E. Humble, eds. (Tata Institute of Fundamental Research, 2005), 1157 (2003). Vol. 1, p. 229. 16. Y.-M. Wang and N. R. Sheeley, Jr., “Solar wind speed and coronal 36. G. P. Zank, G. Li, V. Florinski, Q. Hu, D. Lario, and C. W. Smith, “ flux-tube expansion,” Astrophys. J. 355, 726–732 (1990). Particle acceleration at perpendicular shock waves: model and ob- ” 17. S. K. Antiochos, Z. Mikic, V. S. Titov, R. Lionello, and J. A. Linker, “A servations, J. Geophys. Res. 111, A06108 (2006). “ model for the sources of the slow solar wind,” Astrophys. J. 731, 112 37. E. M. de Gouveia dal Pino and A. Lazarian, Production of the large (2011). scale superluminal ejections of the microquasar GRS 1915+105 by ” – 18. L. Zhao, E. Landi, T. H. Zurbuchen, L. A. Fisk, and S. T. Lepri, “The violent magnetic reconnection, Astron. Astrophys. 441, 845 853 evolution of 1 AU equatorial solar wind and its association with the (2005). “ morphology of the heliospheric current sheet from solar cycles 23 38. L. Drury, First-order Fermi acceleration driven by magnetic reconnec- ” – to 24,” Astrophys. J. 793, 44 (2014). tion, Mon. Not. R. Astron. Soc. 422, 2474 2476 (2012). Research Article Vol. 54, No. 31 / November 1 2015 / Applied Optics F231

39. V. Bosch-Ramon, “Fermi I particle acceleration in converging flows in pointing reconstruction for a deployable x-ray telescope,” Proc. mediated by magnetic reconnection,” Astron. Astrophys. 542, 125 SPIE 7738, 77380Z (2010). (2012). 53. H. Peter, L. Abbo, V. Andretta, F. Auchère, A. Bemporad, F. Berrilli, V. 40. N. Nishizuka and K. Shibata, “Fermi acceleration in plasmoids inter- Bommier, A. Braukhane, R. Casini, W. Curdt, J. Davila, H. Dittus, S. acting with fast shocks of reconnection via fractal reconnection,” Phys. Fineschi, A. Fludra, A. Gandorfer, D. Griffin, B. Inhester, A. Lagg, E. Rev. Lett. 110, 051101 (2013). Landi Degl’Innocenti, V. Maiwald, R. Manso Sainz, V. Martínez Pillet, 41. J. F. Drake, P. A. Cassak, M. A. Shay, M. Swisdak, and E. Quataert, S. Matthews, D. Moses, S. Parenti, A. Pietarila, D. Quantius, N.-E. “A magnetic reconnection mechanism for ion acceleration and abun- Raouafi, J. Raymond, P. Rochus, O. Romberg, M. Schlotterer, U. dance enhancements in impulsive flares,” Astrophys. J. Lett. 700, Schühle, S. Solanki, D. Spadaro, L. Teriaca, S. Tomczyk, J. Trujillo L16–L20 (2009). Bueno, and J.-C. Vial, “Solar magnetism eXplorer (SolmeX). 42. K. Knizhnik, M. Swisdak, and J. F. Drake, “The acceleration of ions in Exploring the magnetic field in the upper atmosphere of our closest solar flares during magnetic reconnection,” Astrophys. J. Lett. 743, star,” Exp. Astron. 33, 271–303 (2012). L35 (2011). 54. E. Renotte, E. C. Baston, A. Bemporad, G. Capobianco, I. Cernica, R. 43. E. L. Olsen, E. Leer, and T. E. Holzer, “Neutral hydrogen in the Darakchiev, F. Denis, R. Desselle, L. De Vos, S. Fineschi, M. Focardi, solar wind acceleration region,” Astrophys. J. 420, 913–925 T. Górski, R. Graczyk, J.-P. Halain, A. Hermans, C. Jackson, C. (1994). Kintziger, J. Kosiec, N. Kranitis, F. Landini, V. Lédl, G. Massone, 44. L. A. Allen, S. R. Habbal, and L. Xing, “Thermal coupling of protons A. Mazzoli, R. Melich, D. Mollet, M. Mosdorf, G. Nicolini, B. Nicula, and neutral hydrogen with anisotropic temperatures in the fast solar P. Orlea´nski, M.-C. Palau, M. Pancrazzi, A. Paschalis, R. Peresty, wind,” J. Geophys. Res. 105, 23123–23134 (2000). J.-Y. Plesseria, M. Rataj, M. Romoli, C. Thizy, M. Thomé, K. 45. S. R. Cranmer, “Non-Maxwellian redistribution in solar coronal Lyα Tsinganos, R. Wodnicki, T. Walczak, and A. Zhukov, “ASPIICS: an emission,” Astrophys. J. 508, 925–939 (1998). externally occulted coronagraph for PROBA-3: design evolution,” 46. R. Steiner, A. Pesch, L. H. Erdmann, M. Burkhardt, A. Gatto, R. Wipf, Proc. SPIE 9143, 91432M (2014). T. Diehl, H. J. P. Vink, and B. G. van den Bosch, “Fabrication of low 55. K. S. Wood and J. V. Breakwell, “Ultra-high angular resolution mea- stray light holographic gratings for space applications,” Proc. SPIE surements in X-ray astronomy by means of occultation techniques,” 8870, 88700H (2013). Acta Astronaut. 15,9–16 (1987). 47. S. Fineschi, M. Romoli, L. D. Gardner, J. L. Kohl, and G. Noci, 56. J. D. Moses and S. Fineschi, “Formation flying instrumentation for re- “Ultraviolet coronagraph spectrometer for the solar and heliospheric mote sensing of the solar corona,” in 40th COSPAR Scientific observatory: optical testings,” Proc. SPIE 2283,30–46 (1994). Assembly, Moscow, Russia, 2014, paper D2.3-7-14. 48. S. Fineschi, L. D. Gardner, J. L. Kohl, M. Romoli, and G. Noci, “Grating 57. Y. K. Bae, “Photonic laser thrusters for spacecraft maneuvering,” in stray light analysis and control in the UVCS/SOHO,” Proc. SPIE 3443, Photonics Tech Briefs (Tech Briefs Media Group, 2014). 67–74 (1998). 58. M. D. Grossi and G. Colombo, “Interactions of a tethered satellite sys- 49. F. B. Berendse, R. G. Cruddace, M. P. Kowalski, D. J. Yentis, W. R. tem with the ionosphere,” in UAH/NASA Workshop on the Use of a Hunter, G. G. Fritz, O. Siegmund, K. Heidemann, R. Lenke, A. Seifert, Tethered Satellite System, Huntsville, Alabama, 1978, pp. 177–181. and T. W. Barbee, Jr., “The joint astrophysical plasma dynamics ex- 59. J. D. Moses, C. Brown, G. Doschek, Y.-K. Ko, C. Korendyke, J. M. periment extreme ultraviolet spectrometer: resolving power,” Proc. Laming, D. Socker, A. Tylka, D. McMullin, C. Ng, S. Wassom, M. SPIE 6266, 62660V (2006). Lee, F. Auchère, S. Fineschi, and T. Carter, “The coronal suprather- 50. C. M. Korendyke, C. M. Brown, R. J. Thomas, C. Keyser, J. Davila, R. mal particle explorer (C-SPEX),” Proc. SPIE 8148, 81480J Hagood, H. Hara, K. Heidemann, A. M. James, J. Lang, J. T. Mariska, (2011). J. Moser, R. Moye, S. Myers, B. J. Probyn, J. F. Seely, J. Shea, E. 60. J. M. Beckers and H. V. Argo, “A solar Lyman alpha coronagraph,” Shepler, and J. Tandy, “Optics and mechanisms for the Extreme- Proc. SPIE 445, 312–317 (1984). Ultraviolet Imaging Spectrometer on the Solar-B satellite,” Appl. 61. S. Fineschi, J. D. Moses, and R. J. Thomas, “Spectro-imaging Opt. 45, 8674–8688 (2006). of the extreme UV solar corona,” Proc. SPIE 5901, 289–297 (2005). 51. L. D. Gardner, J. L. Kohl, P. S. Daigneau, P. L. Smith, L. Strachan, Jr., 62. J. L. Kohl, S. R. Cranmer, J. C. Raymond, J. T. J. Norton, P. J. R. A. Howard, D. G. Socker, J. M. Davila, G. Noci, M. Romoli, and S. Cucchiaro, D. B. Reisenfeld, P. H. Janzen, B. D. G. Chandran, Fineschi, “Advanced spectroscopic and coronagraphic explorer: T. G. Forbes, P. A. Isenberg, A. V. Panasyuk, and A. A. van science payload design concept,” Proc. SPIE 4843,1–7 (2003). Ballegooijen, “The coronal physics investigator (CPI) experiment 52. D. I. Harp, C. C. Liebe, W. Craig, F. Harrison, K. Kruse-Madsen, and for ISS: a new vision for understanding solar wind acceleration,” A. Zoglauer, “NuSTAR: system engineering and modeling challenges arXiv:1104.3817 (2011).