Quick viewing(Text Mode)

N90-18 35

N90-18 35

N90-18 35

Chapter 6

Report from

Panel: A.B.C. Walker, Chair; L. Acton, G. Brueckner, E.L. Chupp, H.S. Hudson and W. Roberts

6.1 The Nature of Solar Physics (especially with -like stars at different stages of evolution, or with stars of different The sun provides a laboratory in which the mass or composition) is a powerful technique for interaction of , magnetic fields and gravi- stellar astronomy. The sun also represents a uni- tational fields occurs on scales, and in regimes of que opportunity to study phenomena which we temperature and density which cannot be dupli- observe elsewhere in the galaxy, in sufficient cated in the laboratory. The phenomena which detail to test fundamental physical laws. The occur on the sun, such as the solar activity cycle, comparative study of such phenomena on the sun the generation of by thermonuclear reac- and elsewhere in the galaxy or the universe can tions, the nonthermally heated corona, the accel- provide insights valuable both to the solar physi- eration of particles to very high energy, and the cist and the stellar or galactic astronomer or generation of the solar are challenging and cosmologist. fascinating problems in fundamental physics. The core of the discipline of solar physics is the study of these phenomena. We have made significant A complete and comprehensive solar pro- progress in identifying the physical laws respon- gram must embrace all four aspects of solar phys- ics (Table 6-1); pure solar studies, solar-terrestrial sible for these phenomena, and in formulating studies, comparative solar/stellar studies, and the more precisely the fundamental questions which study of physical processes, such as particle must be addressed to achieve a deeper under- standing of them. acceleration, which play an important role in astronomical phenomena on many scales. The The sun also represents the major source of latter two aspects are sometimes referred to as energy in the , and controls, or "the study of the sun as a ". strongly influences, events in planetary atmo- spheres, , and , via The outer solar atmosphere becomes unex- direct irradiation, and via the extended atmo- pectedly hot within the first several thousand km sphere of the sun, the . The problems of altitude above the . _(his energiza- presented by the interaction of the sun with the tion also impels the , which flows out- , in particular, solar-terrestrial relations, are wards to form the heliospheric cavity in the inter- of great importance. These problems are no less stellar medium. The flow of the solar wind is challenging or rewarding than those associated regular and has continued over the lifetime of the with the sun itself. They do, however, require sun, but it contains many complexities; these solar observations of a different nature from include a time variable neutral sheet, transient those required to address the physics of the sun disturbances such as streamers, coronal mass itself, along with measurements of the magnetos- ejections, and large scale shock waves, plus a pheric, ionospheric, or atmospheric phenomena component of zodiacal and cometary dust. In a of interest. sense this enormous volume (extending to at least The sun is the only star that we can study in 50 AU) should be considered to be an integral detail; the comparison of the sun and of other part of the volume of the sun, but one that is

31 Table 6-1. The Nature of Solar Physics

* "Pure" solar physics includes the study of: -- Complex interaction of plasma, gravitational, and magnetic fields -- The solar activity cycle -- Coronal heating -- Particle acceleration -- Solar wind generation

• Solar-terrestrial relations make strong demands on our understanding of because: -- Solar radiative and particulate fluxes energize the magnetospheres, atmospheres, and ionospheres of the planets, including earth -- Understanding the nature and causes of solar variability is critical to modeling the variability of the earth's atmosphere, , and

• Study of the sun as a star has important consequences for astrophysics because: -- Activity cycles and coronae are common features of cool stars, therefore comparative studies of the sun and other sun-like stars are mutually beneficial -- Many stellar and galactic phenomena (particle acceleration, winds, flares, etc.) can only be studied in detail by observing their solar manifestations

generally optically thin and that has interesting Future missions optimized for understand- intrusions, such as cosmic rays, the planets and ing the solar-terrestrial relationships should their magnetospheres; comets, etc. emphasize the synoptic (i.e., stable, long-term, systematic measurement of the most significant The steady and varying properties of the parameters). This must include extensive remote heliospheric plasma have many direct effects on sensing or imaging data since truly comprehen- the terrestrial-plasma environment, ranging from sive multipoint observations of heliospheric the auroral displays to the generation of structure would be prohibitively expensive in Earth currents during major magnetic . terms of numbers of spacecraft. The structure of the heliospheric plasma modu- lates the galactic cosmic rays, as well as trans- 6.2 Current Understanding and Anticipated porting the solar energetic particles directly to Near-Term Progress Earth. In these ways the detailed physics within the heliospheric volume plays a role in determin- 6.2.1 A Brief Review of Recent Advances in ing the terrestrial environment which may be as Solar Physics significant as the consequences of the solar irra- diance variability. In the past decade, observations of the sun The present status of heliospheric physics from space and from the ground have led to pro- can best be described as "on hold" between mis- foundly important and frequently unexpected sions; we are hoping for substantial progress via discoveries which have greatly enhanced our SOHO/Cluster and the Wind spacecraft, plus knowledge of solar phenomena and of their con- perhaps new selections of Explorer or small nections to the other disciplines cited above. Explorer experiments, but there is very little Among the most significant of these discoveries observational activity from space at present. are: Indeed, the future missions that have been selected are not optimal for the specific needs of • The first direct experimental confirmation solar-terrestrial research, but are instead oriented of the central role played by thermonuclear more strongly towards the "pure" branches of processes in stars, by the successful detec- solar physics and/or space plasma physics. tion of neutrinos from the sun (l) .

32 More importantly, the disagreement of the which appear to be fundamental to atmospheric observed neutrino flux with that predicted by heating, are beyond the resolving power of pres- standard solar models has resulted in the plan- ent instruments. ning of new observational approaches to test the • The demonstration that the large-scale assumptions and the detailed predictions of the solar- is organized into two resultant models more directly. distinct types of structures: magnetically • The discovery that the 5-minute oscillations closed regions, in which hot plasma mag- of the sun are a global seismic phenomenon netically confined in loops largely generates that can be used as a probe of the structure the x-ray corona; and magnetically open and dynamical behavior of the solar regions, the so-called "coronal holes," interior(2). which are the source of high speed streams in the solar wind (Figure 6-1a)(5). The study of these oscillations, and of longer period oscillations whose existence has been • The confirmation of the evidence (provided recently reported, provides a unique and power- initially by 17th century observations) that ful method to probe solar (and therefore stellar) the cycle and associated active structure and evolution, and the transport of phenomena were largely absent for a energy and the generation of magnetic fields in period of 70 years in the 17th century. the sun's (and therefore in the convection zones of cool stars). More recently, This e_isode is known as the Maunder Min- imum(2a, 6). We now know that such interrup- Space Lab II observations have demonstrated the tions, along with periods of heightened activity, persistence of flow fields at mesogranulation and occur quasi-periodically and that there is a corre- granulation scales in the photosphere. The role of this phenomenon in the evolution of the solar lation between the _eneral level of solar activity and the occurrence(6) of climatic changes on the magnetic field is unknown. Earth (Figure 6-1b). The cause of this modula- • The discovery that the damping of solar tion of the solar activity level is unknown. atmospheric waves driven by convection Recently, SMM observations have shown that cannot account for the energy(3) required the solar luminosity varies as a function of sun- to heat the corona and drive the solar spot activity, and with the ; the impli- wind. cations of these variations for the Earth's climate are not known. The correlation between the level of intensity of coronal phenomena observed in other stars • The recognition, as a result of observations (which, like the sun, have convective envelopes) of hard x-rays, gamma rays, and energetic and their stellar rotation rate has reinforced the neutrons, that the energy released during conclusion (drawn from solar observations) that the impulsive phase of a solar flare is magnetic effects underlie many of the active phe- initially largely, or entirely, c,gntained in nomena observed in stellar atmospheres. nonthermal particles accelerated during • The discovery that, when viewed on a fine magnetic-reconnection processes in the coronal field(8). scale, the solar magnetic field is subdivided into individual flux tubes with field • The discovery that the ejection of large strengths exceeding 1000 gauss. clouds of gas called coronal mass tran- sients(5) can occur in association with some The physical size of these fundamental mag- flares. netic flux elements is smaller than can be • The discovery by SMM that the elemental resolved by any present telescope(4). The cause abundances in x-ray emitting flare plasma of this phenomenon is unknown. More recently, vary during the flare event. rocket observations have shown that the transi- tion region contains very fine scale structures SMM observations have shown that both which are highly dynamic. These structures, and are accelerated promptly

33 • ORIGINAL PAGE PHOTOGRAPH

Figure 6-1a. X-ray Photograph of the Sun

This photo, taken August 21, 1973, with the American Science and Engineering instrument on , shows large structures and many small bright points thought to be loops that are too small to be resolved. A large extending from the north pole across the equator is plainly visible.

during a flare. The acceleration mecha- ¢.a nism demonstrates fine time structure (on a mil- ANNUAt/'t r 1° _= .s _rs f_l[... 9 lisecond scale), and evidence for the production °° mUtU U of bursts of beamed electrons which transport flare energy by propagating along coronal mag- RETREAT \ netic flux tubes. n __ n SPORER MINIMUM: \

_5 A comprehensive review of the current sta- == # tus of solar physics is contained in the three

JU UL rr %= volume set "The Physics of the Sun,'_ 8) which is BE AD recommended to those who wish to pursue in ( 1 I I I I I | [ I I 3000 2500 2000 1500 1000 500 I 500 1000 1500 21100 depth any of the specific topics mentioned above. YEAR

Figure 6-lb. Apparent Correlation of Solar The profound impact of these and other dis- Activity and Climate. The louver curve, based on tree-ring coveries on our appreciation of the complexity daia, represents the rate of production of carbon-14 by cosmic ray and diversity of solar phenomena has led to the bombardment of the upper atmosphere. This production varies inversely v_'i_h solar activity. Plotted above are measures of mean maturing of solar physics as a scientific disci- European climate: the advance and retreat of alpine glaciers, histor- pline. This new maturity has allowed solar physi- ical inlercnccs of mean annual temperature, and the recorded sever- it) of norlhern European winters. The temporal coincidL.nce of low cists to formulate a much more precise theoreti- solar activity and cool European climate sugg&ts a casual connec- cal and observational strategy for their tion between hmg-term solar behavior and climate, although other data indicate a more complex relationship. discipline(9).

34 _ _ _ ,'1 '- _ _ _,

6.2.2 Solar Physics--Expected Accomplish- The consensus from all the studies identify ments Through 1995 and a Plan for the following set of broad scientific objectives: Continuing Advances into the • Structure and dynamics of the solar inte- 21st Century rior which includes problems of the mag- netic cycle and the coupled dynamics of the The current plans for the study of the phys- convective envelope ics of the sun from space are, at best, modest, even considering initiatives of other countries. • Structure and dynamics of the solar atmo- There is now no cohesive plan for extending the sphere and solar activity, including the accomplishments of previous and current major development of active regions and flare and space missions, such as Skylab, P78-1, SMM, post-flare phenomena and Hinotori. Without such a (US) plan, the vital ° Coronal dynamics and coupling to the unsolved scientific problems in solar physics will interplanetary medium, including the origin of the solar wind not be properly attacked. The only near-term programs which can investigate a limited number • Solar-terrestrial relations which go beyond of solar physics problems are the Solar A and basic solar physics and consider the cou- SOHO missions, rocket flights, and the Max 91 pling of (both radiative and balloon program. We therefore first briefly particulate) to Earth. review from a broad perspective the major scien- Table 6-2 lists the basic problems presented tific problems which we believe will remain by these objectives and the tools required for unsolved by 1995. attacking them. Column 5 lists the near-term Several reports which were written over the missions which can make modest advances to last [0 years have identified most of the questions [995. Column 6 indicates the required space that can be studied from the Earth and space. flight capabilities for the vital extension of stu- These are: dies of the sun. The missions listed are described in more detail below. l) The Colgate Reports (Space Plasma Physics: The Study of Solar System Plasmas, 1978, 6.3 Problems and Objectives and The Physics of the Sun, [985). 6.3.1 A Scientific Strategy for "Pure" Solar 2) The Kennel Report (Solar-System Space Physics Physics in the 1980's: A Research Strategy, 1980). The report of the Solar Physics Working, 3) Solar-Terrestrial Research in the 1980's Group of the Astronomy Survey Committee(9) (1981). has recommended three themes or areas of con- 4) The Nature of Solar Physics: Chapter III of centration as potentially the most productive for "Challenges to Astronomy and Astrophysics: solar physics over the next decade. These three Working Documents of the Astronomy Sur- themes (Table 6-3) are: (1) development of obser- vey Committee "(1983). vational techniques capable of probing the inte- rior structure, dynamics, and composition of the 5) National Solar-Terrestrial Research Program sun; (2) study of the "active phenomena" such as (1984). flares, , the activity cycle, the chromo- 6) A Strategy For the Explorer Program for sphere, the corona, and the solar wind, which are Solar and Space Physics (1984). a consequence of solar magnetic variability; and 7) An Implementation Plan for Priorities in (3) study of the role of the sun in shaping the Solar-System Space Physics (1985). three-dimensional structure and dynamics of the 8) The Advanced (Executive heliosphere. The last two themes will have impor- Summary, 1986). tant implications for our understanding of the 9) "Solar and Space Physics" (Space Science in Earth s and climate(10). We the Twenty-First Century: Imperatives for the can formulate a coherent scientific program Decades 1995-2015, 1988). which addresses these themes as a series of seven

3fi Table 6-2. Major Scientific Problems in Solar Physics

I. Objecli_e 2. ,_.rea 3. Problem d. Tools for Solution S. Approved 6. Proposed

Structure and interior a) What i, the rotation profile of the Sular Observation of the sun'_ radial and SO _ GDI:HRTC

Dynamics of d) namies Interior? non-radial oscillations (a,b,e) the _',olar Inlerior b) Are there residual effects from possible Theor) and modeling (a,h,c) -- primordial abundance inhomogeneities in the Observation of the Galiiur

young sun? Spectrum (b,e) c) VChat is the temperature of the ? Determlnatl.n of the S.lar Quadrupole SOHO Solar Probe Momenl Ca,b)

Dynamie_ .f a) What role do con_ection and circulation play Extremely high ',ensitbit_ measures of SOHO OSI. the eorl_ecli_e in the convective transport or energy in the velocit) and rotalion (a.h)

envelope envelope? h) H.,* does large scale circulation operate on High precision measurements of lhe OSL sun and _hat are its effects? brigiHnt_s pattern of the solar surface_ (a,b) Theory and computer modeling ia,h) -- GDI:IfRT(

Magnetic c)cle a) What is lhe origin of the solar magnetic field, Obser_ation'_ of the non-radial SOHO GDI:HRTC and of its ,.ecular variation? oscillations of the sun tn deduce the structure of the eon_ecflon lone (a)

b) _._,hal are the dynamics and energetics of sun- Synoptic observations of solar magnetic SOHO (;DI:HRT( spots and other manifestations of emerging field with modest resolution. _elocily, and flu*.? magnetic field observation., of polar

regions of sun (a,b,c) c) I].w rapidly and in _hat way do coronal ATM data analysis; coordinated synoptic Solar-A HRT( field*, e_.l_e and di,.'_ipate? x-ray/X UV and magnetngraph data with Heliosphere adequale llme resolution (minutes to hours?) (b,c) llighly resobed magnetograms and lEVY OSL and optical data; theory plus time- resolved magnetograms, .ptical data (including velocities) (e)

AIm._pheric Mmospheric a) What is the _elocil) field in the transition XUV and spectroscopy with Solar-A OS|r, HRT( Struclure and Xeti'.e structure and zone and corona? high spatial and spectral resolution

Phennmena d)namics b) What is the rule of magnetic fields in heating? Moderate time re',olution; x-ra_., XUV, c) _,re time-dependent ionDation effect_ radio O (a,b,c,d) important in the dynamically varying quiet High-resolution magnelograph, _isual, Solar-A OS;I,, HRTC sun? uhraviolel, XIV, x-ray data; ATM dala

d) What is the relati+e role of magnetic anal_,si*_ (b,f,e) di,_sipation and wave propagation in healing X ['V and ufiravhdet line ratio obser_a- 0 OSI,. HRTC the and cor,.na? tion_, theory (c) e) _,Vhal is the role of _pcules in the exchange High resolution measurements of the OSL of mass between the chromosphere and photospheric veloeil_ field (d) corona?

f) l_,'hal is the fine scale structure and dynamical behavior of the m_gnetic field?

OSI. Active regions a) _hat is the nature of sunspots; why are sun- Theoretical sfudie,, of basic dynamical spol_ and flare km)ts stable? effects in sunspot structure; observational b) _'hal i_ the role of corlmal bright points in studies of wave fiuxe_ from sunspots;

the emergence or magnetic flux? ver) highl) re,.ohed magnelograms and c) llow are coronal loops heated, how do the) optical data: theory (a,b,c,d) Solar-A OSI., HRT( exchange ma_ with the chromosphere? _re High-r_olutlon visible, uhra_iolet, XI'V

the) chemicafl) homogenous? x-ray, and radio imaging, and d) Vehal is the role of prominences and coronal speetropholomefry (a,b,c,d) Solar-A IfTRf condensation,, in the energetic*, of the corona? Synoptic _tudies at _isible, radio, ultra- violet, and soft x-ra) ranges It,d) Highest resolution El'V, visible, and SOHO HTRC radinbservalions; thenrHicaI studies (a,b,c)

_olar- _ HRTC, tlEIC, Flare_ and a) line* does stored energy build up in c.ronal Temparal and spatial magnetograph data P/OF tran_ieni_ fields and ho_ ix it released? with concurrent sofi x-ray and radio h) What is the site of the impulsive energy imager), theoretical d 3 namieal studies release in flare,.. _hal are the details of (a,h,c) Solar-,_ HRTC, HI:I(, reconnecllon and particle acceleration Ilard x-ray imaging and spectroscopy; P/OF processes? gamma-ray and energetic beam impact- HRTC. IIEIC, ¢) Ho,_ i'. life energy released in flare., Irans- point ohser_atiow,, high-resolution P/OF porled to other part. of Ihe atmosphere and microwave mapping,.: fast meter dissipated? decameter radinhefigraph (FeXXI. while d) '*Vhat is the mechani_-m v.hich trigger_ coronal light flares) (b,d) transients? High-resolution _isihle, ultraviolet, XI.'V, OSL HRTC, P/OI

e) I|.'_ does chemica! and isotopic fractionation x-ra_ and centimeter-wave imaging and in fiare_ occur? spectruphotometry (c)

36 Table 6-2. Major Scientific Problems in Solar Physics (Continued)

I. Objective 2. Area 3. Problem 4. Tools for Solution 5. Approved 6. Proposed

Solar cosmic-ray obsesvations with good Solar-A Heliospherc, Solar elemental and isotopic resolution; gamma Probe ray spectra with high resolution and sensitivity; XUV and soft x-ray abundance studies (lo, e)

The Corona and the Coronal a) What are the processes responsible for heating Imaging and spectroscopy in visible ultra- SOHO, OSL, HRTC, P/OF Interplanetary structure and and mass transport in the quiet corona and violet, XUV, x-rays; high-resolution Solar-A Medium dynamics coronal holes? magnetic fields; fast meter-decameter b) What role do coronal transients play in the radio heliography (a,b,c)

energy and mass balance of the corona and Radio polarization observations, while SOHO P/OF solar wind? light coronagraph polarization studies c) How does chemical fractionation in the (a,b)

corona arise and what is its relationship to Development and coordination of the SOH0, HRTC, P/OF abundance anomalies in flares and in the solar theoretical modeling with empirical Solar-A wind? models; observation of coronal

lempcrature and density structure with white-light and L vman-a coronagrapbs; ultraviolet, XUV and soft x-ray line profiles (a,b,c)

The solar wind a) What is the structure and composition of the Complete coordinated data on corona SOHO HRTC, P/OF solar wind over coronal holes, over light and solar wind parameters; theory and bright points, and over active regions? computer modeling; extend b) How is the solar wind accelerated? interplanetary data closer to sun and out c) What mechanisms are responsible for the of the ecliptic (a,b)

observed variations in the composition and Well-calibrated observations of angular P/OF, Hellosphere temperature of the solar wind? momentum of solar wind, ultimately Solar Probe d) What is the angular momentum of the solar out of the ecliptic (d)

wind and what is its role in the evolution of Out-of-the-ecliptic measurements (e) Ulysses Solar Probe the sun? Better composition measurements of 0 Heliosphere, Solar c) What is the structure of the solar wind and solar wind; theory of ionic diffusion in Probe interplanetary medium at mid and high helio- transition zone and separation in solar centric latitudes? wind (c)

Solar a) What mechanisms are respons_le for the very Observation of the sun's and nonradial SOHO HRTC terrestrial long-term variations in solar activity which oscillations (a,b,d) relations cause phenomena such as the Maunder Proxy studies of the level of solar activity Minimum over ver). long periods, studies of solar b) Are there indicators which allow the llke stars (a) prediction of long-term (activity cycle) vari- Studies of the structure and evolution of SOHO HRTC, P/OF ations in the trends and short-term (flares, the corona coupled with studies of the transients) events on the sun which affect non-radial oscillations (e)

conditions on the earth? Synoptic observations of solar ultra- UARS, SVO, Janus c) What is the best way to monitor the level of violet emission, upper atmosphere, EOS solar activity and the structure of the inler- magnetospheric studies (c)

planetary medium in relation to ionospheric, Improved atmospheric modeling (c) magnetospheric and atmospheric physics? Continue the coordination and statistical SVO, Janus d) What are the mechanisms which are respon- study of solar, interplanetary, and SVO, Janus sible for short-term and long-term variations terrestrial conditions (c,¢) In the solar constant? Precision synoptic observation of total UARS, SVO, Janus e) How do variations on the sun control the solar luminosity (d) EOS structure of the interplanetary medium?

-- Theory Key: SOHO Solar Heliospheric Observalor) SVO Solar Variability Observatory 0 None planned ASO Advanced Solar Observatory Janus A proposed Solar Terrestrial Observatory P/OF Pinhole Occulter Facility (Part of ASO) Gallium Search for low energy solar neutrinos with a gallium based detector HRTC High Resolution Telescope Cluster (Part of ASO) EOS Earth Observation S.vstem HEIC High Energy Instrument Cluster (Part of ASO) GDI Global Dynamic Instruments (Part of ASO) OSL Orbiting Solar Laboratory UARS Upper Atmosphere Research Satellite fundamental questions, which present an over- particular, what is the sun's internal rotation view of the theoretical and observational issues rate, chemical composition, and temperature which should be the focus of solar research over distribution, and what is the detailed process the next decade. of nuclear energy generation and energy trans- 1. What are the fundamental properties of the port? How do these properties relate to current theories of stellar evolution? solar core (where energy is generated) and the radiative interior (through which energy is 2. What is the magnetohydrodynamic structure transported to the sun's outer layers)? In of the solar convection zone, and what is the

37 Table 6-3. "Pure" Solar Physics-- 6. What are the large-scale structure and plasma Scientific Objectives dynamics of the solar corona, including the processes involved in heating various coronal Themes from the report of the Solar Physics Working Group of structures and initiating the solar wind? What the Astronomy Survey Committee are the implications for stellar coronae and Structure and dynamics of the solar interior winds other astrophysical flows? What is the Rotation profile, energy source origin of coronal transients? Nature of solar convection 7. What are the implications of coronal struc- Origin of solar magnetism ture for the three-dimensional structure and Atmospheric structure and active phenomena Velocity fields, magnetic and acoustic dissipation, fine structure dynamics of the heliosphere and what are its Active regions (origin, heating, evolution) implications for cosmic ray modulation and Flares and transients (energy accumulation, trigger mechanism; for the modulation of planetary atmospheres, high energy particle acceleration) ionospheres, and magnetospheres, including Recurrent phenomena (e.g., sun spots) those of Earth? Corona and interplanetary medium Structure and dynamics 6.3.2 Solar-Terrestrial Physics Origin of solar wind The solar output at ultraviolet and x-ray role of the convective scales observed on the has profound effects on the upper sun, the granulation, the , atmosphere of the earth (Table 6-4). The chemis- and the large-scale circulation, in transporting try of the atmosphere, the energy budget, and energy from the solar interior to the solar sur- perhaps the dynamics of the stratosphere and face? Can a generalized theory of stellar con- lower mesosphere, are solely determined by the vection in the presence of rotation and mag- incoming UV in the 120 mm <,a<300 netic fields be developed which describes the nm band, while XUV radiation at 15 <,_<100 structure of the sun's convective zone and pre- nm determines the energy budget of the thermos- dicts the observed convective scales? phere. X-rays are absorbed or scattered in many 3. What physical mechanisms drive the solar layers. The solar cycle variations of the thermos- magnetic field and activity cycle, what result- phere are governed by the strongly variable solar ing variations in the solar radiative and partic- XUV radiation. Ozone concentration in the ulate output follow on various time-scales, and stratosphere is dependent on the solar UV radia- what is the effect of this variability on the tion between 180 and 300 nm. It has been Earth's atmosphere, ionosphere, and magnet- osphere? What causes the long-term variations Table 6-4. Identification of Physical in the solar magnetic and activity cycles, such Mechanisms Long-Term as occurred during the Maunder Minimum? Variability How do these phenomena relate to activity and variability on other stars? Problems in Solar-Terrestrial Physics 4. What processes, involving small scale velocity Radiative coupling sun-upper earth atmosphere and magnetic fields and various wave modes, Energy, chemistry and dynamics of stratosphere, mesosphere determine the thermal struc-ture and dynamics and of the solar photosphere, chromosphere, and Coupling between solar wind and magnetosphere Reconnection processes corona, and what are the implications of such Trigger of magnetic substorms processes for stellar atmospheres in general? Particle acceleration, plasmoid ejection 5. What are the basic plasma-physics processes

responsible for metastable energy storage, Large scale electric field systems , particle acceleration, Sun-earth weather and energy deposition in solar flares and Confirmation that apparent statistical correlations have a related nonthermal phenomena? What are the physical basis implications of these for other high energy Identification of physical mechanisms responsible for any correlations verified processes in the Universe?

38 estimatedthat adecreaseof 5% of the solar radi- tropopause weather patterns. Although it is at ation at 250 nm results in an ozone column den- present only a statistical correlation, solar terres- sity change of approximately 2.5%. trial physics has the mandate to find a plausible mechanism which couples either solar constant Solar cycle induced ozone column density variations, solar UV flux variations or changes in variations are therefore comparable to long peri- solar corpuscular emission with tropospheric odic variations caused by chemicals released weather patterns. from the surface of the Earth. It is impossible to distinguish between the two effects as long as no 6.3.3 Solar/Stellar Relationships precise knowledge exists of the sun's 1 l-year variability at the critical UV . The list One of the major discoveries of the below shows the required precision and accuracy first comprehensive x-ray observatory, the Ein- of the solar UV spectral irradiance over a solar stein Observatory, was that stellar coronae are cycle, a solar rotation, and short intervals, such common phenomena, arising naturally in cool as flare-induced variations. stars as a result of the convective transport of energy in the outer envelopes of these stars, and 0.1.5 - 15 nm 15 - 100 nm 180 - 300 nm by a variety of mechanisms in other circum- Soft X-ra[s XUV EUV stances, such as close binary pairs. Another Time Precision Accurac[ Precision Accuracy Precision Accuracy major discovery of Einstein was that the quasar 11 Years 10% 20% 5% 10% <1% <5% 25 Days 5% 20% 2.5% 10% <0.5% <5% phenomenon, like the coronal phenomena, is _Minutes 5% 20% 2.5% 10% <'0.5% <5% essentially a result of the generation of very high temperature plasmas (107 to 109 K) by nonther- Short-term solar UV variability (days to mal processes, which involve the acceleration of months) is caused in first order approximation particles to very high energy. It is a fact that the by excess radiation of plages (UV intensity is extensive community of astronomers which has modulated by the evolution of the plages, and by formed to study coronal and other nonthermal their passage across the solar disk as a result of phenomena in stars has come essentially from the solar rotation). However, there are strong solar-physics. Clearly, the comparative study of indications that a so-called third component may solar and stellar coronal phenomena, and the significantly contribute to the I l-year cycle varia- relationship of coronal parameters to basic stellar tion. This third component may consist of small, parameters (mass, age, surface temperature, rota- isolated chromospheric brightnings distributed tion rate, etc.) is essential if an understanding of over the whole solar disk, or a uniform variabil- coronal phenomena, and activity cycles in stars is ity of chromospheric temperature. It is obvious to be achieved. The objectives of comparative that total measurements cannot solar/stellar studies are summarized in Table 6-5. distinguish between the two- or three-component Table 6-5. Solar Stellar Properties model, however, the understanding of the solar cycle and its underlying magnetic variations Objectives of Solar/Stellar Studies requires a resolution of this problem. Scaling of coronal properties (temperature, density, filling factor) Therefore, a need exists for synoptic obser- with fundamental stellar parameters vations of the sun over a solar cycle at all UV Mass wavelengths with good spatial resolution (--! arc Surface temperature second) and appropriate time resolution (--1 Surface gravity Rotation rate day). Properties of stellar activity cycles, scaling of activity cycle charac- The correlation of atmospheric parameters teristics with stellar parameters with the solar cycle, taking into account quasi Comparative properties of stellar winds and the solar wind biannual oscillations which were first found by Comparative properties of solar and stellar flares Labitzke(11), has survived severe statistical tests. Flares in main sequence and giant stars Flares in "pathological stars," i.e., flare stars, close binaries Furthermore, it has now also been detected in

39 6.4 PresentProgram high time and spatial resolution of specific solar structures. The present program is summarized in 4. Major missions: The major missions employ Table 6-6. The data base from previous years is large single or multiple instruments, which are an important resource. The near-term programs, expected (with periodic servicing) to remain in and the presently operating missions (SMM, orbit and operation for at least 10 years. The rockets) can address specific problems, however instruments for these major missions may be they do not provide the very high resolution changed and upgraded over the life of mission. (--0.1 arcsecond) necessary for many critical 5. Shuttle attached payloads: This mode of problems. implementation is most beneficial for the Table 6-6. Present Programs--Solar Physics employment of large instruments which do not require long duration (less than 10 days) mis- Resources and Operatinlg/Near-Term Missions sions. This mode is also useful for the testing Data base from previous missions Skylab, OSO series, P78-1 of new instruments and the development of Operating: investigation techniques. Launch schedule SMM Continue acquiring data base for _> one 1 I-year cycle uncertainties are a major problem. Rockets 5 year now desirable to increase 6. Space station attached payloads: This mode of Near-Term (< 1995): implementation (although presently uncertain) SolarA some advances for all objectives but a small explorer may be most attractive for the deployment of high energy very limited SOHO some advances for all objectives but flares multiple large observatory instruments. The

Rockets 5 year desirable to increase space station should allow for the accommo- MAX91 some significant advances possible but limited by dation of instruments requiring large foot- short duration prints, and will provide power, thermal con- Conclusion: Existing program is inadequate! trol, data handling, commanding, and other resources. In addition the ability to recalibrate, 6.5 Potential Implementation Modes repair, and upgrade these instruments will sig- nificantly enhance the scientific return. Prob- In order to accomplish the broad objectives lems which could occur include contamina- of solar physics over the next 25, or so, years a tion, light scattering from station structures, number of modes (Table 6-7) are considered. and disturbances caused by shuttle dockings Following is a list of these modes with a brief and other space station operations. statement of the advantages derived through the use of these modes. 6.5.1 Modeling and Theory Programs

1. Rockets and balloons: These modes are the In order to maintain a balanced program in most responsive in terms of the time from con- solar physics, it is essential that the observational cept to data analysis. They are most suppor- programs be complemented with a significant tive of the "graduate student" approach to modeling and theory component. Such programs developing scientific research, and have proven should include both a "pure theory" emphasis an excellent means for the development of new independent of flight programs, and more con- instrument and investigation techniques. crete efforts aimed at directly understanding and 2. Small Explorer missions: This mode may be predicting observational results. The develop- ment of solar models and theories must be used to support relatively small groups of diagnostic instruments and innovative instru- infused with experimental results, and experi- ments for longer time periods. ments should be developed with an eye toward 3. Moderate Explorer missions: This mode of verifying the models and theories. implementation begins to allow for the devel- 6.5.2 Data Analysis Campaign opment of more comprehensive instrument packages. Single large instruments or multiple To allow effective utilization of the wealth instruments may be included which provide of data which has been obtained from prior

40 Table6-7.PotentialImplementationModes Time Cost* Modes (Conceptto Implementation) InstrumentModality

Rocketsandballons "X_l year (1) Quick response, graduate student support

Smallexplorers '_'5 years (2) Single or multiple small instruments

Explorers "_'10 years (3) Large or multiple instruments

Moderatemissions "_15 years (3) Large or multiple instruments with "strap-ons"

Majormissions "x_20 years (4) Large instrument, long duration, upgrades

Shuttleattached "x_8years (2) Large instruments, development, calibration

Spacestationattached "xd0 years (3) Large instruments, long duration, upgrades

Lagrangianpointorbits %15 years (4) Multiple instruments

Lunarbasing "x,20 years (4) Large instruments, long duration

<1 AUplatforms "x_20years (4) Multiple instruments, steroscopic observations "Eventemphasis"data analysismissions '_1 year (1) Quick response, to include models and theory *Cost (1) _<$1M (3) _<$500M (2) _<$100M (4) >$500M missions such as Skylab, P78-1, SMM and Hino- • Space station utilization: Heavy payloads tori, a series of data analysis campaigns is most with modest SS impact--availability of appropriate. These campaigns might be imple- manned support mented at the rate of one or two per year with • Space shuttle: Flights of opportunity-- the objective of addressing specific solar features verification in space of key instrument through the review and analysis of data compiled advances, calibration, and quick-return from prior missions. Teams of investigators before long-term placement in space could be selected to work on "special emphasis" scientific programs using existing data resources. • Suborbital flights: Same advantages as Teams would include not only experimentalists, space shuttle and in addition low in cost but also specialists in the modeling and theories and good for training of young scientists relating to the features under study. We expect • Solar orbiting-free flyer (--1 AU): Stereos- that the state of knowledge in solar physics could copic observations be significantly enhanced by the implementation and proper management of such an effort. • Lunar basing or Lagrangian point: No atmospheric disturbances--weight and 6.5.3 Observational Modes volume limitations, satisfactory, if manned A broad attack on the basic solar plysics base support problem after 1995 must first consider the possi- • Inner planet observations: 1/r 2 advantages ble available platforms and the unique capabili- for angular resolution, stereoscopic obser- ties and limitations of each platform. These are vations, low energy neutron spectroscopy (with foreign collaboration encouraged): • Heliosynchronous orbit (approximately • Earth orbiting free flyers: Precise pointing, 0.1 AU): l/r 2 advantage for angular reso- long-term undisturbed observations--full lution, stereoscopic observations, low wavelength coverage energy neutron spectroscopy 41 • Near-sunorbit: I/r 2 advantagefor angular on planetary ionospheres, atmospheres and resolution,stereoscopicandin-situobser- magnetospheres, particulary those of the earth. vations,low energyneutronspectroscopy 3. Understanding solar phenomena, such as par- • Solarprobe(oneshortmission):1/r2 ticle acceleration, coronal heating and solar advantagefor angularresolution,stereos- wind generation, in relation to similar phe- copicandin-situobservations,low energy nomena in other astrophysical settings. neutronspectroscopy Note: 1/r2 advantagerefersto theimprove- Each of the objectives will require special- ized programs and specific measurements to mentin resolutionandsensitivityof address the outstanding problems. A basic solar solarstructuresachievedby placingan physics strategy to obtain the necessary observa- observingplatformcloserto thesun than I AU. tions is presented in Table 6-8. The objectives of the missions listed in the table are summarized in A fewremarksregardingShuttleAttached Table 6-9. The study of solar phenomenology, Payloadsareappropriate.After theChallenger for example, requires very high spatial and spec- ,mostof theSpacelabpayloadsin the tra resolution t ° achieve a physical model of the spacephysicsdisciplinewerecancelled.Seven small scale structures which control the flow of yearsof developmentwerediscarded.This mass and energy in the atmosphere. Also resultednot onlyin a tremendouslossof future required are in-situ measurements of the micro- sciencewhichcouldhavebeenobtainedfrom scopic conditions in the heliosphere, and remote multipleflights of scheduledinstruments,but observations of the global properties of the alsoin acrisisof confidencebetweenNASA and heliosphere. theimpactedsectorsof thescientificcommunity. Thistrendmustbereversed.Sustainedefforts The study of solar-terrestrial phenomena requires the precise measurement of solar outputs mustbemadeto find flight opportunitiesfor and their variation, and the understanding of the existinginstrumentsor, if this is not possible, origins of this variation. Finally, direct measure- other means to carry out the investigations. ment of coronal and other nonthermal pheno- However, there exists a class of instruments mena on other stars is essential to an understand- which must be carried by the shuttle because of ing of the sun in an astrophysical context. In the the need for long-term (approximately solar following discussion, we have specified the fun- cycle) measurements which require periodic damental measurements which must be made, reflights and calibrations between flights. Some and commented on a strategy or strategies by of these experiments are scheduled on the which such measurements can be achieved. ATLAS mission which must be flown periodi- cally over the next 10 years simultaneously with 6.6.2 Suggested Missions the UARS satellite for calibration purposes. We briefly describe each of the major goals 6.6 Solar Physics Strategy identified above, and discuss missions by which 6.6.1 Introduction these goals can be achieved.

We have identified three major goals of The Solar Activity Cycle and the Magnetic "pure" solar physics: Field

1. Understanding the phenomenoiogy dis- The thermodynamic structure and dynamics played by the sun, including the activity cycle, of the solar atmosphere are determined by the the generation of the corona and the solar interaction of magnetic field and plasma on a wind, the acceleration of energetic particles in very small scale. The objective of the Orbiting flares, and the structure and dynamics of the Solar Laboratory (OSL) is to provide the angular heliosphere. resolution, sensitivity, and stability to permit 2. Understanding the variability of the radiative study of the fundamental interactions of solar and particulate output of the sun, and its effect surface wave and flow fields with the magnetic

42 Table 6-8. Solar Strategy

Strategy

• Orbital Solar Laboratory (OSL) is the top priority for solar physics • Because Solar Physics has multiple objectives (solar phenomena, the hetiosphere, solar terrestrial relations, astrophysical phenomena obser- vable on the sun), several different types of solar measurements, and hence several types of solar observing capability, are required. The missions which are required are summarized below:

- Very high resolution instruments with diagnostic capability: OSL, High Resolution Telescope Cluster (HRTC), High Energy Instrument Cluster (HEIC) including the Pinhole/Occulter Facility (P/OF)

- Very high precision instruments for the study of the solar output and its variability: Solar Variability Observatory (SVO), Janus - Multiple in situ and remote sensing (for global structure) instruments to study the heliosphere: Solar Probe, "Heliosphere" - Comparative solar/stellar observations of coronal, cycles, etc.

Table 6-9. Summary of Solar Missions

SOHO/ Solar Probe SVO Scout/ Solar Physics Cluster OSL HRTC HEIC Heliosphere Janus Explorer Convection zone

Photosphere/chromosphere J Transition reg.!corona/ J J solar wind

Solar wind/heliosphere ./ Solar Terrestrial Relations

Solar output,'variability J Terrestrial response J Solar Stellar

Stellar and solar corona, J flares, winds

Scoot Expl Mod Maj Shuttle Space Station Nature of Missions Attached Attached

OSL HRTC:P/OF HEIC:P/OF Solar probe J "Heliosphere" SVO/Janus J Solar/stellar field at the scale of 100 km. OSL is complemen- Table 6-10. Orbiting Solar Laboratory (OSL) tary to Solar and Heliospheric Observatory (SOHO), Solar-A, the NOAA x-ray imaging OSL Major Facilities monitor and the ground-based project in the Photospheric magnetic and velocity field (visible light) primary objective of understanding the solar cycle. High spatial, high spectral resolution

OSL is the highest priority mission in the O. I arcsecond ,a / A,a >500,000 discipline of solar physics. The priority has been Long time sequences --days (SS orbit) recently reaffirmed by the Space and Earth Chromospheric and transition zone spectroscopy (UV) Science Advisory Committee. The future strategy for solar physics and the ability of the discipline Spatial resolution 0.5 arcsecond to contribute with fundamental understanding to Spectral resolution " _/A_ --30,000 the "input" side of solar-terrestrial studies depend Coronal imaging and spectroscopy (XUV, x-ray) upon the early implementation of this keystone Spatial resolution 0.5 arcsecond mission. The capabilities of OSL are summarized in Table 6-10. Spectral resolution _/A,a _100 43 A thoroughunderstandingof theconvective Table 6-11. High Energy Instrument Cluster conditionsin thesunwhichunderliethesolar [Thrust through 2002 (next solar activitycyclewill requirethestudyof thesolar maximum after 1991)] oscillations,whichwill beoneof theobjectivesof theSOHOmission. Major objective--advance aggressively to an understanding of high energy particle acceleration High-Energy Investigations of Solar Parameters and measurements Phenomena Accelerated species and maximum energy of each species The most striking accomplishments of the High energy and high time resolution spectra of x-rays, y- SMM and Hinotori missions have been the reali- rays, neutrons Acceleration region (e.g., location, composition...) zation that the very efficient acceleration of indi- Imaging x-rays and y-rays to 1 arc second vidual electrons and to relativistic High energy resolution spectra on short time scales is a fundamental property of Geometry of accelerator solar flares and probably of other cosmic plas- Angular distribution of emissions from: mas. An understanding of this phenomenon High resolution imaging, stereoscopic observations would have ramifications in the broadest astro- Nuclear line Doppler shifts and polarization physical context. The specific parameters and polarization observations needed are as follows:

• The species and maximum energies of and neutron spectrometers which can be accelerated particles. This information can accommodated on a platform with only modest be derived from measuring the spectra of pointing capabilities. This group of instruments x-rays, gamma rays, and high-energy neu- is referred to as the High Energy Facility (HEF). trons with high time and energy resolution. The hard x-ray and gamma-ray imaging instru- • The physical properties of the acceleration ments will make use of coded aperature imaging region, such as its location and composi- techniques, and will be part of the Pinhole/Oc- tion. This requires precise imaging of hard culter Facility (P/OF) described below. x-ray and emissions to MeV Together, the high energy instruments are energies and measurement of gamma-ray referred to as the High Energy Instrument Clus- spectra with the highest energy resolution. ter (HEIC). • The geometry of the accelerator. This High Resolution Studies of Chromospheric requires determining the angular distribu- and Coronal Structure and Dynamics tion of secondary neutral emissions from individual flares and can be accomplished The upper chromosphere, transition region, by high resolution imaging, stereoscopic corona, and corona/solar wind interface span observations, Doppler shifts of gamma-ray temperatures from 5 x 104 K to 107 K. During lines, and polarization measurements of flares, plasma with quasi-thermal temperatures bremsstrahlung and nuclear emissions. as high as 108 K are generated. These plasmas are confined to very small structures, especially By focusing strong effort on one of the most in the early phase of flares. A cluster of high fundamental problems of solar flare physics, one resolution hard x-ray, soft x-ray, x-ray ultravi- can expect to advance to an understanding of olet (XUV) and EUV telescopes able to carry how the flare itself is triggered and how the out diagnostic observations on spatial scales of energy released is distributed in numerous other --50-100 km (--0.1 arc second) is necessary to forms and transported throughout the solar address fundamental issues such as: atmosphere. The properties of the high energy instru- • Coronal heating of active regions loops ments necessary to address these objectives are • Mass transport between coronal and summarized in Table 6-1 !. Some of the instru- chromospheric structures ments required are high resolution gamma-ray • Generation of the solar wind

44 • Acceleration, transport, and thermalization Heliospheric Studies of energetic particles. The solar corona (where the solar wind is Many of the required instruments can be formed) remains one of the most ill-observed incorporated in a High Resolution Telescope regions of the solar-terrestrial environment, in Cluster (HRTC), which can accommodate the spite of the fact that a total eclipse can make required high resolution soft x-ray, XUV, and some of it directly visible to the naked eye. Some EUV telescopes. The measurement of the faint of the observational problems can be eased by structures in the corona/solar wind inteface will new missions that emphasize (a) remote sensing; require occulted telescopes, which are most effec- (b) stereoscopic viewing, to permit tomographic tively incorporated into the P/OF mentioned reconstruction of the corona's three-dimensional above. The hard x-ray imaging observations will geometry; and (c) direct in-situ measurements of require the use of coded aperature techniques, conditions in the heliosphere. These needs can be which can be accommodated on P/OF. met by deep-space observatories (heliosynchro- The Advanced Solar Observatory nous, Lagrangian-point, or planet-based) carry- ing relatively low-resolution solar imaging Many of the problems pertaining to high instruments such as (a) white-light and UV coro- energy phenomena on the sun will require the nal imagers, (b) soft x-ray telescopes, (c) low- combined power of the OSL, and instruments frequency radio receivers and solar energetic par- described in the discussions of High Energy ticle and solar-wind particle measuring Investigations and High Resolution Studies. This instruments, and by a probe of heliospheric con- set of instruments will have the highest resolution ditions as close to the sun as possible. and sensitivity of any of the instruments envisi- oned in this report, and are collectively referred The extended solar atmosphere (the helio- to as the Advanced Solar Observatory (ASO). sphere) is extensive and highly structured. To The ASO instruments are most effectively pack- understand the dynamics of the heliosphere, it is aged into four ensembles: the OSL, a Pinhole/ necessary to observe the physical processes Occulter Facility which makes use of Fourier occurring on microscopic and macroscopic transform imaging techniques and occulted coro- (meters to kilometers) scales, as well as the global nal telescopes which use a remote (--50 meters) structure and dynamics (i.e., coronal mass ejec- occulter/mask; a HEF which incorporates high tions). Accordingly, both in-situ missions such as energy gamma-ray and neutron spectrometers; a Solar Probe, capable of approaching the sun to and a HRTC. The properties of these ensembles within 4 solar radii, and a complex of remote are summarized in Table 6-12. The OSL is sensing platforms which allow stereoscopic imag- already NASA's highest priority for a moderate ing of the far corona (e.g., ISPM) as well as in- mission. The other ASO instrument ensembles situ sampling of heliospheric conditions both in (HEF, P/OF, HRTC) could be deployed on the and above (or below) the ecliptic ar_ essential. manned space station, on a co-orbiting platform, We call the later complex of platforms "helio- or on two moderate size spacecraft, such as that sphere"; heliosphere might include a "helio- planned for the OSL. Perhaps initial deployment synchronous orbiter" and an "interstellar probe" on the space station, and later extended deploy- which could travel beyond 100 AU from the sun. ment on another platform is the most logical approach. The HRTC telescopes, the OSL, and The Solar Probe would carry out the first in- the HEIC [including hard x-ray and gamma-ray situ exploration of the solar corona, penetrating imaging (i.e., P/OF)] must be capable of being to a height of about 4 solar radii above the photo- operated as a single observatory, as described in sphere. This innermost region of the solar wind the Advanced Solar Observatory SWG approaches the "temperature maximum" of the Report(12). Instruments for the HEF are des- corona, where the heating is the strongest, and cribed in a recent publication(13). remains one of the most inaccessible frontiers of

45 Table 6-12. ASO Instrument Ensembles

Resohing Power

,Angular Spectral {g) Temporal Field of Vie_ fare min) [n_l rument Spectral Range Apcrlure {are sec) (EfAE) (see)

High Resolution Tele,,cope Cluster tHRTC) 0.15/0A f 0.01 3.5' x 3.5";12 x 12" Soft X-Ra) Telescopes a 15 - 170 A 40 cm I0,000 _10 0 1/04 f 0.01 3.5' x 3.5":12 x 124 XUV Telescopes b 150 h 40 cm 20,000 x 40 cm O. I/0A f 30,CO0 Ol 3,5" 3 5"12 x 12 1 EUV Telescope 550 II_O A 10 0.001 full tun Gamma Ra.,. Imaging I)clect_r 2 lOng} kcV 60 cm 16 4_) cm 10 I O,O00 0.01 full _un X-Ra} Flare Spcctrtuncter 1.5 25A 1.0 full sun Global D)namics lnstrumentafton c IGDI) 35[X) I I ,O00 A 5t) cm 0.5'5 c 1130,000 c 0.1 3.5" x 35' Ultraviolet Telescope t175 1700 A 60 cm 0 i 30,O00

High Energy Facility IHEF) 400/20 1/0,25 full sun Gamma Ra) Line Spectrcaneters e 10 keV I0 MeV 3() _lr_) Crll 5 0.001 full sun High Energ) Gamma Ray Speytrometer 10 McV If×} MeV I_l cm cm 5 0.001 full sun High Energ) Neutron Spectrometer I0 McV I{_K) McV If_'l 200 1 full sun l Jaw Frequency Radio Spectrograph 1 20 MHz 30JXR) 90

Pinhole/Occulter Facility (P;OF) 70 I0 0.001 full sun Coded Aperture Imager 2 keV IPg3 tm 4.0 IO0 tm 0 2 10 0.001 full sun Fourier Transfiwm Imagcr 2 1000 kcV 1100 50 cm I O 5,000 I 0 full sun White Light Coronagraph 11,(_) A 50 cm 1.O 20,0O0 O. 1 Full sun EUV Coronagraph 3(_) 17(XI A

Orbiting Stflar I.abora0r)_ rOSL) 2[X_) 171_) I iX") cm f) I I0{},000 O. I 1.3" x I.Y/3' x 3 Optical Telesct_pe

4'x4" Ultra,,iolet Tcle,.cc, pe 1175 1700A 30 cm 05 30,000 O. 1 a Our slrawman configuration envisions tu.o _a)ft x ra_ tele,.copes with spectral coverage 15 - 2 5 A and f0 170 ,'_ b Our strawman configuralic, n envisions three XUV tclc'-.copes 'a.ilh ,,peclral coverage 150 IgO _, and 280 310 ,_. c The Global Dynamics Package includes ftmr small tclc,,copcs d Po'-Aihlc hmg term future upgrade for the OSL ultraviolet telescope c Entries refer to high rc'-a.lu6on and high ';ensitivily ,,peclromeler_; I-c_,peuli'*el_ I Entric,, rclk_r Io high resolution and wide /told modes rcslv.:cft',cly g Figure refer'. I. highcsl re',illulit)rl IllllKlC Olhcr Ffll_Jcs illay havc h.v,'er rc,,oftHi_./i corona, where the heating is the strongest, and propose a "Solar Variability Observatory" to remains one of the most inaccessible frontiers of carry out these important observations (Table the heliosphere. The Solar Probe should carry 6-13). Any platform which can be pointed toward instrumentation for the measurement of the the sun is suited. The space station must be magnetic field and of the populations of thermal favored because its instruments can be exchanged and energetic particles, including neutrals; it Table 6-13. Solar Variability Observatory should also carry out spectrophotometry of the Solar Component outer corona by viewing outwards from the sun. Space Solar-Terrestrial Physics SMM UAIRS EOS Station

The sun/ heliosphere/ Earth system forms a Spectral irradiance XX XXX 91- 95- tightly coupled physical system, which must be 120

46 periodically.It isnecessaryto add high precision Polar will acquire detailed coverage of seasonal photometers for the XUV regime because no effects such as , distribution of efforts are ongoing at the time being. Table 6-13 gases, ozone distribution, etc., in both hemis- lists planned solar irradiance measurements, and pheres. The satellite will be equipped with the components of a future Solar Variability appropriate in-situ particle and field sensors to Observatory. trace the magnetospheric effects of the incident streams observed by Janus L1. To carry out a comprehensive measurement of the variation of the solar irradiance and par- Although the Janus mission deserves careful ticle output and of the response of the Earth's and thorough definition, the two spacecraft will ionosphere, magnetosphere, and atmosphere to need to include instruments of the following these variations, we purpose a two-spacecraft types: mission which we call Janus. Janus L1 The Janus mission is named after the Solar viewing: Roman god of gates and doorways who, having Solar constant monitor two faces, looked both ahead and behind. The UV irradiance monitor name is appropriate because the primary space- Soft x-ray photometers craft is to be located at the Lagrangian libration Soft x-ray or XUV imager for point LI between the sun and the Earth and is coronal structure data equipped with both sun viewing and earthward Wide angle coronagraph for mass looking instruments. ejection data The objective of the Janus mission is the Earth viewing: study of solar-terrestrial relationships from the Geocoronal imagers global perspective. It utilizes both in-situ and In-situ particle and field monitors remote sensing techniques to observe the solar Janus Polar input and global response of the earth's atmo- Earth viewing: sphere. Absolute calibration of Janus instru- Auroral imager ments will be maintained by periodic comparison Ozone imager measurements on the space shuttle or space sta- In-situ particle and field monitors tion, and calibrating the Earth's global photome- try by using stars as calibration standards. This The Janus concept is described in Table 6-14 and will produce a record of Earth's luminosity which Figure 6-3. should be extremely accurate and which can be Comparative Solar/Stellar Observations reviewed in 100 years to detect long-term trends. A logical approach to the comparative study Two spacecraft provide the necessary observing of coronal phenomena on the sun, and on other perspective, Janus L1 at the libration point and Janus Polar in high (ca. 18 hour) circular polar Table 6-14. Solar-Terrestrial Mission "Janus" orbit.

The purpose of Janus L1 is to observe the Janus Concept solar electromagnetic, particle and magnetic field I. Earth as a "sun": Earth irradiance photometry input to the Earth vicinity, and to carry remote Global composition and physical properties of the earth sensing instruments to image the sunlit hemis- Global ozone and other minor constituants of the upper atmosphere phere of the Earth to obtain precise albedo mea- Global cloud coverage, surements. The solar instruments are tailored to Global albedo of the solid earth the particular solar-terrestrial task. 2. Earth as a planet: Earth imaging The Janus Polar satellite is intended to pro- Structure, dynamics of the atmosphere Global hydrology oceans and atmosphere vide global remote sensing coverage of the polar Ocean temperature, ice caps, regions, the day-night terminators and the dark 3. Earth as a 'qong-term variable star": Search for global changes side of the earth. In the course of a year Janus

47 Figure6-3.TheJanusMission stars,isthedevelopmentof asmalldedicated wavelengths, and new optical technologies. spacecraftwhichcancomplementIUE (andthe Instruments now under construction for missions projectedLymanmission)by observingboththe such as SOHO, may, therefore, carry considera- sunandnearbystarsat XUV, EUV,andsoft ble risk. Instruments planned for future missions, x-raywavelengths. such as OSL, need a catch-up effort of uncertain outcome. Twenty years ago several industrial 6.7 Advanced Instrument Development efforts were supported with research funds which resulted in useful products. Most of these efforts Most solar space instrumentation planned have disappeared. For example, images obtained for flight on long-duration platforms in the next by the Naval Research Laboratory (NRL) XUV decade is based on technical developments which monitor on board Skylab (flown 15 years ago), are 10 to 20 years old. Ten years ago, NASA which were based on technology developed 20 substantially curtailed large-scale support of years ago, have never been superceded. This is an future technology. Examples of areas urgently in extremely alarming trend that needs to be recti- need of development funds are two-dimensional fied by steady, generous support of future tech- detector arrays for UV, x-ray, and gamma-ray nology developments.

48 References

l a. John Bahcall, "Solar Neutrinos: Theory ver- 7. Gerard Van Hoven, "Plasma Energetics in sus Experiment," Space Science Reviews, 24, Solar Flares," Highlights of Astronomy, 5, 277 (October 1979). 343 (1980). lb. John N. Bahcall "Neutrinos-Electron Scatter- 8. Physics of the Sun (three volumes), edited by ing and Solar Neutrino Experiments," P.A. Sturrock, T.E. Holzer, D.M. Mihalas, Review of Modern Physics 59 #2, April 1987, and R.K. Ulrich (D. Reidel Publishing Com- p. 505-522 pany, 1985). 2a. Gordon Newkirk, Jr., and Kendrick Frazier, 9. "Solar Physics: The Report of the Solar "The Solar Cycle," Physics Today, 35, No. 4, Physics Working Group of the Astronomy 25 (April 1982). Survey Committee," Chapter 1 of Challenges 2b. Advances in Helio and Astroseismology, to Astronomy and Astrophysics: Working IAO Symp, 123, Ed. J. Christensen, Dalo- Documents of the Astronomy Survey Com- gaard and S. Frandsen (D. Reidel Publ. Co. mittee (National Academy Press, Washing- Dodrecht, 1988) Symposium on Seismology ton, D.C., 1982); an overview of the report of of the Sun and Sun-like Stars, Tenerife, the Solar Physics Working Group is con- Spain, Sept. 1988, to be published as ESA tained in the article by A.B.C. Walker, Jr., in Publ. 286 Ed. E. J. Rolfe Physics Today, 35, No. l I (November 1982). 3. R. Grant Athay and Oran R. White, 10. "Solar Variability, Weather, and Climate," "Chromospheric Oscillations Observed with (National Academy Press, Washington, D.C., OSO 8: IV. Power and Phase Spectra for C 1982). II," The Astrophysical Journal, 229, 1147 11. K. Labitzke "Sunspots, QBO, and the Stra- (May 1979). tospheric Temperature in the North Polar 4. Jack Harvey, "Observations of Small-Scale Regions," Geophys. Res. Letters, 14, 535 Photospheric Magnetic Fields," Highlights of (1987) Astronomy, 4, 223-239 (1977). 12. A.B.C. Walker, Jr., R. Moore, and W. 5. Jack B. Zirker, Editor, Coronal Holes and Roberts, The Advanced Solar Observatory, High Speed Wind Stream (Colorado Asso- NASA Technical Publication (1986). ciated University Press, 1977). 13. "High Energy Aspects of Solar Flares," Solar 6. Jack A. Eddy, "The Maunder Minimum," Physics 1988, 118, Editors E.L. Chupp and Science, 192, 1189 (June 1976). A.B.C. Walker, Jr.

49 m _

m

P • Jp_ _o • T_ I _ _ i_ _r