Aspects of future solar stereoscopic observations

Phys. Chem. Earth, Voi. 22, No. 5, pp. 463--468, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0079-1946/97 $17.00 + 0.00

Pergamon

PII: S0079-1946(97)00175-4

Aspects of Future Solar Stereoscopic Observations P. Maltby

Institute of Theoretical Astrophysics, University of Oslo, P.O.Box 1029 Blindern, 0315 Oslo, Norway

Received 18 July 1996; accepted 7 March 1997

Abstract. It is argued that the Sun offers an unique place to pursue studies that aim for a deeper understanding of the generation and the activity of magnetic fields in the Universe. It is well known that magnetic fields play an important role in physical processes occurring both in quiet and active regions in the solar atmosphere. The magnetic field is also an essential ingredient in the physics of solar activity. Recently inversion techniques have been applied to acoustic travel-time helioseismology observations. The results reveal large-scale subsurface structures and flows related to active regions. This indicates that the generation and activity of magnetic fields in the solar interior may be studied, preferably by combining helioseismology observations from different vantage points. The objectives for a few stereoscopic mission scenarios are discussed and the importance of simultaneous observations from different vantage points is emphasized, both for studies of the solar interior and for investigations of the solar atmosphere and its environment. © 1997 Published by Elsevier Science Ltd 1

aspects, but they all involve magnetic fields in either a primary or a supporting role, strongly suggesting that the magnetic field is a main source or channel of unrest in the Universe. The Sun offers an unique place to pursue the mysteries of magnetic field generation and activity. This implies that high priority should be given to understanding the effects and nature of magnetic fields in any future solar space mission. Some key issues in solar physics, all of which emphasize the importance of magnetic fields, include: - origin of magnetic fields and their interaction with the solar plasma, -

role of magnetic fields in internal transport of energy and angular momentum,

- origin of active regions, including sunspots and flares - origin of the hot corona, - origin and development of cool prominences embedded in the hot corona, - driving force and the mass transport in fast solar wind streams.

Introduction

Although the magnitude of the Sun's activity cannot rival that of some active stars, the display of magnetic activity is spectacular, even outside active regions where the magnetic field exhibits the remarkable property of concentration into isolated flux tubes of approximately 1500 Gauss (0.15 Tesla). The concentration is most likely caused by the interaction between the magnetic fields and the convective motion. Our knowledge about the internetwork magnetic field is limited, but recent observations suggest predominantly horizontal magnetic flux structures (see Lites et al. 1996 and references therein). In order to maintain the chromosphere and the corona at temperatures higher than the photosphere the solar

It is often argued that the importance of solar physics is related to the fact that the Sun is the only star that may be studied in detail. This argument is correct, but it does not make the Sun and its environment the Rosettastone of astrophysics. Let us therefore start by noting that one of the most fascinating aspects of astronomy is that nature often proves to be more imaginative than the scientists. The discoveries of exotic objects such as active supernova remnants, pulsars, X-ray sources, gamma-ray sources, active galaxies and quasars show that the Universe contains a fascinating ingredient in the form of activity. The exotic objects differ in several

Correspondence to: P. Maltby 463

464

P. Maltby

atmosphere must be heated by non-radiative sources. Although acoustic energy in the form of shocks are observed in super-granular cell interiors (Carlsson & Stein 1995), heating mechanisms in the form of magnetic reconnection in closed magnetic regions and MHD waves in open field regions are presently favored, but need firm observational confirmation. Studies should be aimed at understanding the heating mechanisms or in more general terms to the non-equilibrium behavior of magnetic fields in nature. The active Sun exhibits a variety of phenomena, such as sunspots, faculae, prominences, and furthermore explosive events such as flares and coronal eruptions whereby material is thrown into the solar wind. Knowledge obtained from solar activity studies is extensively used in discussing the physics involved in other branches of astrophysics. This transfer of knowledge is not limited to the activity of other stellar objects, but reach as far as the magnetic field of galaxies. Knowledge of magnetic reconnection theory combined by realizing the importance of the continuous generation of cosmic rays in our galaxy led Parker (1992, 1995) to revise the theory for the Galactic magnetic field by suggesting that reconnection of magnetic loops is a major effect in the structuring of the Galactic halo. So far we have concentrated the discussion to the solar atmosphere and its environment. The time may be ripe to ask: What is the role of magnetic fields in the solar interior? Numerical studies of how magnetic ~2-1oops rise to the solar surface with an orientation compatible with sunspot observations suggest magnetic fields of 0.5 - 1 × 105 Gauss (5 - 10 Tesla) at the bottom of the convection zone (e.g. Choudhuri & Gilman 1987, D'Sflva & Choudhuri 1993). Recent attempts on tomographic imaging of the Sun's interior based on inversion of the acoustical travel-time helioseismology data promise to give both sound speed and material velocity at different depths as function of solar latitude and longitude (D'Silva & Duvall 1995). The results presented by Kosovichev (1996) show large changes between active and quiet regions on the Sun and suggest that simultaneous observations of different latitude intervals are required. This points to a stereoscopic mission as the opportunity to investigate the role of the magnetic field in the solar interior. Several aspects, such as the role of the magnetic fields in the internal transport of energy and angular momentum are of interest. Realizing the importance of magnetic field activity also makes clear that the space comprised of the Sun and its interior, the heliosphere and the Earth's magnetosphere is a unique laboratory for studies of plasmas in magnetic fields. Accordingly joint efforts such as in the Solar Terrestrial Science Programme (STSP), comprised of SOHO and a re-flight of the CLUSTER mission, is an obvious way to obtain a better understanding of the magnetized plasma and its activity. Hence, when discussing the STEREO concept below, let us keep an open

mind to a possible joint effort with a mission mainly aimed at studying the environment of the Earth and its magnetic activity. In the following we will first consider a large stereoscopic mission scenario where the aims and requirements for studies of the solar interior are combined with studies of the solar atmosphere and its extension into the solar wind. Next, we will consider smaller to medimn size mission scenarios, aimed at seeking solutions to specific topics, such as the dynamics and role of magnetic fields in the solar interior.

2

S T U D I E S OF S T E R E O S C O P I C

MISSIONS

Images of the Sun are usually observed from a single vantage point on the Earth or in space. One may consider using the Sun's natural rotation to change the viewing angle, but experiments have shown that solar features, such as coronal streamers are persistent but not stationary (Poland 1978). This means that the temporal evolution of most solar features is too fast to obtain a quasi three dimensional picture in this manner. The obvious solution is to use simultaneous measurements from spacecraft positioned at different vantage points in order to obtain stereoscopic observations of the Sun. This approach has been tried in solar radioastronomy (Steinberg & Coroubalos 1976; Poquerusse & Steinberg 1978) and in X-ray observations of flares (Garcia & Farnik 1991; Kane et al. 1992). During recent years several options for a solar stereoscopic mission have been suggested (e.g. Hudson & Hildner 1990; Cooper & Burks 1991; Grigoryev 1993; Schmidt & Bothmer 1996). Also brief accounts of the studies carried out by space agencies have appeared, i.e. the SYSTEM project, of the Russian space agency (Oraevsky & Fomichev 1996), SPINS of NASA (Pizzo 1994; Kalkofen 1996), and STEREO of ESA (Battrick 1994; Huber 1996). Whereas the SPINS and STEREO concepts concentrate on spacecraft in the ecliptic plane, the SYSTEM mission concept may include one or more spacecraft which rise out of the ecliptic plane and allows a view of the polar region of the Sun. 2.1

The STEREO Study

Scenarios with 2, 3 and 6 spacecraft in a 1 AU solar orbit were considered in the 1994 ESA study, see Figure 1 (top) for the 2 and 3 spacecraft options. It was assumed that the payloads were financed nationally and that the cost for ESA, including a full Ariane 5 launch, was kept within the envelope assigned to larger, so-called cornerstone projects. The available payload mass reduces quickly with the number of spacecraft if each spacecraft is equipped with identical instrumentation. Whereas the available payload mass for each spacecraft would be in the range 600 - 713 kg for the 2 spacecraft option, the

Aspects of Future Solar Stereoscopic Observations

465

Fig. I. Relative positions of 2 and 3 spacecraft with respect to the Sun and Earth in the 1994 S T E R E O one of the discussed hehoseismology mission scenarios.

nlasS would reduce to 73 - 180 kg for the 6 spacecraft option. After discussing the scientific objectives the study group (P. Bochsler, J.L. Culhane, B. Fleck, C. Fr6hlich, R. Harrison, P. Maltby, E. Priest, G. Racca, & R. Schwenn) set the following aims for the mission: -

-

T a b l e 1. S T E R E O 1994 model p a y l o a d for each spacecraft Instrument Coronagraph X-ray/EUV/UV imager EUV/UV spectrometer Optical telescope Helioseismology Magnetometer + particle detector Whlte-light sky i m a g e r Radio wave i n s t r u m e n t Total

To understand the 3-D structure and evolution of magnetic fields throughout the solar atmosphere, from the photosphere to the corona. To investigate the large-scale 3-D structure and evolution of the corona and the heliosphere, especially with respect to the effects on the terrestrial environment.

-

To understand the cause of the solar coronal mass ejections and their propagation through the hellosphere and to predict geomagnetic effects.

-

To understand the relationship between solar activity and solar irradiance variability.

Mass(kg) 100 60 100 200 15

Telcmctry(kbit/s) 8 10 8 10 0.1

25 20 10 530

2.5 0.5 1 40.1

The available payload mass depends on the speed in the cruise phase. The duration of the cruise phase ranges from 425 days to 1156 days. Hence, even with a relatively short cruise phase of 425 days, each spacecraft could have a payload mass of 530 kg.

8

These alms comply with the key issues in solar physics mentioned above, all of which emphasize the importance of magnetic fields. Note that these objectives are extensions to and complementary to the anticipated results from the SOHO mission. The magnetic field observations imply a relatively heavy payload (see Table 1) on 3-axis stabilized observing platforms. Accordingly the emphasis in the ESA study was given to the option with 2 spacecraft positioned in a 1 AU solar orbit, one close to Earth and the other at 450 to 600 following the Earth.

mission scenario (top) and in

NEEDS

FOR

FURTHER

STUDIES

The STEREO concept was studied by ESA for a brief period in the summer of 1994, having in mind a large, cornerstone mission. Within the present ESA planning a smaller, medium-class mission in solar physics is foreseen, provided, that the quality of the proposal is eminent. This means that the cost for ESA must be limited to that of a medium-class mission, which may be obtained either with a smaller mission of by a joint venture with one or more space agencies.

466 3.1

P. Maltby Requirements for Solar Instrumentation

As discussed above the magnetic field is the key parameter in understanding the Sun and its environment. Since the velocity field in the photosphere alters the magnetic field configuration in higher layers we need to measure both the velocity field and the magnetic field in the photosphere. Unless an ingenious method for magnetic field measurements is presented in the near future, an optical telescope with a diameter of, say, 0.50m, equipped with, for example, an advanced Stokes polarimeter must be included in the model payload. Our knowledge about the magnetic field in the transition region and corona is limited, but some information about the magnetic field configuration in higher layers may be deduced from 1) observations of EUV and Xray images, combined with measurements of the photospheric magnetic field, 2) solar radio astronomy techniques (e.g. Kundu, 1990), 3) depolarization of emission lines (Hanle effect) in prominences and 4) observations of Zeeman sensitive coronal fines with new infrared detectors, which promise to give us more reliable values for the coronal magnetic field strength in the near future (Penn & Kuhn 1994, Kuhn 1995).

T a b l e 2. R e q u i r e m e n t s set by t i m e v a r i a b i h t y AA Photosphere Visible+IR Chromosphere UV+Visible Transition Region UV Corona XUV+Visible Radio Corona mm-lcAxi "rob s = O b s e r v e d t i m e v a r i a t i o n "rTeq = R e q u i r e d t i m e resolution

"robs

"rreq

rain. rain. 10 s 20s ms

15 s 10 s 1s 5s? ms

The requirements for instrumentation in future solar physics missions have been discussed recently for the X-ray (Culhane 1993), UV (Brueckner 1993) and visible wavelength regions (Title 1993). Ideally the instrumentation should have a spatial resolution of 0.1 arc-sec and a temporal resolution of 1 second from x-rays to the visible, combined with a spectral resolution A/&A higher than 5 x 104, 1.5× 104, and 2 x 103 in the visible, UV and X-ray region, respectively. Data from the SOHO spacecraft indicate that insufficient attention has been given to the requirements for temporal resolution. The estimates of observed time variations and required time resolutions given in Table 2 should therefore be regarded as preliminary. 3.2

Joint Venture

Consider that the planning of a stereoscopic solar physics mission starts by looking into the 1994 STEREO concept. Several topics would have to be studied in order to bring the STEREO concept ready for competition with other mission proposals. These studies could focus on:

An update and sophistication of the model payload composition should be discussed, including the readiness of intefferometry, an advanced Stokes polarimeter and imaging Fourier Transform spectrometers. Options with 2 and 3 spacecraft should be reviewed. One option with a full Ariane 5 launch is 3 spacecraft, positioned 600 apart in a 1 AU solar orbit, see Figure 1 (top). With a cruise phase of 1032 days each spacecraft could have a payload mass close to 400 kg. The question of selecting the optimum aspect angles is difficult, since the optimum range of aspect angles differ from one instrument to another and very probably also from one particular type of solar features to another. With large differences in aspect angle we will be able to derive a quasi 3D picture of the overall structure of sunspots and prominences, but it will be difficult even to identify the same fine structures from widely separated vantage points. Since the study of fine scale structures may be essential for understanding the physics involved in the solar atmosphere, studies with aspect angles ranging from a few degrees to, say, 200 will be required. This points to selecting a long cruise phase and, in contrast to most other missions, to operate the instruments during the cruise phase, except during the burns and other type of calibration. In fact, the amount of gas needed for scientific operations during the cruise phase is so small that it hardly influence the available payload mass. Special attention should be given to the telemetry rate and data storage in the distant spacecraft. For spacecraft positioned in a 1 AU solar orbit, 600 away from the Earth, the telemetry rate considered in the 1994 study was 40 kbit/s, whereas the capacity for the nearby spacecraft was several Mbits/s. The question of telemetry rate is closely linked to the choice of model payload. It would certainly be of interest to include radio observations, but one should be aware that a feasibility study is needed to establish that antenna oscillations do not interfere with the stability required for the imaging instruments or with the field of view or stray fight capabilities of other instruments. In the STEREO concept one of the spacecraft is in orbit relatively close to the Earth. One could therefore consider the possibility that this spacecraft also carried instrumentation for magnetospheric studies of the Earth. There are certainly both advantages and disadvantages in combining a solar physics mission with another mission, but a brief study may clarify the pros and cons.

Aspects of Future Solar Stereoscopic Observations Assmning that these items have been satisfactory studied and satisfactory answers have been found, we would be in a position to evaluate the proposal and eventually to submit it to the space agencies, underlining the advantages of simultaneous observations from different vantage points. 3.3

Medium-Class Mission

The STEREO concept as discussed above has to be considerably descoped in order to be proposed as a mediumclass mission. This implies the use of a small to medium size launcher that is less expensive than Ariane 5. Alternatively one may consider an add-on opportunity to another mission to be launched by Ariane 5, see Pgtzold et al. (1996) for a solar radio astronomy proposal. One should ask which branch of solar physics does have the greatest potential for scientific discoveries and also which topic would have general support from outside the field of solar physics. One may consider focusing the mission to: - coronal extended structures, - solar wind studies, - helioseismology. Evidently the objectives for these options differ and are limited in scope compared to those given above for the STEREO concept. Regarding the first of these three topics consider, for example, a mission concept with the objective to obtain 3D distribution of extended structures in the corona with some, but limited information on temperature and density. Limiting the instrumentation to a normal incidence multi-layer coronal imager with a few spectral band-passes and a white light coronagraph, the instrument package could be 50 kg in each spacecraft (see Harrison, this issue). Combining the first two topics above Schmidt & Bothmer (1996) have presented a stereoscopic mission concept for coronal and interplanetary activity. They intend to image coronal structures in EUV and soft Xrays, while interplanetary events such as Coronal Mass Ejections (CMEs) would be seen as density enhancemerits in white light. CMEs could be tracked from their start in the corona until their arrival and impact on the Earth's magnetosphere. In order to carry out such a mission they suggest an instrument package of 80 kg, comprised of coronal instruments and equipment for in situ measurements of the solar wind. Finally, let us consider a sterescopic mission with helioseismology instruments, aiming for a better understanding of the solar interior. To the author's knowledge the proposals for sterescopic missions have hitherto regarded the helioseismology instrument package as an add-on instrument. Let us go to the other extreme and regard the helioseismology instrument as the core

467

instrument. The reason for presenting this as a valid option now is the recent development in helioseismology observation and data reduction technique. Duvall et al. (1993, 1996) have shown that it is possible to separate the effects of sound speed variation from the effects of material flow by measuring the travel time of waves propagating in opposite directions along the same ray paths. It is of interest to note that D'Silva et al. (1996) have presented a method for deducing the sub-surface magnetic fields from time-distance helioseismology. Recently presented inversion techniques of the acoustical travel-time data show that both sound speed and material velocity at different depths vary with the presence of an active region on the solar surface (Kosovichev 1996). Hence, solar interior magnetic field are very probably isolated in solar latitude and longitude and simultaneous observations from different vantage points are required to obtain further insight. This points to the need for a stereoscopic mission in order to investigate in more detail the structure and motion in the solar interior and to explore the effects of the internal magnetic field. In passing we note that one of the helioseismology instruments on SOHO, the Michelson Doppler Imager also measures the photospheric magnetic field with a spatial resolution of 1.2 or 4 arc-see, depending on the selected field of view. Accordingly, such a mission could also give a nearly complete picture of the photospheric magnetic field. Consider a mission concept with 3 spacecraft separated by, say, 120°, in a 1 AU solar orbit (see Figure 1, bottom). Time-distance helioseismology observations require instrumentation in each spacecraft with a weight of, say, 50 kg, i.e. comparable to the weight of the the Michelson Doppler Imager, which has a total weight of 56.5 kg. Both distant spacecraft would have to be equipped with a large on-board computer capacity for data pre-analysis, data compression and a computer mere ory of such magnitude that the data can be transmitted to the Earth at a speed adjusted to the capacity of the receiving equipment available for the mission. This raises the questions of selecting the optimum aspect angles and time resolution in the observations. Hence, also this proposal need further studies on several topics before an eminent solar physics proposal emerges, hopefully in the near future.

4

Acknowledgments

The author wishes to thank Drs. Bo Andersen, Richard Harrison, Philip Judge, Giuseppe Racca, and Professor Olav Kjeldseth-Moe for helpful informations and discussions. References

Battrick, B. 1994, HORIZON 2000 plus, ESA SP-1180

468

P. Maltby

Brueckner, G.E. 1993, in Scientific Requirements for Future SolarPhysics Space Missions, eds. P. Maltby, & B. Battrick, ESA SP-1157, 171 Carlsson, M. & Stein, R.F. 1995 ApJ 397, L59 Choudhuri, A.R. & Gilman, P.A. 1987, ApJ 316, 788 Cooper, R.A., & Burks, D.H. 1991, Space Physics Mission Handbook, NASA Culhane, J.L. 1993, in Scientific Requirements for Future SolarPhysics Space Missions, eds. P. Maltby, & B. Battrick, E S A SP-1157, 147

D'Silva, S. & Choudhuri, A.R. 1993, A & A

272,621

D'Silva, S. & Duvall, T.L.Jr. 1995, ApJ 438,454 D'Silva, S. & Duvall, T.L.Jr.,Jcfferics,S.M., & Harvey, J.W. 1996, ApJ 471, 1030 Duvall, T.L.Jr., D'Silva, S., Jefferies, S.M., Harvey, J.W., & Schou, J. 1996, Nature 379, 235 Duvall, T.L.Jr., Jefferies,S.M., Harvey, J.W., & Pomcrantz, M.A. 1993, Nature 362,430 Garcia, H.A., & Farnik, F. 1991, Solar Phys. 131,137 Grigoryev, V.M. 1993, Solar Phys. 148, 389 Hudson, H., & Hildner, E. 1990 in Astrophysics from the Moon, eds. M.J. Mumma & H.J. Smith, (New York, Am. Inst. Phys.), 584 Huber, M.C.E. 1996, Adv. Space Res. Vol. 17, No 4/5,355 Kalkofen, W. 1996, Adv. Space Res. Vol. 17, No 4/5,363 Kane, S.R., Hurley, K., McTieraan, J.M., Slocum, T., Laros, J.G., Fenimore, E.E., Klebsadel, R.W., Sommcr, M., Yoshlmuri, M., & Ohki, K. 1992, BAAS 24, 760 Kosovichev, A.G. 1996, ApJ 461, L55 Kutm, J.R. 1995 in Infrared Tools for Solar Astrophysics: What's Next? Proc. 15th NSO/SP Summer Workshop (Singapore, World Scientific), pp. 89-93 Kundu, M.R. 1990, Mcmorie Soc. Astron. It~liana, Vol. 61, No. 2, p. 431 Lites, B.W., Leka, K.D., Skumanich, A., Martinez Pinet, V., & Shimizu, T. 1996, ApJ 480, 1019 Oraevsky, V.N., & Fomichev, V.V. 1996, Adv. Space Res. Vol. 17, No 4/5,359 Piltzold, M., Neubauer, F.M., Hiiusler, B., Eidel, W., & Bird, M.K. 1996, Adv. Space Res. Vol. 17, No 3, 57 Parker, E.N. 1992, ApJ 401,137

Parker, E.N. 1995 in Frontiers of Astrophysics, Proc. Rosseland Centenary Syrup., eds. P.B. Lilje & P. Maltby (Oslo; Inst. Theor. Astrophys.), pp. 115-124 Penn, M.J., & Kuhn, J.R. 1994, ApJ 434, 807 Pizzo, V.J. 1994, The Findings of the SPINS Science Workshop, Space Environment Laboratory Report Poland, A.I. 1978, Solar Phys. 57, 141 Poquerusse, M., & Steinberg, J.L. 1978, A & A

65, L23

Schmidt, W.K.H., & Bothmer, V. 1996, Adv. Space Res. Vol. 17, No 4/5,369 Steinberg, J.L., & Caroubalos, C. 1976 in Study of the Sun and Interplanetary Medium in Three Dimension, Goddard Space Flight Center Proc., 65 Title, A. 1993, in Scientific Requirements for Future Solar-Physics Space Missions, eds. P. Maltby, & B. Battrick, ESA SP-1157, 161