Abstract.
The International Solar Terrestrial Physics (ISTP) program has coordinated the activi-ties of an armada of spacecraft, ground-based observatories, and theoretical modeling centers. This constellation represents the first grand observatory for space physics, providing both a global, telescopic view of the Sun-Earth system, as well as a magnificent microscopic view of its physical processes. In just two years of coordinated observations, ISTP has already reaped a rich scientific harvest. However, the future holds even more opportunities to pursue a new, profound understanding of the Sun and the Earth and the interplanetary space between them. Continuing the mission through solar maximum (2000-2001) and beyond offers potentially prodigious re-wards for a remarkably low cost.
Introduction
Since the dawn of history, humans have been fascinated by the Sun and its relationship to Earth. Every civilization has speculated about the place of our planet in the realm of the stars and about our relationship to our own star. Stonehenge and sundials and the folklore of eclipses are testi-monials to that fascination. Yet it has only been within the past few hundred years since Co-pernicus and Galileo that we have closely examined the changing face and place of the Sun in our skies (ancient Chinese and Greek observers saw sunspots centuries before telescopes proved they were there). And the role of the Sun in driving magnetic disturbances at Earth has only been appreciated in the last 150 years. It has only been in the 20th century and primarily the last 40 years that we have arrived at a relatively clear picture of solar variability and its effect on Earth. And what we have found through astronomical observations is that our Sun is rather common and ordinary, akin to the many variable stars in the universe. In essence, our Sun-Earth system is the physical prototype for stellar systems throughout the cosmos. It is also the only one we can study up close.
As our appreciation of the Sun-Earth system has grown more sophisticated, so too has our tech-nology. Today, a tangled web of electrical and communication links has been woven across Earthıs surface, while fleets of spacecraft work in the electric space above us. By using electro-magnetism to enhance communication, navigation, reconnaissance, and weather prediction and generally make the world safer, we have also put ourselves in harms way. Every tool and gadget that relies of radio waves, conducting wires, and sensitive transistors and processing chips can be affected by disturbances in the solar-terrestrial system. And many more disturbances lie in our immediate future.
The Sun reaches a maximum of activity every 11 years, and as it reaches the peak it is capable of expelling huge magnetic clouds of material (called coronal mass ejections, or CMEs) which can move outward at speeds sometimes approaching 2000 km/s. The shock waves preceding such clouds can accelerate particles to tremendous energies-sometimes more than 100 million electron volts. If the CMEs and the shock waves they produce strike Earthıs magnetosphere, they can precipitate forceful geomagnetic storms that can disturb power systems, communication links, and the constellations of spacecraft on which society increasingly relies. The appreciation of CMEs as the primary drivers of such disturbances has only come about in the past few years, and this paradigm shift has had a far-reaching impact on how we perceive solar-terrestrial rela-tionships [e.g., Gosling, 1993].
Given the many thousands of years we have waited to achieve our current understanding of the Sun, the Earth, and the space in between, we now enjoy a most remarkable situation. The Inter-national Solar-Terrestrial Physics (ISTP) program has put into place an amazing array of space-craft and ground facilities for studying the Sun-Earth environment [Baker and Carovillano, 1997]. Sensitive telescopes in space examine the Sunıs many layers in unprecedented detail. Other spacecraft sample the hot, high-speed plasmas flowing past the Earth from the expanding solar corona. Still more satellites continuously monitor the plasmas which ebb and flow within the magnetosphere as it is buffeted by the solar wind. There is even an international network of ground stations recording the magnetospheric and ionospheric signatures of interaction between Sun and Earth. Merged as they are into a unified, comprehensive mission, the many components of ISTP afford an opportunity to stretch our understanding of the physics of solar-terrestrial processes.
The Astrophysical Connection: The Sun as a Star
In the past century, massive ground-based telescopes expanded and refined our view of the solar system, then began to reveal the origins of the galaxy and universe. Improvements in the spatial and spectral resolution of ground-based telescopes brought astronomy to a point where the mys-teries of the distant cosmos -- such as black holes, quasars, active galaxies, and developing plane-tary systems -- are being discovered and explored at an exciting pace.
Astronomical studies have reached a crescendo with the space-based "Great Observatories": the Hubble Space Telescope (HST), Compton Gamma-Ray Observatory (GRO), Advanced X-ray Astronomy Facility (AXAF), and the Space Infrared Telescope Facility (SIRTF). Each has pushed back the cosmic frontiers (or soon will) in their respective wavelength regimes. Figure 1a, for example, shows the distant galaxies of the "Hubble Deep Field" study [Williams et al., 1996]. In this project, long observations of small regions of the sky have brought us to the edge of the expanding universe.
But the image from HST also illustrates the limitations of astronomy. In studies with astronomi-cal telescopes, researchers can gather magnificent views of large segments of the cosmos. Yet the details of the physical processes in the far-reaches of space may never be directly visible. Im-mense modeling efforts and subtle detective work are needed to tease out the physics of distant, obscure objects. It is only in our own "cosmic backyard" that can we study directly, and in de-tail, the physical processes that drive much of the observable universe.
Within our own solar system, we can explore a typical main sequence star and a wide variety of planets in remarkable detail. Figure 1b, for example, shows the exquisite structure of the active regions around the Sunıs equatorial belt. This TRACE image reveals the small-scale magnetic structures that can persist for long periods. Such images show intense, dynamic energy-conversion events that even the largest astrophysical observatory will never be able to observe with such spatial (and temporal) resolution. Therefore, it is likely that only by detailed examina-tion of our Sun can we ever hope to understand the physical processes operating in other stellar atmospheres.
Observing Life Cycles: Events from Cradle to Grave
While it is useful to consider the relevance of solar-terrestrial observations to astrophysical re-search, the greatest impact of ISTP lies in something far more familiar and relevant: the chroni-cling of solar-terrestrial events from start to finish. From origins deep in the solar interior of the Sun to collisions with the magnetosphere to the dissipation of energy in aurora and plasmoids, the ISTP constellation can analyze Sun-Earth connections from a global perspective.
A series of events from May 1998 provide a useful case study in how ISTP is changing and im-proving our understanding of the solar-terrestrial system. The sequence began with a series of observations from the Solar and Heliospheric Observatory (SOHO) spacecraft at ~1000 UT on 2 May 1998. Figure 2 shows a set of observations taken by the EIT experiment on SOHO (cour-tesy J. Gurman and J.-P. Delaboudiniere). The images are recorded in the FeXII line (195Ċ) and show a bright, intense flare on the Sun (lower right quadrant). This was an X-class event that produced copious quantities of energetic particles at Earth [see Baker et al., 1998]. The second and third images in Figure 2 show successive differences that reveal a disturbance wave propa-gating across the solar surface.
The disturbance seen in the SOHO/EIT data rapidly spread outward from the solar surface into interplanetary space. Figures 3a and 3b (courtesy of R. Howard and G. Brueckner) show SOHO coronagraph (LASCO) images of material being expelled from the Sun: Figure 3a (taken at 1503 UT on 2 May) is from the LASCO C2 coronagraph and shows a huge, bright CME moving out-ward toward the right of the image. Figure 3b is from the LASCO C3 coronagraph and shows a larger scale image of the CME evolution (taken at 1642 UT on 2 May). The solar disturbances on 2 May 1998 were clearly of great power and immense physical scale (G. Brueckner, private communication, 1998).
The active solar event on 2 May discussed above produced powerful streams of solar wind plasma that were detected upstream of the Earth a few days later. Figure 4a shows the solar wind speed (VSW), the total interplanetary magnetic field (IMF) strength (BIMF), and the IMF north-south component (Bz) for 20 April to 20 May 1998 (DOY 110 to 140), as recorded by the Wind spacecraft (courtesy K. Ogilvie and R. Lepping). In the period from DOY 121 through DOY 140, there were four separate streams in which VSW reached peak values >~ 600 km/s. Such streams are very effective at producing highly relativistic electron (HRE) events in the magnetosphere [Baker et al., 1994, 1998]. A particularly notable solar wind stream occurred on Day 124 (4 May), when VSW went to ~850 km/s. This is the highest solar wind speed that has been measured near 1 AU in several years (A. Lazarus, private communication, 1998).
When high solar wind speed occurs in combination with large BIMF, and especially when Bz is strongly negative, then we expect significant electron acceleration [e.g., Blake et al., 1997] and intense geomagnetic activity. Indeed, the planetary magnetic index Kp reached 9 on Day 124 (4 May). The Dst index on that day reached 218 nT (a major geomagnetic storm) and the provi-sional auroral electrojet (AE) briefly exceeded 2500 nT (WDC-C2, Kyoto University). All of this information is indicative of powerful, global geospace disturbances on 4 May.
Energetic particle data revealed a strong acceleration of relativistic electrons exceptionally deep in the magnetosphere. The powerful solar wind streams and southward IMF illustrated in Figure 4a produced disturbed auroral conditions as seen by the Polar auroral imaging system, VIS [Visible Imaging System; Frank et al., 1995]. Figure 4b shows an auroral image (courtesy L.A. Frank and J. Sigwarth) taken at 0731 UT on 4 May 1998. The image shows an auroral oval that was greatly expanded equatorward. Moreover, auroras were particularly active and intense that day. As noted above, the AE index was >2500 nT on 4 May, indicating an unusually strong auro-ral electrojet.
Figure 5a is a plot of particle fluxes measured by an array of solid-state detectors (Heavy-Ion Large Telescope, HILT) on the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft. Flying in a high-inclination (82°), low-altitude (~600 km) orbit, SAMPEX samples magnetic field lines across nearly the entire magnetosphere every 100 min [see Baker et al., 1994 and references therein]. It carries sensors capable of measuring very ener-getic ions and electrons of both solar and magnetospheric origin, and the HILT channel shown in figure 5a has an electron energy threshold of 1 MeV. This channel also has sensitivity to >4 MeV protons (which is especially important during solar particle events and during surveys of the inner radiation belt).
Figure 5a shows particle flux plotted on a global map according to the color bar to the right of the figure. The data show rather quiet conditions on Day 121 (1 May), with a modest, variable flux of electrons with E>1 MeV in the outer radiation zone (which mapped to rather typical latitu-dinal positions during that interval). Figure 5b shows the global map for Day 123 (3 May), when a large solar proton event filled the polar cap with energetic particles [see Baker et al., 1998].
The most striking and notable event in the interval occurred on DOY 124 (4 May). On that day, SAMPEX observed a huge increase of the flux of HREs very deep in the magnetosphere (L<~ 3). The "slot" region between the inner and outer radiation zones was filled, and a new radiation belt feature appeared at L~ 2.2 ħ 0.2 [Baker et al., 1998]. The relativistic electrons remained high throughout the outer zone for at least two weeks. Electrons filled a broad region from L~ 2 to be-yond L~ 7 over the next several-day interval. The relativistic electron enhancement, seen for Day 126 (6 May) in Figure 5c, was as intense, long-lasting, and spectrally hard as any event seen in the magnetosphere over the past several years (c.f., Baker et al., 1998).
Figure 6a shows the daily average flux (electrons/cm2-sr-day) of E>2 MeV electrons from 21 April to 20 May 1998, as recorded by National Oceanic and Atmospheric Administrationıs GOES satellites. Electron flux was low (104/cm2-sr-day) on 21 April, but the flux then rose pro-gressively over the subsequent week or so, reaching a maximum on 29 April. The electron intensi-ties then were lower for several days (1-4 May). The average electron intensity jumped by two orders of magnitude of 5 May and stayed high for the subsequent 10 days. On 16 May the elec-tron flux diminished by a factor of 2-3, but it remained well above 107 until the end of the plot-ting sequence.
The Human Consequences of Solar-Terrestrial Disturbances
As reported in the newspaper headlines in Figure 6b, at approximately 2200 UT on 19 May 1998, PanAmSat Corporationıs Galaxy IV spacecraft experienced a catastrophic failure in its attitude control system. Unfortunately, the backup system also had failed -- either at that same time or earlier -- so the operators were unable to maintain stable Earth-link [Space News, 25-31 May 1998, p. 3]. PanAmSat and Hughes have been working to determine the exact cause of the Galaxy IV failure [Space News, ibid., p. 18]; in August 1998, company officials proclaimed the failure "an isolated incident" and "a one-time, random event" (Cable News Network, 11 August 1998). In the past, long-duration HRE enhancements such as those observed in August have been convincingly associated with spacecraft failures [e.g., Baker et al., 1994, 1996].
Regardless of whether the space environment played a role in the failure of Galaxy IV, the event puts into sharp focus just how much society has come to rely on satellites and space-based communications. Galaxy IV was a heavily subscribed communications satellite at geostationary orbit; its sudden failure caused the loss of pager service to some 32 million customers, as well as numerous other communications outages [USA Today, p. 1, 21 May 1998]. Doctors and fire-fighters could not be reached for emergencies. Stock brokers, business executives, and radio an-nouncers were suddenly incommunicado. In an ironic twist, even the illicit drug trade was slowed by the loss of paging relay systems. In essence, the Galaxy IV failure provides a hint of what we might see during the enhanced solar-terrestrial interactions that come at solar maximum.
With modern technological society so reliant on space-based communications -- systems that are vulnerable to the plasmas and radiation environments studied by ISTP -- we need more reliable information about the dynamics of the space environment. Yet many current models of the radiation belts are based on information that is 30 years old. New observations from ISTP could help scientists and engineers create more accurate models of Earth's radiation environment, and thereby build heartier spacecraft.
The International Space Physics Program: A Telescope and a Microscope
Earthıs space environment traditionally has traditionally been explored as a system of independ-ent parts the interplanetary region, the magnetosphere, the ionosphere, and the upper atmos-phere. Consequently, past science missions have advanced the understanding of these geospace components individually.
Yet even from the earliest studies, we have known that geospace is composed of highly interactive elements. To understand the system as a whole, we needed to plan a program of si-multaneous space and ground-based observations and theoretical studies. It would require that we assess the production, transfer, storage, and dissipation of energy across the entire solar-terrestrial system. In essence, we needed a comprehensive, quantitative study of the energy chain from the Sunıs interior to Earthıs magnetic tail [Baker and Carovillano, 1997].
The InterAgency Solar-Terrestrial Physics (IASTP) Program was created out of this need to obtain a comprehensive, global understanding of the generation and transfer of energy from Sun to Earth. IASTP was (and is) coordinated by the InterAgency Consultative Group (IACG), which includes representatives from the U.S., European, Japanese, and Russian space agencies. The stated goal of IASTP has been to establish cause-and-effect relationships between key re-gions and processes within the solar-terrestrial system.
Figure 7 shows many of the space-based assets now operating in the IASTP. This pro-gram represents a multi-billion dollar investment toward understanding the Sun-Earth system in unprecedented scope and detail. When these spacecraft are linked with the ground-based elements and the theoretical modeling tools of ISTP, the result is a true "great observatory" for space physics. The ISTP observatory provides not only a telescopic view (like its astronomical coun-terparts), but also a microscopic view. Figure 8 shows the sort of global solar image that ISTP can provide, as well as the exquisite detail that can be obtained from particular active regions. This coupling of microscopic and telescopic views is crucial to understanding the physical proc-esses -- such as magnetic reconnection and particle acceleration -- that drive our solar system.
But this dynamic and complex system of interacting plasmas, magnetic fields, and electri-cal currents also might serve as an astrophysical prototype. Plasma physics determines the be-havior of matter in the solar-terrestrial system on spatial and temporal scales, and with particle densities vastly different from those produced earthbound laboratories. Thus, solar-terrestrial space is a unique and readily accessible laboratory for investigating the natural plasma processes of astrophysics.
Future Directions
As measured by sunspot number, this next solar maximum will most likely be a large one, per-haps among the most active of the modern era. Figure 9 suggests that the next peak of solar ac-tivity is likely to occur in the year 2001, and as with past maxima, this one should bring solar dis-turbances of great power and geoeffective potential. Thus, it is an historic occurrence that the ISTP constellation should be operating as the solar maximum approaches.
Never before have we had such a complete set of tools with which to study the beginning of a new solar cycle (number 23). And never before have we had tools of such power and preci-sion to study our most important star the Sun and our most important planet the Earth. We have a chance to study all aspects of the solar maximum and its consequent effects on near-Earth space, and we can do it for modest costs. An investment in extended operations of ISTP and its affiliated spacecraft can give us the perfect vantage point to finally understand the disturbed Sun and how it disrupts the geospace environment. Responsible stewardship of the Sun-Earth investment demands operation of ISTP for as long as possible.
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