D.N. Baker1 and R. Carovillano2
1Laboratory for Atmospheric and Space Physics, University
of Colorado, Boulder, CO 80303, USA,
2NASA Headquarters, Washington, DC 20546
The InterAgency Solar-Terrestrial Physics (IASTP) Program developed out of an international need to obtain a comprehensive, global understanding of the generation and transfer of energy from the Sun to the near-Earth space and atmospheric environment. The goal was (and is) to establish cause-and-effect relationships between key regions (and processes) within the solar-terrestrial system. The program is coordinated by the Inter-Agency Consultative Group (IACG) consisting of the heads of the U.S., European, Japanese, and Russian space agencies. This paper presents the IASTP program from the American viewpoint. It briefly discusses the program evolution and several of the key milestones. It describes the program as it now is, including scientific goals and mission objectives, program elements, and examples of science achievements to date. Emphasis is placed on the Global Geospace Science (GGS) portion of the overall program.
Exploration of the Earth's space environment has revealed a dynamic and complex system of interacting plasmas, magnetic fields, and electrical currents. This region, formed by solar wind plasma impacting the magnetized Earth is called "geospace." Plasma physics determines the behavior of matter in geospace on spatial and temporal scales and particle densities vastly different from those that can be duplicated in Earth-based laboratories. Geospace thus affords a unique and readily accessible laboratory for investigation of natural plasma processes that are important both for astrophysics and for laboratory plasma physics. Through these plasma processes, energy expelled by the Sun is fed into the Earth's environment, constituting a small but highly significant part of the total atmospheric energy budget. The study of these processes, ranging from the Sun to the Earth, is the concern of solar-terrestrial physics.
The near-Earth space environment has traditionally been explored and studied as a system of independent component parts -- the interplanetary region, the magnetosphere, the ionosphere, and the upper atmosphere. From such early explorations, it was known that geospace is a complex system composed of highly interactive parts. While previous programs advanced the understanding of these geospace components individually, an understanding of geospace as a whole required a planned program of simultaneous space and ground-based observations and theoretical studies keyed to a global assessment of the production, transfer, storage, and dissipation of energy throughout the solar-terrestrial system. Prior understanding of the various geospace components plus the availability of advanced instrumentation was hoped to allow, for the first time, the definition and planning of a comprehensive, quantitative study of the solar-terrestrial energy chain.
The Global Geospace Science (GGS) mission featured the terrestrial portion of the solar-terrestrial connection. It was a key element of the ESA/ISAS/NASA/IKI InterAgency Solar-Terrestrial Physics (IASTP) Program and consisted originally of four spacecraft supplied jointly by the United States and Japan, an international ground-based segment, and a coordinated theoretical effort aimed at developing realistic models of the geospace environment.
The purposes of GGS stated in the GGS Mission Science Plan (May 1984) were: 1) to trace the flow of matter and energy through the geospace system from input by the solar wind to ultimate deposition into the atmosphere; 2) to understand how the individual parts of the closely coupled, highly time-dependent geospace system work together; 3) to investigate the physical processes that control the origin, entry, transport, storage, energization and loss of plasma in the Earth's neighborhood; 4) to assess the importance to the terrestrial environment of variations in atmospheric energy deposition caused by geospace plasma processes; and 5) to provide input to other heliospheric and solar-terrestrial studies by observing the solar particles and fields output near the Earth's orbit.
It was recognized that the geospace system has two major plasma sources -- the solar wind and the terrestrial ionosphere -- and two major storage regions -- the geomagnetic tail and the near-Earth plasma sheet and ring current. These four basic geospace regions are interconnected by transport processes which determine the highly interactive behavior of the system as a whole. Hence, simultaneous measurements of each of these key regions were required to meet the objective of a global assessment of the energy flow through the geospace system. Observing platforms were to be stationed to observe each of the two source and two storage regions. These laboratories were to be implemented by four satellites:
GEOTAIL was to have an orbit adjust capability to optimize the study of the tail energy storage and plasma energization region. Figure 1 illustrates the general observational configurations that could be used to study the global energy transfer problem. The figure shows WIND measuring the input conditions to the magnetospheric system. GEOTAIL, in the distant geomagnetic tail, would observe solar wind and ionospheric plasma entry and the effects of varying solar wind input on plasma entry, storage, energization, dissipation and transport throughout the distant tail regions. Simultaneously, POLAR and EQUATOR were to perform complementary observations covering the high latitude and near-tail regions. In such a case, not only would the overall problem of plasma entry be addressed, but the connections between the regions would be established by the simultaneous observations from the GGS laboratories.
Fig. 1. Example of expected GGS spacecraft configuration. In this configuration, WIND would measure solar wind input originally planned; POLAR and EQUATOR would measure entry, energization and transport while GEOTAIL would measure entry, energization and transport in the distant tail regions.
With the passage of time as explained further below, the US program was divided into two parts: the GGS program (consisting of WIND and POLAR) where NASA is the lead agency, and the Collaborative Solar-Terrestrial Research (COSTR) program where our international partners provide the lead role. COSTR consists of GEOTAIL (Japan/US), SOHO (ESA/US and Cluster/Phoenix (ESA/US). GGS and COSTR together constitute the International Solar-Terrestrial Physics (ISTP) program. Though EQUATOR as originally defined dropped from the program, it has re-emerged as the smaller mission Equator-S (German/US) which should be thought of as part of COSTR and ISTP.
Because plasma systems occur throughout the universe (Figure 2) knowledge gained from the ISTP/GGS program could be applied directly to planetary, stellar, and astrophysical studies. While the boundary conditions and scale sizes vary significantly from one system to another, a firm understanding of the overall geospace system would contribute fundamentally to studies of other cosmic plasma systems. In space plasmas (planetary, stellar, or even larger scales) there are bulk motions of magnetized plasma, irregular motions of energetic charged particles, beams of suprathermal ions and electrons, turbulence and particle acceleration processes. The ISTP Program was to provide a timely opportunity for an organized effort to study fundamental problems common to both the fields of solar-terrestrial relations and astrophysics.
Fig. 2. Comparative magnetospheres. Fundamental similarities characterize the magnetospheric configurations of the planets in the solar system and some other celestial objects in the universe at large, but their scales are vastly different because of differences in their intrinsic magnetic fields and associated plasmas (courtesy of NRL, based on earlier figures by the NASA Goddard Space Flight Center and L. Lanzerotti).
Knowledge gained about the geospace system has been used to provide useful and specialized services to societal systems affected by geospace perturbations. For example, alerts, warnings and forecasts of conditions throughout the geospace system have been supplied to national defense systems and to communication, power, and oil exploration industries to aid their day-to-day operations. Significant variations in energy deposition in the upper atmosphere may have important consequences on the Earth's weather and climate. Development of a more complete understanding of the physical processes that determine the geospace energy budget may allow assessment of the potential practical benefits.
The use of multi-satellite programs to study large-scale geospace phenomena has long been a concept with notable successes. The initial effort in this direction was the dual spacecraft IMP-7 and IMP-8 program. Multi-satellite programs were also used to study more localized phenomena such as magnetospheric boundaries or ionosphere-magnetosphere coupling. The International Sun-Earth Explorer (ISEE) program measured the position, velocity and geometric features of numerous boundaries found in the geospace system, showed the importance of these boundaries in the energy transport process, and measured plasma structures from a few hundred kilometers to approximately one Earth radius in size throughout some parts of geospace. Dynamics Explorer (DE) combined remote sensing (via auroral imaging from high altitudes) and two-point in-situ measurements to study the global energy deposition into the upper atmosphere and its relation to precipitating particles observed at low altitudes. The great success of these multi-satellite programs in attacking both localized and mesoscale geospace problems was also expected to contribute greatly to the GGS/ISTP program as it addressed the geospace system as a whole.
The recognition of the need for a multi-satellite approach to attack large-scale geospace problems gained impetus from a study conducted by the Space Science Board of the National Academy of Sciences in 1974. The Committee on Space Physics (F.S. Johnson, Chairman) foresaw a need for multiple spacecraft program to make coordinated measurements in different parts of the magnetosphere during the 1980s. The necessity for these spacecraft to operate as an integrated system was also stated. A year later the Committee on reaffirmed Physics (R.A. Helliwell, Chairman) reaffirmed this recommendation to the Board.
In 1977 the Space Science Board undertook a comprehensive study to identify future objectives of research in space plasma physics. The study committee (S.A. Colgate, Chairman) reported that space plasma physics is of fundamental importance for the development of general plasma physics and other astrophysical studies, and of practical importance for terrestrial communications and for meteorology. This report gave high priority to the development of a comprehensive understanding of the cause and effect relations between time-dependent magnetospheric processes. These objectives were again emphasized by two National Academy of Sciences reports concluded in 1980 and 1981. The report of the Committee on Solar and Space Physics developed a research strategy for the overall field through the 1980s, and the report of the Committee on Solar-Terrestrial Research presented recommendations, developed in coordination with the former report, for specific solar-terrestrial physics research.
In 1977 NASA formed a study group of space plasma physicists and charged them with creating a plan for a program that would be responsive to the recommendations of the National Academy of Sciences. This plan would involve both theory and observation in a meaningful, interactive relationship and complement parallel programs in upper atmospheric research and solar physics to form a major thrust in the understanding of solar terrestrial relations. The result of that study was the formulation in 1979 of a plan for the Origins of Plasmas in the Earth's Neighborhood (OPEN) program. Following a peer review of competitive science investigation proposals in 1980, an OPEN Science Working Group was selected in 1981 to conduct detailed science definition studies in 1982-83. In 1983, NASA and the National Science Foundation convened a workshop involving over 100 solar-terrestrial scientists to assess the status and future directions of the field. Central to the conclusions of that workshop was a reaffirmation of the importance of the goals and objectives of OPEN. Concurrent with the NASA studies, science teams in Japan and Europe worked to develop plans for complementary missions to study the middle magnetosphere (ISAS/OPEN-J), the sun (ESA/SOHO), and the microphysics of space plasmas (ESA/Cluster). From those early deliberations the IASTP program evolved.
After the OPEN concept evolved at NASA in the early 1980s under the leadership of E. Schmerling and D. Cauffman, there were several unsuccessful attempts at Congressional new starts (see Table 1). There was also the selection of a prime contractor (RCA/Astro) for the spacecraft design and development. By this time, S. Shawhan arrived at NASA Headquarters and worked to combine the GGS program elements to form the ISTP program. In the course of this, the EQUATOR spacecraft was eliminated as a cost cutting measure. Over time SOHO and CLUSTER were added to the set of missions that complement the core ISTP missions (see Table 2). Led by IKI, the Interball-Tail Probe and Interball-Auroral Probe (each consisting of a pair of satellites) have been launched. Most recently, the FAST mission (U.S.) has joined the fleet and Equator-S will do so in late 1997. Today, an absolutely unprecedented constellation of spacecraft and facilities comprises the IASTP program.
Table 1. Origins of Plasma in Earth's Neighborhood (OPEN): A Brief History
|1976-79||OPEN concept devised|
|1979||Announcement of opportunity|
|1980||Science proposals submitted; selections announced|
|1981-82||New start briefings (unsuccessful)|
|1983||Name change -- ISTP|
|1984||Selection of spacecraft contractor (RCA Astro Division, East Windsor, NJ)|
|1985-86||New start attempts (unsuccessful)|
|1987||Contractor Change (RCA/Astro acquired by General Electric)|
|1988||New start achieved: Geotail S/C to Japan; Equator S/C eliminated|
|1991||Contractor change (Astro Division sold to Martin Marietta by GE)|
|1995||Contractor change (Martin merges with Lockheed)|
Table 2. InterAgency Solar Terrestrial Physics Program
|Ground-based measurements||NSF/NASA and other non-US agencies|
PRESENT PROGRAM: RESULTS AND POSSIBILITIES
Given the intense interest in understanding the acceleration and transport of high-energy magnetospheric particles and their dependence on solar and magnetospheric conditions, we have used data in the early part of 1996 from several IASTP (and operational) spacecraft to examine global solar-terrestrial connections. These data are employed to provide a broad context for simultaneously observing the changes occurring at the sun, in the interplanetary medium, and in the geospace environment under solar minimum conditions. The IASTP spacecraft armada and coordinated ground-based measurements yield a comprehensive view of the complex processes that modulate energetic charged particles in the magnetosphere; the importance of these particles is underscored in this study through the possible association of a major satellite failure with the radiation environment within which it was operating.
Figure 3 shows the daily Ap geomagnetic index values (Mayaud, 1980) for the first three months of 1996. Small, recurrent storms with peak Ap activity levels in the range of 20-40 are evident. The Ap enhancements peak every 12-15 days. The overall pattern is one of relatively weak activity with no major storms during the interval. The largest event (Ap = 38) occurred on Day 80. Figure 4 shows 2-6 MeV electron intensities measured by the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft at low-Earth, polar orbit (see Baker et al., 1993). A subset of the Proton-Electron Telescope (PET), data averaged each day for L=5 (± 0.1) is plotted for the period Day 1 to Day 100 of 1996 [see Baker et al., 1996]. The SAMPEX data show relatively brief flux peaks associated with solar wind streams in January and February, but larger and longer lasting flux enhancements in late-March are quite clear. The occurrence of relativistic electron enhancements associated with solar wind streams (but delayed by some days) has been known for some time (e.g., Paulikas and Blake, 1979).
Fig. 3. Daily Ap magnetic index data for the first three months of 1996.
Fig. 4. Daily-avereage fluxes of 2-6 MeV electrons at L=5 from the PET sensor on SAMPLEX
Given the condition that the largest solar wind streams originate from solar coronal holes (e.g., Feldman et al., 1978), available solar soft X-ray (Yohkoh) and extreme ultraviolet (SOHO) data have been examined to identify solar wind stream source regions. Figure 5 shows a SOHO image of the sun taken at 1333:37 UT on 19 March 1996. In general, the solar corona was seen in SOHO data to be extremely quiet and unstructured during the early part of 1996. However, a large, trans-equatorial coronal hole is discernible near central meridian on 19 March. This hole undoubtedly gave rise to the solar wind stream which the Earth encountered on ~22 March. This same coronal hole was seen earlier in February and again returned in April of 1996.
Fig. 5. An image of the Sun taken by the SOHO EIT experiment (at 195Å) at 1333:37 UT on 19 March 1996. A coronal hole can be seen near central meridian extending from the south polar region across the equatorial plane (courtesy of J. Gurman and J.-P. Delaboudiniere).
Enhanced geomagnetic activity and subsequent relativistic electron events within the magnetosphere are produced by a combination of high-speed solar wind and strongly southward IMF. Since the launch of the WIND spacecraft in November 1994, there are nearly continuous solar wind data. Figure 6a shows the solar wind speed (VSW) measured by the Solar Wind Experiment (SWE) onboard WIND (Ogilvie et al., 1995). Data are shown as hourly averages for Day 60 (March 1) to Day 100 (April 9) of 1996. Figure 6b shows hourly averages of the IMF north-south component (Bz) for Days 75-86 in GSM coordinates measured by WIND (see Lepping et al., 1995). It is seen that the IMF was persistently southward from Day 77 (17 March) to near the end of Day 81 (21 March). This negative Bz was "piled up" at the leading edge of the high-speed solar wind stream which began to be seen at Earth on ~Day 80 (20 March). As seen in Figure 3, the southward IMF and high value of VSW produced a clear increase in the global magnetic index, Ap. The Ap value peaked on 20 March when the IMF was most negative.
Fig. 6. (a) A detail of solar wind speed for Days 60-100 of 1996. (b) Hourly averages of the interplanetary magnetic field Bz component for Days 75-86. (Data courtesy of R.P. Lepping and K.W. Ogilvie.)
In Figure 7, data from the geostationary orbit GOES-8 spacecraft are shown for electrons with E>2 MeV. Note that the GOES data are shown as daily fluence averages. Looking at the low frequency temporal patterns, it is seen from Figure 7 that each solar wind stream was associated with a large and relatively brief enhancement of >2 MeV electron fluxes (e.g., in mid-January, in February, and in early March). On the other hand, the strong and complex solar wind variations of late March produced a long-lasting (>2 week) enhancement of relativistic electrons at geostationary orbit beginning on ~12 March; this persisted until essentially the beginning of April.
Fig. 7. GOES-8 measurements of electrons with E>2 MeV for January to early April of 1996. (Data courtesy of NOAA and J.H. Allen.)
Very much the same electron behavior is seen in the data from Los Alamos National Laboratory (LANL) sensors onboard another series of geostationary spacecraft. Also, a more extensive examination of the SAMPEX data at low-Earth orbit fully supports the idea that relativistic electrons were substantially enhanced throughout the outer trapping zone for nearly two weeks in late March. Figure 8 shows data from a dosimeter onboard a spacecraft at ~60Å inclination and having a highly elliptical orbit extending out to 6.6 RE. Several selected energy channels as labeled are shown and the data have been averaged for each day. The dosimeter shows exactly the same features as the SAMPEX measurements: Each individual brief flux enhancement in January and February looks the same in both data sets. Moreover, the intense and long-lasting flux increases of late March and April of 1996 are also clear. Thus, the magnetospheric electron increases are a truly coherent phenomenon.
Fig. 8. Data from a dosimeter on a spacecraft in a highly elliptical, 60°-inclination orbit with apogee at 6.6 RE for days 1-150 of 1996 (courtesy of J.B.Blake).
A final view of the electron environment in the outer zone is provided by exciting new data from the Comprehensive Energetic Particle Pitch Angle Distribution (CEPPAD) investigation onboard the recently launched POLAR spacecraft (Blake et al., 1995). The High-Sensitivity Telescope (HIST) sensor on POLAR measures 0.4 to >10 MeV electrons. In Figure 9, we show electron fluxes versus L for two similar passes through the outer zone. Data for 10 March 1996 (Day 70) were taken when the radiation belts were rather weak. It is seen that HIST data (covering 3.5 MeV E 5 MeV) reached a peak counting rate of about 500 counts/sec. Later, when the radiation belts were much more intense, the flux of electrons measured by HIST was much greater. For example, a similar pass through the radiation belts on 26 March 1996 at L values and local times comparable to the pass on 10 March shows that the peak 3.5-5 MeV counting rate was ~7 x 103 c/s and the entire outer zone profile was much broader in time. This indicates a much more substantial outer zone electron population on 26 March.
Fig. 9. POLAR spacecraft data for high-energy electrons measured by the CEPPAD/HIST detector comparing 10 March 1996 data with 26 March 1996 data. The spacecraft cuts in L and local time (LT) were similar for these passes. The fluxes are shown versus L value for two channels and clearly demonstrate the much higher fluxes and greater radiation belt width on 26 March (from Baker et al., 1996).
ISTP/GGS represents a major advance in space plasma physics. It is a program totally defined by the physics inherent in its major goal of performing a global assessment of the energy balance in the geospace system. It utilizes a team of experimentalists and theorists to define the coordinated observations required to attack that scientific goal. As suggested by Figure 10, the following tasks are among those which the new capabilities of GGS make possible:
The first truly global assessment of geospace energy balance.
The first comprehensive study of the distant geomagnetic tail and the plasma sheet.
The first comprehensive study of the equatorial magnetosphere, including a search for near-Earth acceleration processes.
The first comprehensive global study of geospace dynamics incorporating global auroral imaging.
The first comprehensive study of plasma transport in the polar cusps.
The first comprehensive study of various magnetospheric boundaries at equatorial latitudes.
Fig. 10. Schematic diagram showing the solar-terrestrial energy chain and the IASTP spacecraft contributing to the understanding of this coupled system.
Furthermore, GGS employs new technological capabilities that have not been available until now. Complete plasma and energetic particle distribution functions with high time resolution and ion composition measurements accompanied by full magnetic and electric field and plasma wave measurements are being obtained for the first time. Ground data handling will be accomplished through a distributed computer network that contains the entire ISTP data base. This approach is mandatory in order to permit effective use of theoretical modeling and analytical studies required for the overall quantitative description of geospace.
The practical consequence of global observations is emphasized in the context of the Sun-Earth connection to demonstrate use of the ISTP capabilities. The Telesat Canada Anik E1 communication satellite located at 111°W at geostationary orbit suffered a severe operational problem on 26 March 1996 (Baker et al., 1996). The Anik satellite lost all power from its south solar panel array when the array was effectively disconnected from the satellite payload at 2047 UT. The 50% power loss reduced the spacecraft's capacity from 24 C-band channels to nine channels and it reduced the Ku-band capacity from 32 channels to 10. It is not expected that the lost solar panel can be recovered; thus this was a permanent degradation of communication capability for Telesat Canada. It affected a broad range of video, voice, and data services throughout North America. Service to Telesat Canada customers was restored after about six hours by link switches to other spacecraft and by using backup systems such as fiber optics ground links.
In summary, a remarkable array of scientific and operational spacecraft, as described here, demonstrates that the high-energy electron environment was quite elevated throughout late-March 1996. The satellite and ground-based data suggest that the observed space weather conditions could have caused, or at least exacerbated, the conditions onboard Anik E1 that led to the power failure and crippled the spacecraft through the mechanism of deep-dielectric charging. Moreover, data from the vast ISTP complex allows the study of relativistic electron acceleration -- a problem of great astrophysical interest, importance and application -- with unprecedented completeness.
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