Fleet of Satellites and Ground-Based Instruments Probes the Sun-Earth System

Space physicists are embarking on a new voyage of discovery of both the inner and outer frontiers of the chain of processes that connects the Sun with the Earth and the other planets. With the successful launch of the POLAR satellite on February 24, 1996, all the elements of the long-awaited Global Geospace Science (GGS) program are in place. GGS is the first phase of an exciting NASA science initiative called the Solar Connections Program, and it is also the primary contribution of the United States to the International Solar Terrestrial Physics (ISTP) Program.

Humans have always known that life on Earth is dependent on the relatively steady flow of light and heat from the Sun. But undetected by our senses, corpuscular radiations from the Sun intermixed with electromagnetic fields also link the Sun to the Earth in a more dynamic way. This source of solar energy, called the solar wind, generates and characterizes the near-Earth environment that protects our lifeforms and is manifest by the awe-inspiring optical displays known as the aurora (Figure 1). Recently, solar radiations have been found to affect the increasingly complex technological systems on which society is becoming more dependent.

Fig. 1. A global image of Earth's northern auroral oval observed with the ultraviolet imaging photometer onboard the Dynamics Explorer 1 satellite. The observed intensities are due predominantly to atomic oxygen emissions at 130.4-nm and 135.6-nm. The bright dayglow can be seen as a crescent. A coastline map is superimposed on the image.

A chain of individual links connects the Sun and Earth (Figure 2); it begins at the Sun's convective zone beneath the photosphere and passes through the corona and interplanetary space and on to Earth's magnetosphere, ionosphere, and atmosphere. These links have been explored and studied as interacting adjacent components by a number of satellite missions. However, it became apparent from these earlier explorations that not only is this complex system composed of highly interactive parts but its behavior differs significantly from simple linear relationships between the individual components. Near the end of the 1970s the space physics community proposed a new tack to understanding the Sun-Earth system in a quantitative and definitive way. It suggested a systems approach, placing satellites at key locations in the system to obtain simultaneous, coordinated data (Figure 2). The measurements would guide theorists and modelers in the development of predictive capabilities of the response of the system to the variable solar corpuscular output. The study recognized the value of international collaborations for this ambitious program. The late Stan Shawhan, then Director of the Space Physics Division of NASA, was very instrumental in successfully establishing the ISTP program.

Fig. 2. Regions or cells of the Sun-Earth system, with interfaces between the cells. The flow of energy, mass, and momentum through the cells and interfaces will be studied by the ISTP satellites, as shown.

GGS will supply a wealth of data on the Sun-Earth system. Its first component, the WIND satellite launched in November 1994, is providing detailed quantitative observations of the solar wind in interplanetary space and upstream signatures of the magnetopause and bow shock. The POLAR satellite measures the entry of plasma into the polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere. NASA is also collaborating with ESA on two ISTP missions. The Solar and Heliospheric Observatory (SOHO), launched in December 1995, is investigating the interior dynamics of the Sun from where the heated plasma is accelerated outward to form the solar wind. The Cluster mission, comprised of four satellites to be launched next summer, will conduct three-dimensional investigations of small-scale spatial structures and plasma turbulence throughout the polar and middle magnetosphere. Japan's satellite, GEOTAIL, is producing outstanding quantitative measurements of the entry, storage, acceleration, and transport of energy and particles into and within the geomagnetic tail [Nishida et al., 1992].

Key data from regions of geospace not sampled by ISTP will be obtained by satellites of collaborating agencies, the National Oceanic and Atmospheric Administration and the Department of Energy, and from the former InterCosmos organization in Russia and the Max Planck Institute for Extraterrestrial Physics in Germany, as well as from NASA's venerable IMP 8 (Figure 2). NASA's Advanced Composition Explorer and the Small Explorer satellite, Fast Auroral Snapshot, are expected to join the GGS program once they are launched.

Russell [1995] provides a detailed description of the GGS project, the spacecraft, international collaborations, the instruments, ground-based and theory investigations, and the ground data processing system. Additional information can be obtained from the World Wide Web site: http://www-istp.gsfc.nasa.gov/ISTP/ggs_project.html.

Scientific Mission of GGS

Geospace is defined as the region surrounding the Earth where the heliosphere is disturbed by the Earth's magnetic field. This region is not static because of variations in the solar wind density, speed, and magnetic field as a result of changing conditions in the outer atmosphere of the Sun. The direction of the interplanetary magnetic field vector determines the extent of electromagnetic coupling of energy from the solar wind to the magnetosphere, where energy is dissipated through a variety of coupling processes to the ionosphere, upper atmosphere, and within the magnetosphere. The momentum carried by the solar wind determines the shape of the magnetosphere, and some of the solar wind particles enter into the magnetosphere to be stored, energized, and precipitated into the atmosphere. The ionosphere in turn provides ions at a highly variable rate to the magnetosphere. The first scientific objective of the GGS program is to acquire a global understanding of the effects of the variable solar activity on the flow of energy, momentum, and mass through the Sun-Earth connected system.

The major components of the system—interplanetary space, the parts of the magnetosphere, the ionosphere, and atmosphere—can be compared to cellular structures separated by thin layers. The bulk of the mass, momentum, and energy resides in large-volume cells that play different roles in the transport, storage, and evolution of these quantities within the system. However, the physics of the boundary layers determine the coupling between the cells. Acquiring a physical understanding of the cause and effect relationships within and between the cells of the system is the second objective of the program.

A large fraction of the solar wind energy captured by the magnetosphere, measured in millions of megawatts, is sporadically deposited in the ionosphere and upper atmosphere during events called auroral substorms. The most apparent evidence for the deposition of this energy comes from observations of the optical emissions from the resulting excited atmospheric constituents, or the aurora (Figure 1). The third objective of the program is to determine the control of this energy input to the Earth atmosphere by plasma processes in geospace.

While the ISTP program science pertains to the connection through corpuscular radiations from our star, the Sun, to our planet, Earth, the scientific results from the program are expected to be applicable to other astrophysical systems involving plasma couplings that cannot be studied in the detail and with in situ measurements that our own system allows.

An understanding of the Sun-Earth system will also lead to applications close to everyday life. Many human activities, both civil and military, are becoming more dependent on the use of space, spacecraft, and the long-distance transmissions of electromagnetic energy, whether it be for power distributions, communications, or other signal transmissions. Unfortunately, these systems are susceptible to disruptions and damaging effects due to natural electromagnetic disturbances in geospace or exposure to the space environment itself. The vulnerability of this modern infrastructure has spurred the establishment of a U.S. multifederal agency collaboration dedicated to understanding geospace and developing predictive capabilities. This program, called Space Weather, will use GGS data for its initial research on the development of global predictive modeling. Then real-time data like those from the GGS spacecraft will be assimilated into operational models to predict the times and locations of natural disturbances.

The Spacecraft and Their Orbits

WIND and POLAR are cylindrical spacecraft of traditional design, 2.4 m in diameter and 1.8 m high. Because both spacecraft are spin stabilized, the prime mechanical difference between them comes from the requirement for POLAR of a three-axis (despun) stable platform for high-resolution imaging of the aurora. Both spacecraft use long wire antennas for AC and DC electric field measurements. Particular attention was placed on building magnetically, electrostatically, and electromagnetically clean spacecraft to allow accurate measurements of very low energy plasmas and weak electric and magnetic fields. Long booms place several instrument sensors away from the spacecraft body to keep interference to a minimum.

The orbital characteristics of the WIND satellite were selected to enable radial mapping of the near-Earth interplanetary medium. The spacecraft was placed into an eccentric orbit with apogee on the dayside just beyond the orbit of the moon. This orbit, called "double lunar swing-by" [Farquhar, 1981], uses the gravitational attraction of the Moon during periodic encounters to keep the orbit roughly aligned with the Earth-Sun direction. Finally a lunar swing-by will boost the spacecraft into an orbit from which, in early 1997, the on-board propulsion system will place it into a "halo" orbit around the forward Lagrangian point. It will remain there with infrequent adjustments through the combined gravitational effects of the Earth and the Sun and centripetal forces.

The POLAR spacecraft was placed in an 86Ý inclination orbit with apogee at 9 R_E and perigee at 1.8 R_E geocentric distances. Apogee is located and will stay over the northern polar region. This orbit was selected for excellent long duration imaging of the northern auroral oval and for sampling at high altitudes the characteristics and flows of the ion distributions emanating from the auroral and polar regions of the ionosphere. In the southern hemisphere the high perigee will allow the spacecraft to cut across auroral field lines at altitudes where auroral particle acceleration and precipitation processes are thought to occur. The locations of the flotilla of ISTP satellites within the geospace system are depicted in Figure 3.

Fig. 3. A flotilla of international spacecraft will study Sun-Earth connections. GEOTAIL was built in Japan, WIND and POLAR in the United States, and SOHO and Cluster in Europe.

Science Instruments and Investigations

The GGS spacecraft carry full complements of instruments that represent the state-of-the-art in experimental space plasma physics. Both new technology detectors and advanced data processing techniques have been incorporated within the instruments. The measurement capabilities of the GGS instruments and of instruments on key collaborating spacecraft are listed in Table 1. WIND carries eight instruments and POLAR has 11. The names of the GGS investigations, the principal investigators who provide the instruments, and their institutions are given by Russell [1995] and in the WWW site.

Fields

Both spacecraft can measure magnetic and electric fields in the quasi-static (DC) and wave (AC) regimes. They are also able to treat the data on board with new techniques such as burst capture, Fast Fourier and wavelet transforms, and neural network event identification. These techniques allow spacecraft resources to concentrate on the phenomena found to be most important by earlier missions, especially high temporal resolution measurements during appropriate intervals.

The fields instruments on board WIND primarily measure the interplanetary fields upstream of the magnetosphere, particularly its direction and strength, or equivalently, the magnetic flux carried to the magnetosphere in a unit of time. These quantities are important for placing other GGS measurements into context. The AC fields instruments will allow remote sensing of shocks and energetic particles from about 3 to 4 solar radii out to about one AU through their radio wave signatures.

The POLAR fields instruments are able to probe small structures such as double layers, cavitons, and field-aligned currents that are relevant to the acceleration of auroral particles and the coupling of the solar wind first to the outer magnetosphere and then to the ionosphere. The wave instrumentation and charged particle detectors will determine quantitatively for the first time the role of wave-particle interactions in the acceleration, transport, and loss of particles in the magnetosphere.

Charged Particles

Both spacecraft contain plasma and energetic particle instruments that measure the energy range from below 1 eV to over 1 MeV. Because of the very different plasma regimes encountered by the POLAR and WIND, the missions differ in their measurement emphases.

Measuring the thermodynamic properties of the solar wind is a primary goal of the WIND mission. Knowledge of the velocity distributions and the elemental, isotopic, and ionic-charge composition of this radiation will allow the study of how the solar wind is generated and modified as it is transported to the magnetosphere. By making the first long-term measurements of suprathermal particles, WIND is also expected to provide new understandings of how particles are accelerated to high energies, which is ubiquitous throughout the solar system and beyond.

Measurement of the thermal plasma in the magnetosphere is fundamentally important to the POLAR mission; it will reveal an unknown regime of thermal plasmas known as the hidden ion population [Olsen, 1982]. This difficult measurement will be accomplished by using an instrument with a large geometric factor and by controling the spacecraft potential through the emission of xenon ions from the spacecraft.

The many signatures of time-dependent plasma processes in the magnetosphere can be observed in the polar regions because its magnetic field lines thread the various plasma regimes of the magnetosphere. New insights into a large variety of energization, transport, and precipitation mechanisms are expected from the complete measurement by POLAR of the distributions of electrons and all major magnetospheric suprathermal ion species of auroral energies. Measurement of the even more energetic particles provide probes of regions remote from the spacecraft by virtue of the spatial scale of their adiabatic motion.

Photons

Instruments aboard both WIND and POLAR measure photon fluxes, but with much different scope and objectives. The WIND instruments study gamma ray bursts, although solar flares are also observed in the hard X ray band, permitting the study of the coupling between the active corona and the photosphere. More important, they allow direct study of particle acceleration and the impulsive release of energy in an astrophysical plasma. Space scientists are eagerly awaiting auroral images from the three imagers on POLAR, which cover wavelengths from ultraviolet through visible down to X rays. The imagers' primary goal is to quantitatively assess for the first time the dissipation of magnetospheric energy into the auroral and polar atmospheres. The images also provide an instantaneous reference system for the in situ measurements from the variety of ISTP spacecraft. From the image data the ionospheric conductivity patterns over the auroral and polar regions will be calculated, which, when combined with in situ measurements and modeling, will yield the global ionospheric convection and electric current patterns. Finally, since these images are a visible manifestation of the energy flow from the Sun through geospace, they will provide striking educational and public relations tools.

The far ultraviolet (FUV) and visible imaging instruments are complementary in a number of features. In the FUV the aurora can be imaged at all times, regardless of sunlight or darkness. The narrow band interference filters developed for the mission allow resolution and measurement of several specific emission features and thus quantitative interpretations of the data. For several hours around apogee on periodically spaced orbits, the FUV imager will observe the entire auroral oval during moderately sized auroral substorms and lesser activity, but not the entire planet. At other times the imager will be pointed at critical features and regions of the oval.

Even higher spatial resolution data will be obtained with two cameras at visible wavelengths. A mosaic of a high-resolution instantaneous field-of-view will provide a full image of the nightside auroral oval. An ancillary camera for far-ultraviolet wavelengths within the visible imaging instrument will complement the FUV images by providing full planetary views of Earth during 10-hour periods every orbit. These images will help place in situ data into the context of global geophysical activity and are especially useful for the public's education.

X rays emanating from the atmosphere in the auroral region will also be imaged. These X rays are created as electrons with energies extending well above those that produce the visible and UV aurora bombard the atmosphere. The measurements can be used to estimate the rate of electron energy deposition and the energy distribution of the higher-energy electrons, complementing the visible and UV derivations of the same quantities.

Ground-Based Instruments

The GGS program is the first spacecraft program to contain an extensive ground-based element. It consists of four sets of instrumentation. Ground-based observations, especially from the global-scale network of HF and VHF radars and from large-scale arrays of remote sensing equipment (for example, the Canadian Canopus array) will acquire instantaneous pictures of the two-dimensional distribution of plasma flow, ionospheric currents, particle precipitation, and auroral luminosity in the polar ionosphere. VLF and ELF magnetic activity will also be measured. These data will complement the satellite observations.

Mission Oriented Theory

Although the ISTP program boasts an impressive flotilla of satellites, the early recognition that multiple single point measurements alone cannot accomplish the program objectives remains valid. From the first inception of the ISTP program, theory development has been recognized as an essential component of the program.

The role of mission-oriented theory is to develop techniques and models that can be used by experimenters and theorists to interpret spacecraft measurements, making deductions from the available information about both local and large-scale dynamics. The global interaction of the solar wind with the magnetosphere and transport within the magnetosphere will be studied using three-dimensional MHD simulations. Kinetic simulations will help in interpreting satellite measurements in regions of strong spatial inhomogeneities. To study especially the transition or boundary regions between the cells of the system, intermediate or mesoscale processes will be analyzed using a broad range of techniques, including two fluid, hybrid, particle-in-cell and test particle simulations. Finally, these models and simulations combined with the data from the space and ground-based GGS sensors will lead to the next generation of global and local models with predictive capabilities.

Operations Coordination

Coordination of these missions must be maintained through their flight phases to optimize the opportunities for major advances in our understanding of the Sun-Earth system. An international body, the Inter-Agency Consultative Group (IACG), promotes such coordination through scientific campaigns that address questions that can only be answered by observations from the multiple spacecraft. Workshops have been held to define the strategy for each campaign. The Science Planning and Operations Facility develops the detailed science operations plan for the GGS spacecraft, thus implementing the campaigns defined by the IACG. The Geospace Energy Model program of the National Science Foundation and the Solar Terrestrial Energy Program administered by the Scientific Committee on Solar Terrestrial Physics (SCOSTEP) are coordinating the involvement of the broader scientific community and especially the correlative ground observations.

Data System and Products

A comprehensive ISTP ground science data handling system promotes efficient data processing, analysis and distribution. The Central Data Handling Facility (CDHF) is the one place where measurements from the variety of geospace and ground-based instruments and theoretical models are brought together. It takes in the telemetry data, orbit, attitude, and command histories and provides a computational platform for parameterized modeling. To easily survey the vast quantities of scientific data for scientifically interesting events worthy of detailed analysis, the concept of key parameters was born. Key parameters are low-resolution time series data computed in the CDHF for each instrument. Some additional key parameters, such as from ground facilities and non-ISTP spacecraft, are externally generated and transferred to the CDHF [see Mish et al., 1995].

The key parameters, low-order telemetry products, and associated ancillary data are organized by the Data Distribution Facility into products on CD-ROM for distribution to researchers. The science teams will continue to process data to higher level products and perform analyses on their local systems. The key parameters and processed data will be archived at an on-line facility, the NASA Data Archive and Distribution Service of the National Space Science Data Center, to provide rapid data access to the scientific community. The GGS program is working to make these data available to the community as early as possible.

Acknowledgments

The auroral image was provided by L. A. Frank. Comments by many members of the GGS Science Teams and especially by J. H. Clemmons and M. Hesse were very helpful.

References

Farquhar, R. W., and D. W. Dunham, A new trajectory concept for exploring the Earth's geomagnetic tail, J. Guidance and Control, 4, 192, 1981.

Mish, W. H., J. L. Green, M. G. Reph, and M. Peredo, ISTP science data systems and products, The Global Geospace Mission, edited by C. T. Russell, Kluwer Academic Publishers, Dordrect, The Netherlands, p. 815, 1995.

Nishida, A., K. Uesugi, I. Nakatani, T. Mukai, D. H. Fairfield, and M. H. Acuna, Geotail mission to explore Earth's magnetotail, Eos Trans., AGU, 73, 425, 1992.

Olsen, R. C., The hidden ion population of the magnetosphere, J. Geophys. Res., 87, 3481, 1982.

Russell, C. T., (Ed.), The Global Geospace Mission, Kluwer Academic Publishers, Dordrect, The Netherlands, 1995.

Return to Science and Society
Return to Starting Point