THE GEOTAIL MISSION

Surrounding the Earth -- including its upper atmosphere, ionosphere, and magnetosphere -- is an area called geospace. The Geotail spacecraft, scheduled for launch in July 1992, is designed to explore the tail of the magnetosphere.

The comprehensive instrument package on the Geotail spacecraft will investigate the energy dynamics (transport, storage, and conversion of energy) of the Earth's magnetotail and mid-magnetosphere region extending from near-Earth to distant-tail regions.

The launch of Geotail, developed by the Japanese Institute for Space and Astronautical Science (ISAS) and managed by NASA's Goddard Space Flight Center, Greenbelt, Maryland, inaugurates the Collaborative Solar-Terrestrial Research Program (COSTR). COSTR provides the NASA contributing elements to the Geotail, the Solar and Heliospheric Observatory Mission (SOHO) and Cluster international cooperative missions.

Along with Geotail, NASA's Wind and Polar spacecraft, to launch in 1993 and 1994 respectively, will provide an overlap in coverage enabling the missions to make coordinated science measurements in key energy source and storage regions for the U.S. Global Geospace Science (GGS) Initiative. In addition, major orbital changes can be made for certain spacecraft, allowing study of the global behavior of the geospace system for many combinations of satellite orbit configurations.

The GGS Initiative will provide the Wind and Polar satellite to perform simultaneous and closely coordinated measurements of key geospace regions, supplemented by data from equatorial missions along with ground-based and theoretical investigations.

THE SCIENCE

The solar wind, emanating from the Sun, injects plasma into the magnetosphere and transfers energy to it. Several times a day, the magnetosphere undergoes a disturbance called a substorm. As the substorm grows, most of the solar wind energy is dissipated within the magnetosphere, ionosphere and upper atmosphere.

This disturbance ultimately causes auroral displays, the acceleration of charged particles to high energies, the emission of intense plasma waves and electromagnetic waves, and the generation of strong ionospheric currents that produce significant changes in the upper atmosphere. These waves and currents often result in severe problems on Earth with communications, power supplies, and spacecraft electronics.

The information gathered during the Geotail mission will allow scientists to model and more accurately predict Sun-Earth interactions and their effects on space exploration, communications and in ground technology systems.

THE MISSION

ISAS constructed the Geotail spacecraft for launch on a Delta II expendable launch vehicle from Cape Canaveral Air Station in July 1992. Geotail will use the gravity of the Moon to assist its orbit on the night side of the Earth, where the magnetotail is stretched out as a result of the impact of the solar wind encountering the Earth. Geotail's orbit will extend from 220 Earth radii (1,401,620 km) at its farthest point to 8 Earth radii (50,960 km) at its nearest point.

The objectives of the Geotail Mission are to:

Determine the overall plasma, electric, and magnetic field characteristics of the distant and near geomagnetic tail;
Determine the role of the distant and near-Earth tail in substorm phenomena and in the overall magnetospheric energy balance and relate these phenomena to external triggering mechanisms;
Study the processes that initiate magnetic field reconnection in the near-Earth tail and observe the microscopic nature of the energy conversion mechanism in the reconnection region'
Study plasma entry, energization, and transport processes in interaction regions such as the inner edge of the plasma sheet, the magnetopause and the bow shock, and investigate associated boundary layer regions.

THE GEOTAIL INSTRUMENTS

Geotail has a complement of seven instruments, with the United States providing two and the Japanese providing three complete instruments. The remaining two instruments are shared. The principal roles of the seven experiments onboard the Geotail spacecraft are divided into magnetic-field measurements, electric-field measurements, plasma and plasma wave analysis and energetic particles.

Comprehensive Plasma Investigation (CPI): This investigation is supplied by Louis Frank, University of Iowa, Iowa City. The CPI will obtain complete three-dimensional plasma measurements in Earth's magnetotail. Plasma parameters, including heat flux and field-aligned current density, will be measured by a Hot Plasma and Ion Composition Analyzer and a Solar Wind Ion Analyzer. The plasma data will be correlated with the magnetic field, plasma waves, energetic particles, and auroral imaging data to determine the magnetotail plasma dynamics. Studies will be made to distinguish the source of plasma and the mechanisms and efficiency of the coupling of the solar-wind energy (measured by instruments on the Wind satellite) into the magnetosphere as a function of the upstream solar-wind conditions.

Energetic Particle and Ion Composition Experiment (EPIC): EPIC is supplied by Donald William, Applied Physics Laboratory, John's Hopkins University, Laurel, Maryland. The EPIC investigation uses an ion composition spectrometer and a telescope to measure the charge state, mass, and energy of ions. These measurements will be used to study the relative importance of ion sources and mechanisms for acceleration, transport and loss of particles, the formation and dynamics of magnetospheric boundary layers and their influence on magnetospheric behavior will be studied. Especially important will be the determination of particle sources in the large-scale structures such as bubbles of plasma called plasmoids.

Electric Fields Detector (EFD): This investigation is provided by Koichiro Tsuruda, ISAS. Geotail's measurement of the electric field in the tail is key to developing a theory about the formation of the magnetotail. Electric fields in the near-Earth magnetosphere are closely coupled with the ionospheric electric field. EFD will study this coupling, especially during substorms, using electric-field antennas sampling at 64 samples per second and an electron beam technique at 2 samples per spin. In addition, the merging of magnetic fields in the plasma sheet generates inductive electric fields that help to accelerate particles, which can be measured by other instruments on board the spacecraft.

High Energy Particles Experiment (HEP): HEP is supplied by Tadayoshi Doke, Waseda University, Tokyo. Measurements of high-energy particles can indicate plasma boundary surfaces and reflect whether magnetic field lines are open or closed. The composition and charge state of energetic particles provide rich information on where particles originate, and on solar events that produce different energetic-particle signatures. The origin and acceleration of galactic cosmic rays and their modulation in our galaxy also will be investigated.

Low Energy Particles Experiment (LEP): Toshifumi Mukai, ISAS, provides the LEP experiment. Low-energy electrons will be observed in the magnetotail and in the interplanetary medium to study the nature and dynamics of magnetail plasmas, analyze the plasma conditions under which particle acceleration takes place, and study plasma circulation and its variability in response to fluctuations in the solar wind and in the interplanetary magnetic field. Particles from Earth's ionosphere will be identified and the entry of plasmas into the magnetosphere from the magnetosheath will be studied to improve our understanding of open versus closed magnetospheres.

Magnetic Fields Experiment (MGF): Susumu Kokubun, University of Tokyo, Japan, is the Principal Investigator in this investigation, with Ronald Lepping, of NASA's Goddard Space Flight Center, supplying the flux gate magnetometer. Information about the dynamics of the transport of mass, momentum, and energy between the magnetospheric and ionospheric plasma can be inferred from monitoring changes in the magnetic-field configuration in various regions. MGF will investigate magnetic merging in the magnetotail, which is thought to produce a plasmoid, that flows down the tail during the active periods. Also, MGF will observe the distant tail to determine its magnetic-field structure -- whether well ordered or filamentary, for example -- and its dynamic changes associated with substorms.

Plasma Waves Investigation (PWI): Hiroshi Matsumoto, Kyoto University, Japan, provides this investigation which has a Multi-Channel Analyzer (MCA) component from Roger Anderson, University of Iowa. During Geotail's excursions from the near-Earth to the distant-tail regions, PWI will measure plasma waves to sample phenomena related to plasma dynamics in the different regions on various scales. These phenomena include magnetic-field-line-merging, moving plasmoids, and particle acceleration via wave-particle interaction within the magnetotail.

DATA COLLECTION AND DISTRIBUTION

The NASA Deep Space Network (DSN) will collect the data from the spacecraft via radio link and the Goddard Data Capture Facility (DCF) will assemble the data into computer files. The International Solar-Terrestrial Physics (ISTP) Central Data Handling Facility (CDHF) will store, catalogue, and organize the data. The CDHF will also create summaries, called key parameters, of the raw spacecraft data which will be used by investigators as a kind of index to the bulk data.

In addition, ISAS will acquire real-time science data from the Japanese instruments from their Usuda tracking station in Japan. This data will be processed at the Sahamihara operations center and distributed to the Japanese investigators.

All COSTR investigators are connected by the NASA Science Internet (NSI). Using the NSI, investigators will use Remote Data Analysis Facilities at their home institutions to access the data at the CDHF and use key parameters to determine which data are most useful to their particular investigation.