GEOTAIL is a collaborative mission of Japan/USA [ISAS (Institute of Space and Astronautical Science)/NASA (National Aeronautics and Space Administration)] within the program ISTP (International Solar-Terrestrial Physics). The spacecraft was built and integrated by ISAS, the launch was provided by NASA/GSFC. Responsibilities are shared in
science instruments, telemetry data acquisition, and data processing and archiving. GEOTAIL inaugurates the Collaborative Solar-Terrestrial Research Program (COSTR).
COSTR defines the NASA contributions to the GEOTAIL, SOHO, and Cluster missions.
The GEOTAIL objectives are: study of the structure and dynamics of the geomagnetic tail.
In particular: 1) 2) 3) 4) 5)
· Determine the overall plasma, electric and magnetic field characteristics of the distant
and 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
· 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.
GEOTAIL measures the flow of energy and its transformation in the magnetotail created
by the interaction between the solar wind and the Earth. GEOTAIL also determines, in detail, the overall plasma characteristics of the distant geomagnetic tail - a comet-like tail of
plasma millions of km long created by the solar wind.
The spacecraft structure has the shape of a drum and is spin-stabilized (cylinder of 2.2 m
diameter and 1.6 m in height). The spin axis is oriented nearly perpendicular to the ecliptic
plane (85-89º), and the nominal spin rate is 20 rpm. S/C mass of 970 kg (330 kg propellant,
105 kg science payload). The dual-spinning satellite, equipped with two pairs of wire antennas of 50 m in length (100 m tip-to-tip) and two 6 m long masts, will contribute to deeper
understanding of fundamental magnetospheric processes. Power is provided by surface-
mounted solar cells. The design life of GEOTAIL is 4 years. The satellite employs Indium-
Ion emitters (a multi-emitter module containing four individual ion emitters of the liquid
metal indium field emission type) for S/C charge compensation. 6) 7)
Figure 1: The GEOTAIL spacecraft (image credit: ISAS/JAXA)
RF communications: The communication system, built under prime contract to NEC, comprises S and X-band systems for command, telemetry, and ranging (mechanically despun
antennas are utilized). Spacecraft communication is compatible with both the UDSC (Usuda Deep Space Center) and NASA's Deep Space Network (DSN). There are two on-board
recorders at 450 Mbit each which allow daily 24 hour data coverage. Real-time/playback
transmission rates at 16.384 kbit/s, 65.536 kbit/s, or at 131.072 kbit/s. Ground data reception
at Usuda and Kagoshima stations (Japan) and at the NASA's DSN (playback only). ISAS is
responsible for spacecraft operations.
A GEOTAIL launch took place on July 24, 1992 from Cape Canaveral with a Delta 2 vehicle. GEOTAIL is the first spacecraft launched within the ISTP (the others are: WIND,
POLAR, Cluster, SOHO, Equator-S, Interball).
Orbit: GEOTAIL objectives require spacecraft measurement in two orbits: a nightside
double lunar swingby GeoTail orbit to distances from 80 to 220 RE, and a low inclination
orbit at geocentric distances of about 8 to 30 RE. GEOTAIL uses 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. In this phase, GEOTAIL's orbit
extends from 220 RE (1,401,620 km) at its farthest point to 8 RE (50,960 km) at its nearest
· Distant tail orbit: 1.75 years in distant tail configuration (double lunar swingby to an 8 x
220 RE orbit). This orbit also allows the study the boundary region of the magnetosphere as it skims the magnetopause at perigees. In the first two years the double lunar
swing-by technique was used to keep apogees in the distant magnetotail.
· Near tail orbit: 1.45 years in near tail configuration (reduced to an 8 x 30 RE orbit, 7.5º
The apogee was lowered down to 50 RE in mid November 1994 and then to 30 RE in February 1995 in order to study substorm processes in the near-Earth tail region. In June 1997, the
perigee was slightly lowered to 9-9.5 RE in order to increase the probability that the spacecraft is just inside the dayside magnetopause. The near-tail orbit of 9 RE x 30 RE (with an
inclination of -7º to the ecliptic plane) has allowed extensive study of the magnetosheath,
the bow shock and the upstream region as well..
Operational status: GEOTAIL is operational as of 2005 (and is expected to be operational
until 2007/8). During its mission, the GEOTAIL satellite has identified numerous new phenomena in the magnetosphere. It has collected important data in particular on the issue of
magnetic reconnection, taking advantage of in situ observations. In space plasma physics,
GEOTAIL is providing data on the generation of non-thermal particles by shock waves and
the process of energy reduction by wave-particle interaction.
Figure 2: Illustration of the GEOTAIL spacecraft with booms and antennas
Science background: 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 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 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 regard to communications, power supplies, and spacecraft
GEOTAIL is the first mission making extensive observations in the distant geotail beyond
the lunar orbit and in the neutral sheet of the near-Earth tail region.
Sensor complement: (EFD, MGF, HEP, LEP, PWI, EPIC, CPI)
The GEOTAIL spacecraft carries seven instruments: 8) 9)
EFD (Electric Field Detector), PI: K. Tsuruda, ISAS. Objectives: study of the coupling of
the E-Field in the near-Earth magnetosphere and in the ionosphere (in particular during
substorms). EFD uses electric-field antenna sampling at 64 samples/s, and an electron
beam technique at 2 samples per spin. 10)
The instrument consists of two orthogonal double probes, each of which is a pair of separated spheres on wire booms that are located in the satellite spin plane and whose difference
of potential is measured. The separation distances between the pair of sensors are variable
and as great as 160 m tip-to-tip. One operating mode involves length ratios of the two
antennas of about 2:1 in order to verify instrument operation through showing that the electric field signature is proportional to the boom length. A second reason for two pairs of wire
booms in the satellite spin plane is the requirement for measurements having a time resolution far better than the satellite spin period.
MGF (Magnetic Field Measurement), PI: S. Kokubun, U. of Tokyo, R. Lepping, GSFC,
instrument sponsored by ISAS. Objectives: study of the transport dynamics of mass, momentum, and energy between the magnetospheric and ionospheric plasma (frequency
range < 50 Hz). Study of merging in the magnetotail. Instrument: MGF uses fluxgate and
search coil magnetometers for DC and AC measurements, respectively. Two sets of the fluxgate magnetometers were deployed at distances of 4 and 6 m along the 6 m mast. MGF also
contains the GEOTAIL Inboard Magnetometer provided by the US. The fluxgate magnetometers operate in seven dynamic ranges to cover various regions of the Earth's magnetosphere and the solar wind: ±16 nT, ±64 nT, ±256 nT, ±1024 nT, ±4096 nT, ±16384 nT,
and ±65536 nT, and supply 16 vectors/s. The search coil magnetometer system consists of
three sensors, preamplifier, amplifier, filter, multiplexer, and an A/D converter. The search
coil magnetometers operate in a frequency range of 0.5-1 kHz, and supply 128 vectors/s.
HEP (High Energy Particles Experiment), T. Doke, Waseda University, Tokyo, instrument
sponsored by ISAS. Objectives: measurement of high energy particles up to 25 MeV for
electrons, 35 MeV for protons, and 210 MeV/charge for ions. Measurements may indicate
the plasma boundary surfaces and reflect whether magnetic field lines are open or closed.
There are three scientific objectives to be studied by this investigation: (1) plasma dynamics
in the geomagnetic tail, (2) solar flare particle acceleration and propagation, and (3) the
origin, lifetime and propagation of cosmic ray particles. There are five instruments that
make up this investigation: Low-energy particle Detector (LD), Burst Detector (BD), Medium-energy Isotope detectors (MI-1 and MI-2), and High energy Isotope detector (HI).
LD and BD are mainly dedicated to magnetospheric studies. MI and HI concentrate on solar flare and cosmic ray studies.
· The HEP-LD sensor system consists of three identical Imaging Ion Mass spectrometers
which use time-of-flight/energy measurement, and covers 180 degrees in polar angle
over the energy range 20-300 keV for electrons, 2 keV-1.5 MeV for protons, and 2
keV-1.5 MeV per charge for ions. LD provides distribution of electrons and ions with
complete coverage of the unit sphere in phase space, and electron and proton flux in 4
azimuth sectors, helium and oxygen flux at an azimuth of 0º. 11)
· The HEP-BD sensor consists of three delta-E x E telescopes which identify particles by
their energy loss and residual energy over the energy range 0.12-2.5 MeV for electrons, 0.7-35 MeV for protons, and 0.7-140 MeV for helium. The three telescopes
each have an opening angle of 30º x 45º with look directions of 30, 90, and 150º to the
spin axis. BD provides count rates for high energy electrons, protons and helium, as well
as electron and proton fluxes in four 90º azimuth bins.
· The HEP-MI and HEP-HI instruments are all silicon semiconductor detector telescopes utilizing the well-known dE/dx x E algorithm for isotope identification: mass and
nuclear charge. The MI instrument measures elemental and isotopic compositions of
solar energetic particles and energetic particles in the heliosphere with 2<Z<28 in the
2.4-80 MeV/nucleon energy range, and measures the elemental composition of solar
energetic particles heavier than iron. The HI instrument also measures elemental and
isotopic compositions of solar energetic particles and galactic cosmic rays with
2<Z<28 in the 10-210 MeV/nucleon energy range.
LEP (Low Energy Particles Experiment), PI: T. Mukai, ISAS. Objectives: study of the dynamics of the magnetotail plasmas, plasma circulation and its variability in response to fluctuations in the solar wind and in the interplanetary magnetic field. Measurement of electrons from 6 eV to 36 keV, and ions from 7 eV to 42 keV/ charge. The LEP consists of three
sensors: LEP-EA, LEP-SW, and LEP-MS, with common electronics (LEP-E).
· LEP-EA measures the 3-D velocity distributions of hot plasma in the magnetosphere.
EA consists of two nested sets of quadrispherical electrostatic analyzers. The inner analyzer measures electrons in the energy range from 6-36 eV, and the outer one measures
positive ions from 7 eV/Q to 42 keV/Q. The field of view for each quadrispherical analyzer covers 10º x 145º, where the longer dimension is parallel to the satellite spin axis.
· LEP-SW measures the 3-D velocity distributions of solar wind ions in the energy range
from 0.1-8 keV/Q with a 270º spherical electrostatic analyzer with a field of view of 5º x
· LEP-MS is an energetic ion mass spectrometer, which provides the 3-D determinations
of the ion composition in 32 steps over the energy range of 0-25 keV/Q. All sensors operate continuously as long as the spacecraft power budget can allow, except for the orbit/attitude maneuvering operation.
When spacecraft power budget is not sufficient to fully operate the instruments, priority is
given to LEP-EA and LEP-E.
PWI (Plasma Waves Investigation), PI: H. Matsumoto, Kyoto University, instrument sponsored by ISAS. Objectives: study of the wave phenomena related to plasma dynamics in the
different regions on various scales (phenomena include magnetic-field-line merging, moving plasmoids, and particle acceleration). Measurement of plasma waves in the frequency
range of 5 Hz - 800 kHz. PWI contains also the Multi-Channel Analyzer provided by the US.
The instrument measures electric fields over the range 0.5 Hz to 400 kHz, and magnetic
fields over the range 1 Hz to 10 kHz. Triaxial magnetic search coils are utilized in addition to
a pair of electric dipole antennas. The instrument contains two sweep-frequency receivers
(12 Hz to 400 kHz and 12 Hz to 6.25 kHz), a multichannel analyzer (5.6 Hz to 311 kHz for
the electric antenna and 5.6 Hz to 1.0 kHz for the magnetic coils), a low frequency waveform
receiver (0.01 to 10 Hz), and a wideband waveform receiver (10 Hz to 16 kHz). 12)
EPIC (Energetic Particle and Ion Composition Experiment), PI: R. McEntire, APL, Johns
Hopkins University, instrument sponsored by NASA. Objectives: measurement of the
charge, mass, and energy of ions. Study of the relative importance of ion sources and mechanisms for acceleration, transport and loss of particles, the formation and dynamics of magnetospheric boundary layers. - The EPIC instrument is actually composed of two separate
sensor and processing assemblies: 13)
· STICS (Supra-Thermal Ion Composition Spectrometer). Objective: Measurement of
ions. STICS uses a quadrispherical electrostatic analyzer followed by a foil/solid state
detector time-of-flight (TOF) telescope to measure charge state, mass and energy of
ions with energies of 30 - 230 keV/charge. It uses an electrostatic analyzer with a geometry factor of 0.05 cm2 sr, time of flight and energy analysis.
· ICS (Ion Composition Subsystem). The objective is to measure mass and energy properties of energetic ions with energies of less than 50 keV to 3 MeV. ICS uses a pair of
collimators with sweeping magnets to reject electrons, followed by TOF and energy
analysis, with a geometry factor of 0.2 cm2 sr. A thin foil/solid state detector electron
telescope measures electrons higher than 30 keV.
Figure 3: Illustration of the EPIC instrument (image credit: JHU/APL)
CPI (Comprehensive Plasma Investigation), PI: L. A. Frank, University of Iowa, instrument sponsored by NASA. Objectives: measurement of the 3-D plasma in the Earth's magnetotail. The plasma data will be correlated with the magnetic field, plasma waves, electric
particles, and auroral imaging data to determine magnetotail plasma dynamics.
Instrument: measurement range of 1 eV - 50 keV for the Hot Plasma and Ion Composition
Analyzer, and 150 eV - 7 keV energy/unit charge for the Solar Wind Analyzer. Plasma parameters, including heat flux and field-aligned current density, are measured. 14)
The instrument contains three sets of quadrispherical analyzers with channel electron multipliers. These three obtain 3-D measurements for hot plasma and solar wind electrons, for
solar wind ions, and for positive-ion composition measurements. The positive-ion composition measurement includes five miniature imaging mass spectrometers at the exit aperture
of the analyzer, and covers masses from 1 to 550 u/Q at 100 eV, and 1 to 55 u/Q at 10 keV. The
hot plasma analyzer measures electrons and ions in the range 1-50,000 eV/Q. The solar
wind analyzer measures ions from 150 to 7,000 eV/Q. Sequencing of the energy analyzers
and mass spectrometers, and other control functions, are provided by two microprocessors.
Figure 4: Illustration of CPI (image credit: University of Iowa)
1) "The GEOTAIL Mission," in NASAFacts, GSFC, June, 1992
2) "Delta Launches GEOTAIL," Space News, July 27-Aug. 9, 1992, p. 12
5) Note: ISAS (Institute of Space and Astronautical Science) became part of JAXA (Japan Aerospace Exploration
Agency) in Oct. 2003
6) "New Japanese Spacecraft to Study Solar Flares, Effects of Sun on Earth", Aviation Week and Space Technology,
Aug. 27, 1990, pp. 76-81
7) M. Fehringer, F. Rüdenauer , W. Steiger, "Space-Proven Indium Liquid Metal Field Ion Emitters for Ion
Microthruster Applications," 33 rd AIAA Joint Propulsion Conference, July 6-9, 1997, Seattle, WA, USA
8) "GEOTAIL Instruments and Initial Results," Foreword by A. Nishida, Journal of Geomagnetism and
Geoelectricity, ISSN 0022-1392, Vol. 46, 1994,
10) K. Tsuruda, H. Hayakawa, M. Nakamura, T. Okada, A. Matsuoka, F. S. Mozer , R. Schmidt , "Electric Field
Measurements on the GEOTAIL Satellite ," Journal of Geomagnetism and Geoelectricity, Vol. 46, 1994, p. 693
11) MPAe contribution: The Geotail HEP-LD intrument; URL:
14) L. A. Frank, K. L. Ackerson, W. R. Paterson, J. A. Lee, M. R. English, G. L. Pickett, "The Comprehensive Plasma
Instrumentation (CPI) for the Geotail spacecraft," Journal of Geomagnetism and Geoelectricity, Vol. 46, No 23,
This description was provided by Herbert J. Kramer from his documentation of:
"Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.