Overview

Geotail was launched by a Delta II rocket on July 24, 1992, from Cape Canaveral, Florida, USA, to study the structure and dynamics of the Earth’s magnetotail. The project was a joint program between the Institute of Space and Astronautical Science (ISAS) of Japan and the National Aeronautics and Space Administration (NASA) of the United States, with ISAS providing about two-thirds of the satellite development and scientific instruments, and NASA providing the launch and the remaining scientific instruments.

The satellite is cylindrical in shape with a mass of approximately 1000 kg, a diameter of 2.2 m, and a height of 1.6 m. It has two 6 m long extension masts for magnetometer sensors and four 50 m long antennas.

Orbit

The first two years were planned to follow a double lunar swing-by orbit, keeping the apogee on the night side of the Earth to observe the distant region of the magnetotail (80 to 220 times the Earth radius). The apogee was lowered to 50 times the Earth radius in mid-November 1994, and then to 30 times the Earth radius in February 1995 to study substorm processes in the near-Earth tail. The perigee was set at about 10 times the Earth radius, and the orbital inclination with respect to the ecliptic plane was set at minus 7 degrees to ensure that the spacecraft would be in the neutral plane of the magnetotail at apogee around the time of the winter solstice.

The above orbit plan was very successful, and the area of the magnetotail from 10 to 220 times the Earth’s radius was studied in detail. This orbit also allowed the spacecraft to graze the diurnal magnetospheric interface when the perigee was on the diurnal side, and in June 1997, the perigee was lowered slightly to 9 to 9.5 times the earth radius to increase the probability of the spacecraft coming just inside the diurnal magnetospheric interface. The orbits of 9 times (perigee) and 30 times (apogee) the Earth’s radius allow both the magnetosheath, the gulf-shaped shock wave, and its upstream regions to be studied in detail.

Observation Instruments

Geotail is equipped with the following seven instruments. Plasma and particle measurements are very important in magnetospheric studies, and Geotail is equipped with four plasma and particle instruments, one of which can measure up to the energy range of galactic cosmic rays. These instruments provide detailed analysis of plasma density, temperature, velocity, and composition.

Magnetic Field Experiment (MGF)

The MGF is an instrument for measuring the Earth’s magnetic field with high accuracy. It uses a fluxgate magnetometer and a search coil magnetometer to measure the magnetic field below 50 Hz.
The fluxgate magnetometers are mounted at 4 m and 6 m from the satellite’s extension mast to reduce noise generated by the satellite.
Time derivative values of the magnetic field in the frequency range of 1 to 50 Hz (128 vector samples/second) are acquired by a three-component search coil magnetometer (mounted on a separate mast) 4 m away from the spacecraft.
This allows detailed analysis of the magnetic field variations in the magnetosphere.

Electric Field Detector (EFD)

The EFD measures electric fields in two different ways: the probe method and the electron beam method. The probe method (EFD-P) measures the electric field in the plane perpendicular to the satellite spin axis by measuring the voltage difference between two spherical probes placed 50 m from the satellite. The electron beam method (EFD-B) determines the electric field by measuring the drift motion of the center of rotation of artificially emitted electrons.
The EFD also has the ability to measure the satellite’s potential relative to the surrounding plasma and to control the satellite’s potential by emitting ions.
The simultaneous use of probe and beam technology greatly improves the reliability of the electric field measurement, and the satellite potential control allows measurement of low-energy ions that would otherwise be repelled by the satellite’s positive potential.

Low Energy Particle Experiment (LEP)

The LEP is designed for comprehensive observations of plasma and energetic electrons and ions in the Earth’s magnetosphere (mainly in the magnetotail) and interplanetary space with fine temporal resolution and consists of three units of sensors (LEP-EA, LEP-SW, LEP-MS) and common electronics (LEP-E) The LEP-EA, LEP-SW, and LEP-MS are designed for observation.
Energy-per-charge Analyzers (EA) measure the three-dimensional velocity distributions of electrons (EA-e) and ions (EA-i) simultaneously and separately in the energy-per-charge range from a few eV/q to 43 keV/q. The design of the EA allows for the measurement of small plasmas in the magnetotail with sufficient counting statistics for high-resolution measurements. The EA design focuses on large geometric factors to measure the small plasma in the magnetospheric tail with sufficient counting statistics for high-resolution measurements.
The Solar Wind ion analyzer (SW), on the other hand, has a small geometric factor but fine angular and energy resolution to measure the energy per charge spectrum of solar wind ions.
With both the EA and SW sensors, the full 3D velocity profile is only available at 4 spin periods, but velocity moments up to third order are calculated on the machine every spin period (nominally 3 seconds).
The Energetic-ion Mass Spectrometer (MS) is capable of determining the ion composition in three dimensions.
This allows us to understand the distribution and dynamics of the plasma in the magnetosphere.

Comprehensive Plasma Instrument (CPI)

CPI measures the three-dimensional velocity distribution and mass/energy spectrum of ions and electrons and consists of three plasma analyzers: a high-temperature plasma analyzer, a solar wind ion analyzer, and an ion mass/energy analyzer.
The energy per unit charge (E/Q) ranges of these analyzers are 1.3 V - 48.2 kV, 145 V - 6830 V, and 1.3 V - 48.2 kV, respectively, providing detailed data on solar wind ions and plasma in the magnetosphere.

About CPI, more detailed information, the QL plot, and digital data are archived at the Space Plasma Physics Research Group, University of Iowa.

Energetic Particles and Ion Composition Instrument (EPIC)

EPIC is designed to measure particle population properties important for understanding the composition and dynamics of the Earth’s geomagnetic tail and consists of five sub-assemblies: the Supra-Thermal Ion Composition Spectrometer (STICS) sensor, STICS analog electronics The system consists of five sub-assemblies: a Supra-Thermal Ion Composition Spectrometer (STICS) sensor, an Ion Composition System (ICS) sensor, ICS analog electronics, and a Data Processing Unit (DPU).
The STICS sensor covers an angle of about 4 π, determines the charge state of all ions from 30 keV to 230 keV/e, and measures the mass per charge above 7.5 keV/e for composition and spectral observations.
The ICS sensor provides fluxes, compositions, and spectra of elemental species from protons to iron from ?50 keV to 3 MeV, angular distributions over two polar angles, and electron fluxes above 32 keV and 110 keV in one plane.
The DPU provides the capability for a large number of operating modes, selected from a small number, to optimize data collection throughout the many phases of the Geotail mission.

About EPIC, more detailed information, the QL plot, and digital data are archived at the Johns Hopkins University Applied Physics Laboratory.

High Energy Particle Experiment (HEP)

HEP measures high-energy ions and electrons and consists of five spectrometers (LD, BD, MI-1, MI-2, and HI).
LD (Low energy particle Detector) and BD (Burst Detector) are mainly used to measure electron, proton, helium, and oxygen ions that reflect plasma dynamics in the magnetotail region.
MI-1, MI-2 (Medium energy Isotope telescope-1, -2) and HI (High energy Isotope telescope) are used to measure isotopic abundances of solar flare particles and cosmic ray particles, reflecting the physical conditions of interplanetary space and the reflect the physical conditions of interplanetary space and the origin of these particles.

Plasma Wave Instrument (PWI)

PWI is an instrument for characterizing wave phenomena generated by various plasma processes in the Earth’s magnetosphere, and consists of three receivers: Sweep Frequency Analyzer (SFA), Multi-Channel Analyzer (MCA), and Wave-Form Capture (WFC). The SFA and MCA are used to characterize waveforms.
The SFA and MCA are dedicated to wave spectrum measurements, while the WFC is used to capture the actual waveforms of the two electric and three magnetic field components of the measured plasma wave radiation.
The plasma wave is measured in the frequency range from 5.62 Hz to 800 kHz for the electrical component and from 5.62 Hz to 12.5 kHz for the magnetic component.

About PWI, more detailed information and the QL plot are archived at the Research Institute for Sustainable Humanosphere, Kyoto University, and the radio and plasma wave research group, University of Iowa.

Achievements

We have discovered some major clues in the plasma sheet, the source of the auroral electrons, as to what causes auroras to suddenly start shining brightly on a global scale. Working with satellites in many countries, we are discovering many new facts about where and when magnetic reconnection, an explosive energy release phenomenon, occurs.