The primary purpose of the GEOTAIL satellite is to study the structure and dynamics of the tail region of the magnetosphere.For this purpose the orbit is designed to cover the magnetotail over a wide range of distances: 8 Re to 210 Re from the earth.


The GEOTAIL satellite was launched on July 24, 1992. The primary purpose of this mission is to study the structure and dynamics of the tail region of the magnetosphere. For this purpose the orbit is designed to cover the magnetotail over a wide range of distances: 8 Re to 210 Re from the earth. This orbit also allows us to study the boundary region of the magnetosphere as it skims the magnetopause at perigees. The orbit in the first year after the launch is illustrated here. The technique of the double lunar swingby is used to keep apogees in the distant magnetotail.

The configuration of GEOTAIL is illustrated above. There are a pair of 100 m tip-to-tip antenna and two 6 m-long masts. The spin axis makes an angle of 87 degrees to the ecliptic plane, and the nominal spin rate is 20 rpm. Data are received at a rate of 64 kbps in the real time mode, and at the same time they are recorded on board at a rate of 16 kbps. GEOTAIL carries seven sets of instruments, which are described in detail here. They measure the magnetic field, electric field, plasma, energetic particles, and plasma waves. Special attention is paid to make the spacecraft electromagnetically clean.

Name (pre-launch in parentheses) GEOTAIL
International Designation Code 1992-044A
Objectives Research on structure and dynamics of the earth's magnetotail
Participation in ISTP (International Solar-Terrestrial Physics)
Lunch Date July 24, 1992
Launch Location Cape Canaveral (Florida, U.S.A.)州)
Launch Vehicle Delta 2
Weight 1,009 kg (at launch)
Shape Cylinder of 2.2m diameter and 1.6m height
Equipped with two 6m-long extendable masts for magnetic sensor and four 50m-long antennas
Orbit Altitude Perigee 57,000 km, Apogee 200,000 km
Orbit Inclination 29°
Type of Orbit Double lunar swing-by
Scientific Instruments 1. Magnetic field monitor
2. Electric field monitor
3. Two sets of plasma monitors
4. Two sets of high-energy particle monitors
5. Plasma wave instrument
Operation For the first two years, the satellite was planned to orbit and perform double lunar swing-bys to observe the magnetotail's far region (80 to 220 times the radius of the earth) while keeping apogee in the earth's night side. In mid-November, 1994, the apogee was lowered to a point 50 times the earth's radius and, on February, 1995, even further to 30 times the earth's radius to study the process of magnetotail sub-storms near the earth. The perigee was set to about 10 times the earth's radius and orbital inclination to the ecliptic plane was set to be -7° so that the apogee of the satellite would be positioned in the magnetotail's neutral plane around the winter solstice.

The above orbit was a great success and the region of 10 to 220 times the earth's radius of the magnetotail was closely investigated. In addition, due to the orbit arrangement, GEOTAIL was able to pass close by the boundary plane of the magnetosphere in the day side when the satellite's perigee was also in the day side. In June 1997, the perigee was slightly lowered to 9 to 9.5 times the earth's radius in order to increase the probability that GEOTAIL would be positioned just inside the magnetosphere's boundary plane in the day side. The orbit of 9 times (perigee) and 30 times (apogee) the earth's radius allowed us to study the magnetosheath, bay-shape shock wave and its upstream area in detail.
Results We found several important clues in the plasma sheet, the source of aurora electrons, to explain why the aurorae suddenly brighten on a global scale. Cooperating with other nation’s satellites, we also discovered many new facts concerning the location and timing of magnetic reconnection, an explosive energy-release phenomenon.