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Mission to the magnetotail

The GEOTAIL mission is a collaboration between the Japanese and American Space Adminstrations to study the dynamics of the Earth's magnetotail from the near-Earth region (8 Earth radii (Re)) to the distant geomagnetic tail (about 200 Re). The GEOTAIL spacecraft was designed and built by the Japanese and was launched on July 24, 1992. The Institute has contributed the HEP-LD designed to study plasma dynamics in the geomagnetic tail, solar flare particle acceleration and propagation, and the origin, lifetime and propagation of cosmic ray particles.

> Science objectives of the Geotail mission
> MPS contribution: The Geotail HEP-LD intrument
Co-I: Berend Wilken (deceased), contact Q.-G. Zong
> Science objectives of the Geotail HEP-LD instrument
> Related links
> Geotail publications by MPS members

Science objectives of the Geotail mission

The Geotail mission measures global energy flow and transformation in the magnetotail to increase understanding of fundamental magnetospheric processes. This includes the physics of the magnetopause, the plasma sheet, and reconnection and neutral line formation (i.e., the mechanisms of input, transport, storage, release and conversion of energy in the magnetotail). Geotail, together with Wind, Polar, SOHO, and Cluster projects, constitute a cooperative scientific satellite project designated the International Solar-Terrestrial Physics (ISTP) program which aims at gaining improved understanding of the physics of solar terrestrial relations.

The Geotail mission is divided into two phases. During the initial two-year phase, the orbit apogee was kept on the night side of the Earth by using the Moon's gravity in a series of double-lunar-swing-by maneuvers that resulted in the spacecraft spending most of its time in the distant magnetotail (maximum apogee about 200 Re) with a period varying from one to four months. In February 1995, phase two was commenced as the apogee was reduced to 30 Re to study the near-Earth magnetotail processes.


MPS contribution: The Geotail HEP-LD intrument

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 10--100 keV for ENA (Neutral Particles), 30 keV--1.5 MeV for Protons, 80keV--1.5 MeV for Helium ions, and 160 keV -- 4.0 MeV for CNO (Oxygen) Ions.

The instrument is not sensitive to the ionic charge state of nuclear particles; the mass resolution is marginal for separating carbon, nitrogen and oxygen ions. HEP-LD provides distribution of different ions with complete coverage of the unit sphere in phase space.

The Technical Parameters of the HEP-LD

Flight Path S
Deflection Voltage
34 mm
0-10 KV
Energy Range (KeV)
Angular Coverage:
ENA (KeV) 10-100
Geomatric Factor


Science objectives of the Geotail HEP-LD

By using the HEP-LD data sets, the following scientific objectives are investigating:

  • Particles dynamics in the geomagnetic tail
  • CIR, CME and other Solar events related geomagnetic activity
  • Oxygen ion as a "Tracer Ions" in the substorm dynamical process
  • The acceleration process of energetic ion in upstream and downstream of the bow shock

The classic picture of substorm

The classic picture of substorm and plasmoids formation assumes the existence of an X line (NENL) tailward of the dipolarization region and earthward of the thinning region where the Bz component is essentially nullified. Eventually, part of the plasma sheet is pinched off as a plasmoid, the residual plasma sheet snaps back towards Earth injecting energetic plasma into the inner magnetosphere.

An Energetic Oxygen Beam was observed by HEP-LD

HEP-LD instrument is an advanced energetic particle spectrometer with time-of-flight(T) and energy(E) detection ystems which determine the mass of incident nuclear particles. The novel design of the sensor head as a 'projection' camera allows the linear imaging of particle distributions in the polar angle: 12 contiguous angular intervals over a range of 180 degrees. Combined with the sectored spin plane of the unit sphere in the phase space is covered with a total of 192 contigous angular pixels. The oxygen ions were flowing tailward in a highly collimated beam.


Related links

> Geotail publications by MPS members
> ISTP Home Page
> Geotail Homepage
> Geotail EPIC Summary Plots
> GeoTail Comprehensive Plasma Instrument Observations

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