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Low-Energy Ion Composition

NSSDCA ID: 1977-029A-03

Mission Name: ESA-GEOS 1
Principal Investigators:Dr. Helmut R. Rosenbauer
Principal Investigators:Prof. Johannes Geiss


This instrument (ESA experiment S-303) measured the energy, angular distribution, and composition of positive ions using a cylindrical electrostatic analyzer followed by a crossed electric and magnetic field analyzer to select the energy and velocity. The energy (per unit charge) ranged from 0.001 to 17.2 keV in 32 steps with a delta E/E of 0.03 and a mass range of 1 to 140 u in 64 logarithmically spaced steps. There was a thermal mode in which a retarding grid in the entrance slit was used for analysis below 0.1 keV. All particles that overcame this grid voltage were accelerated to 3 keV before entering the electrostatic analyzer in its lowest energy step, where both analyzers were transparent. The device viewed perpendicular to the spin (Z) axis. For low-energy ions the acceptance angles were plus or minus 6 deg in azimuth and plus or minus 30 deg in elevation (referenced to the Z axis). For the highest energies, these angles decreased to 3.5 and 7.1 deg, respectively. Three percent of the ions leaving the electrostatic analyzer were counted by a channeltron. The remaining 97% entered the crossed electric and magnetic field analyzer and the output was detected by an electron multiplier. This signal was pulse-height analyzed by one fixed and one variable discriminator to obtain better mass discrimination. The main purpose of this investigation was to identify the sources of low energy particles in the magnetosphere. Time variations of the helium/hydrogen ratio, the degree of ionization of helium and oxygen, and the isotopic abundance ratio of helium 3/helium 4 could be measured to determine these sources. Early in the life of the satellite, a correlative experiment with the cesium ion neutralization gun on ATS 6 was performed when the two satellites were within several km and on the same magnetic field line. The ATS 6 gun was fired for some period commencing about 1 h prior to the ESA-GEOS 1 satellite crossing the magnetic field line so that cesium ions would have time to populate the flux tube and, subsequently, be detected by this experiment. This was the first of this type of controlled active experiment between two satellites.

Alternate Names

  • ESA-GEOS1/Low-EnergyIonComposition
  • S303

Funding Agency

  • European Space Agency (International)


  • Space Physics: Magnetospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Prof. Peter X. EberhardtOther InvestigatorUniversitat
Dr. David T. YoungOther InvestigatorLos Alamos National
Prof. Hans R. BalsigerOther InvestigatorUniversitat
Dr. Arthur G. GhielmettiOther InvestigatorUniversitat
Dr. Hans LoidlOther InvestigatorMax-Planck-Institut fur Extraterrestrische Physik
Dr. Helmut R. RosenbauerPrincipal InvestigatorMax-Planck-Institut fur
Prof. Johannes GeissPrincipal InvestigatorUniversitat
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