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DC Electric Field and Gradient B Electron Beam Deflection

NSSDCA ID: 1977-029A-08

Mission Name: ESA-GEOS 1
Principal Investigators:Dr. F. Melzner
Principal Investigators:Dr. Gerhard Haerendel


The prime objective of this experiment (ESA experiment no. S-329) was the measurement of the dc electric field in the plane perpendicular to the local magnetic field (B). The experiment also measured the spatial gradient of B in the vicinity of the spacecraft. With this data, a mapping of the electric fields in the equatorial magnetosphere linked magnetically to the auroral zones could be achieved, as well as determining plasma convection and particle flow within the plasma sheet.

The instrument consisted of four electron guns spaced logarithmically from the electron detector. Two of the guns were mounted on one of the 3-m radial booms. The guns were used one at a time to generate an electron beam of about 1.E-8 amps and energy about 1 keV. Both parameters were varied by telecommand. Deflection plates associated with each gun received a sinusoidal signal from the magnetometer experiment to insure that the beam was always at right angles to B, in spite of the angle of the spin vector to B. The electron detector consisted of deflection plates that removed the elevation correction given to the beam by the magnetometer signal, a curved plate energy filter, and a photomultiplier tube. Since the maximum displacement would occur when the beam made an angle of 0 or 180 degrees to the electric field, all possible displacements less than this would occur twice during a spin period. Consequently, the beam would sweep across the detector twice per spin period provided the maximum displacement was less than the distance between the gun and the detector. The values of the spin angle at which the beam was detected after one gyration, and the distance between the gun and receiver, allowed the determination of the electric field. A possible contribution from the gradient of B could be determined by varying the energy of the beam.

The experiment relied entirely on real-time control by a ground-based computer. It had four basic modes of operation: a search mode, an adjustment mode, an optimization mode, and a normal mode. The search mode was designed to find the signal at nominal beam parameters. If this was not achieved, the adjustment mode was used to vary these parameters systematically. Once the beam was detected, the optimization mode determined the best compromise between beam current and received signal quality. Then the normal mode started, which consisted of a continuous measurement of the electric field and the gradient of B, using the most appropriate of the four guns. Unfortunately, the three innermost guns ceased operation early in the life of the experiment, so only the more intense electric fields could be studied.

Alternate Names

  • DC Electric and Magnetic Fields, and Grad B By Electron Beam Deflection
  • ESA-GEOS1/DCElectricFieldandGradientBElectronBeamDeflection

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. Heinrich J. VolkOther InvestigatorMax-Planck-Institut fur
Dr. Gerd MetznerOther InvestigatorMax-Planck-Institut fur Extraterrestrische
Dr. F. MelznerPrincipal InvestigatorMax-Planck-Institut fur Extraterrestrische Physik
Dr. Gerhard HaerendelPrincipal InvestigatorMax-Planck-Institut fur Extraterrestrische
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