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Electron Beam Experiment

NSSDCA ID: 1992-064A-07

Mission Name: Freja
Principal Investigator:Dr. Goetz Paschmann


The electron beam experiment was intended to measure the local DC electric field vector. It is designed on the basis that if a beam of electrons are shot from a gun exactly perpendicular to the local magnetic field vector, it will arrive after one gyration at the exact location of the emitting gun, if the E-field is zero, however, in the presence of a finite electric field, the drift velocity (vD = (E X B)/B^2), and T x VD (T is one gyroperiod) will be the separation of the emitting point and the arrival point. Because of the varying attitude of the spacecraft, three pairs of orthogonal guns and detectors were used. Each gun consisted of an emitting cathode at -4.0 kV, focussing and deflecting electrodes, and a deflection-amplifying quadrapole magnet. The emerging 1 degree wide beam was swung through a 180 degree deflection in a plane at a rate of 10 kHz, so that during a fraction of this swing period the emission will be perpendicular to the local magnetic field. Flanking each gun was a pair of position-sensitive anode detectors for the returning beam. Each gun emisssion was coded so as to be sure that the arriving beam was from the same emitting gun. Each anode pair (in wedge geometry) was an array of 10 x 0.8 cm strip of discrete microanodes, so that the position of returning beam could be determined. Suitable low-voltage grids ahead of the anodes blocked entry of the thermal electons and ions. A typical gun current was 1 microamp, and the returning current at the anode was about 1 nA. In standard operation, 64 samples/s of beam-arrival positions, and auxiliary information, each sample consisting of 4096 bits were made, totaling a telemetry rate of 64 x 4096 = 262 kbps. The estimated sensitivity of the deflection of the arriving beam was 0.5 mm, corresponding to about 5 mV/m at an apogee of about 1700 km. The sensitivity degrades as the inverse of the square of the magnetic field at the lower altitudes. Especially troublesome was the unavoidable electric field of V x B origin, amounting to 200 mV/m which had to be extracted away.

Alternate Names

  • F6
  • Freja/ElectronBeamExperiment

Facts in Brief

Mass: 2.4 kg
Power (avg): 1.5 W
Bit rate (avg): 262 kbps

Funding Agency

  • Max-Planck-Institut (Germany)


  • Space Physics: Ionospheric Studies
  • 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
Dr. Gerhard HaerendelCo-InvestigatorMax-Planck-Institut fur Extraterrestrische
Dr. Wolfgang BaumjohannCo-InvestigatorMax-Planck-Institut fur Extraterrestrische
Mr. Per-Arne LindqvistCo-InvestigatorRoyal Institute of
Dr. Goran T. MarklundCo-InvestigatorRoyal Institute of
Dr. Roy B. TorbertOther InvestigatorUniversity of New
Prof. Craig A. KletzingOther InvestigatorUniversity of New
Dr. Goetz PaschmannPrincipal InvestigatorMax-Planck-Institut fur Extraterrestrische
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