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Electron Spectrometer and Correlator

NSSDCA ID: 1992-064A-08

Mission Name: Freja
Principal Investigator:Dr. Manfred H. Boehm


The electron spectrometer and correlator instrument was designed to investigate the fine structure of field aligned precipitaion over auroral arcs. The spectrometer consisted of a ``top hat'' spherical electrostatic analyzer, feeding 32 channeltrons. The field of view was biconical, 30 x 360 degrees, and the geometric factor 0.025 sq cm sr. The sweep was made in 32 energy steps from about 10eV to 25 KeV. At any energy step, each channeltron monitored an angular sector of 360/32 = 11 degrees; the pitch angle resolution was often better than 11 degrees, depending upon the orientation of the field of view with respect to the local magnetic field. Each energy step took 1 ms, but was interwoven with other modes (such as fixed energy modes) of another total of 32 ms. This 64 ms overall cycle corresponded to a spatial resolution of about 0.5 Km. Besides providing the high resolution pitch angle distribution, the output from each channeltron was also used in two correlation modes to edify high frequency phenomena which are temporal rather than spatial: wave-particle, and particle-particle correlations. In the wave-particle mode, a 16 MHz sampling grid was imposed on the analog output from the electric field antenna (see Wave Experiment), and the preamp putput of each channeltron. The electric field information was just a one-bit item (i.e, the sign of the electric field: 1 for up-directed instants and 0 for down-directed). Likewise, the sampling grid on the electron channel also provided a one-bit information (i.e, whether there was or not an electron event at each instant of 1/16 microsecond. The correlations were computed during a period of about 2 ms. The energy channel that provided the maximum correlation then provided the electron velocity, which was therefore the phase velocity of the dominant HF wave. (Candidates were the Langmuir and cyclotron waves.) This wave-particle correlation arises from Landau growth/absorption of wave energy by electrons that move the phase velocity of the wave. The particle-particle correlator essentially addressed the time delay between successive arrivals of individual electrons, so as to compute the modulation frequency of the electron stream imposed by the ambient electric waves. An 8-MHz sampling grid imposed on each energy channel provide a sequence of one-bits (1 if there was an electron event; 0 for no event); 256 kilosamples, from 8 energy channels, could be stored and Fast-Fourier-Transformed (1024 transforms in 650 microseconds) for telemetry. The FFT provided the frequency and band width the wave that had modulated the electron stream. Either type of correlator output was telemetered only when the data had significant outputs, well above randomness. The instrument is inoperative since 19 September 1994.

Alternate Names

  • F7
  • Freja/ElectronSpectrometerandCorrelator

Facts in Brief

Mass: 7.4 kg
Power (avg): 13.6 W
Bit rate (avg): 44 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. Charles W. CarlsonCo-InvestigatorUniversity of California,
Dr. Gerhard HaerendelCo-InvestigatorMax-Planck-Institut fur Extraterrestrische
Dr. Goetz PaschmannCo-InvestigatorMax-Planck-Institut fur Extraterrestrische
Dr. Paul M. KintnerCo-InvestigatorCornell
Dr. Gerhard KremserCo-InvestigatorMax-Planck-Institut fur Aeronomie
Mr. Bengt HolbackCo-InvestigatorSwedish Institute for Space
Prof. Rickard LundinCo-InvestigatorSwedish Institute for Space
Dr. Michael Paul GoughCo-InvestigatorUniversity of
Dr. Lars EliassonCo-InvestigatorSwedish Institute for Space
Dr. Hans LaucheCo-InvestigatorMax-Planck-Institut fur
Dr. Robert E. ErgunCo-InvestigatorUniversity of California,
Dr. Manfred H. BoehmPrincipal InvestigatorMax-Planck-Institut fur Extraterrestrische
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