Plasma Density experiment

The main objective of the French Electron density experiment on ISEE-1 was to provide the ambient electron density by two complementary radio techniques: propagation and resonance.
The plasma density, which in most of the regions studied by the spacecraft equals the plasma density, is one of the most important key plasma parameter, essential in large scale MHD (Magneto Hydro Dynamic) studies as well as for understanding microscopic (kinetic) processes.

Propagation experiment:
The propagation experiment provides the total electron content between the 2 spacecraft by measuring the phase shift hhe Faraday rotation of a plane wave emitted at 2 frequencies (683kHz and 272.5 Mhz) from Isee-1 and received on Isee-2. It was built by the DESPA (now LESIA) of Observatoire de Paris at Meudon France, under the responsibility of C.C Harvey, who was also the Principal Investigator of the whole Density Experiment.
Sounder experiment:
The resonance sounder on Isee-1 measured the electron density in the vicinity of this spacecraft by emitting short radio pulses in the frequency range of the expected electron plasma frequency, from which the electron density is easily derived. Used in passive mode (emitter off), it provided a survey of natural waves emissions. It was built by the CRPE at CNET-Issy Les Moulineaux France (the team is now part of LPP in Palaiseau (France) under the responsibility of J. Etcheto. Both instruments were built, operated, and the data processed, thanks to the financial and computer support of the French Space Agency (CNES).
The resonance sounder relies on the observations that when the plasma surrounding a spacecraft is excited by an emitter, a strong resonance (echo like) is observed at some characteristic frequencies of the plasma, Fpe, the electron plasma frequency, Fuh the upper hybrid frequency, nFce, the harmonics of the electron gyrofrequency, the Fq, maximum above Fuh(zero group velocity) of the so called Bernstein modes. These resonances can be differentiated from natural plasma emissions by their greater strength, narrow frequency bandwidth and fast decay with time (a few tens of ms). The first resonance sounder in a magnetosphere was flown on Geos-1 at the geostationary orbit, a few months before Isee-1 launch, but the sounder on Isee-1 was the first to observe resonances in key regions like, solar wind, magnetosheath and magnetospheric tail.

References :

   Ogilvie, K. W., T. von Rosenvinge and A. C. Durney, IEEE Transactions on Geoscience Electronics ( Volume: 16, Issue: 3, pp. 231-238, July 1978 ) DOI: 10.1109/TGE.1978.294553
   C. C. Harvey, J. Etcheto, A. Mangeney, Space Science Reviews, Mars 1979, Volume 23, Issue 1, pp 39-58 DOI: 10.1007/BF00174110