Electron density measurement in space plasma, comparison between two techniques: Sounder by relaxation and mutual impedance probe

In space plasma, the determination of the electron density is an important parameter to understand the wave propagation and energy transfers between wave and particles. Usually, the plasma density and debye length are much different in solar wind and planetary environment than laboratory plasma and it is needed to sound large volume. Several ways exist to estimate the electron density that can separate into two main categories: the passive and active techniques. In this presentation, we propose to discuss the active techniques and we describe on sounder by relaxation instrument and mutual impedance measurement. The both types consist to emit in the plasma a monochromatic frequency. Then, plasma response is recorded for several frequencies, scanning a frequency range including the characteristic plasma frequencies like plasma frequency Fpe, electron gyro-frequency Fce, gyro-harmonics Fcen and berstein's modes Fqn which are perpendicular electrostatic resonances. However, the way to record and the energy used are completely different following the technique.