Numerical modelling of mutual impedance probes and quasi thermal noise spectroscopy in a magnetized plasma

Mutual impedance probes and quasi thermal noise spectroscopy are two in situ plasma diagnostic techniques for the measurement of the plasma density and electron temperature in space. Both techniques rely on electric antennas to measure the electric fluctuations coupled to plasma dynamics. Mutual impedance experiments are active techniques. They use emitting antennas to perturb the plasma and receiving antennas to measure the plasma response. Quasi-thermal noise spectroscopy is a passive technique. It uses only receiving antennas to measure the intrinsic electric potential fluctuations of the unperturbed ambient plasma.

Both techniques are included in the scientific payload of past, current, and future NASA and ESA space missions, such as Ulysses, Rosetta, BepiColombo, JUICE, and Comet Interceptor. New versions of the instruments are being designed to be installed onboard nanosatellite platforms (e.g. CIRCUS - LESIA space laboratory).

Mutual impedance and quasi thermal noise instrumental responses have been modeled assuming a homogeneous unmagnetized plasma. Particular focus was put on the impact of different electron distribution functions on their measurements (e.g. Maxwellian, double Maxwellian, or Kappa distributions). The impact of a magnetized plasma,i.e. the regime where the plasma and the electron cyclotron frequencies are of the same order, on the instrumental response, instead, was never modelled. We note that this magnetized regime is expected to be explored by ongoing and future planetary missions, such as the BepiColombo and JUICE.

In this context, we extend our understanding of these two techniques by modelling the instrumental response of mutual impedance and quasi thermal noise experiments in a magnetized plasma.

For this purpose, we use a numerical model with the hypothesis of a Maxwellian plasma. The electric potential generated by the emitting mutual impedance antenna is computed in space as a function of the emitted frequency and the plasma parameters. The expected autocorrelation of the electric potential measured in a magnetized plasma is also computed, in order to derive the quasi thermal noise spectrum. From the main features of these two numerical calculations, a diagnostic is derived for the plasma density, electron temperature, and external magnetic field.