Rosetta spacecraft influence on the mutual impedance probe frequency response in the long Debye length mode

Abstract

During the Rosetta flyby of comet 46P/Wirtanen from 2011, plasma observations will be obtained from a number of instruments, the mutual impedance probe (MIP) is one of them. The mutual impedance technique is based on the measurements, as a function of the frequency, of the quasi-electrostatic coupling between an emitter and two receivers separated from the emitted source by at least twice the local plasma Debye length. The electron number density is deduced from the electron plasma frequency at which the transfer function reaches its maximum, and the Debye length, λ D, is deduced from the positions of the minima above the plasma frequency. The long Debye length (LDL) mode, which operates in the 7–168 kHz range, is a secondary mode of MIP that has been designed to probe cometary plasmas when λ D is longer than 70 cm. In that case, the Rosetta spacecraft presence cannot be neglected due to its conductive structures, the dimensions of which are of order of the emitter–receiver distance. A numerical simulation of the LDL mode is then necessary. The discrete surface charge distribution (DSCD) method, which is well adapted to the electric antenna problems in a kinetic plasma, would be suitable for the Rosetta flyby measurements. Here, all the conductive surfaces (spacecraft, solar panels and antennae) are compared with an alternating charge distribution that contributes to the LDL mode transfer function. The preliminary results show that in the early parts of the Rosetta mission, the cometary plasma can reasonably be considered to be Maxwellian, homogeneous, isotropic, collisionless and unmagnetized in the range of λ D from 0.7 to 2.5 m. The numerical results are compared with those obtained by ignoring the spacecraft influence. It appears that the resonance peak at the plasma frequency is sharpest and strongest when the spacecraft influence is considered. Moreover, the antiresonance frequencies which occur on both sides of the plasma frequency depend on the Debye length of the surrounding plasma. Hence, the LDL mode should be able to measure the electron temperature in the range from about 10 4– 2×10 5 K . Plasmas with electron number densities lying between 22 and 180 cm −3 should also be probed. This will allow the LDL mode to distinguish the boundary between the Wirtanen's cometary plasma and the Solar Wind in the prime investigations of the Rosetta mission, and this will complete the measurements made by the principal MIP mode in the non-magnetic cavity of the comet.