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

During the Rosetta flyby of comet 46P/Wirtanen from 2011, plasma observatio ns will be obtained from a number of instruments, the mutual impedance prob e (MIP) is one of them. The mutual impedance technique is based on the meas urements, as a function of the frequency 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, lamb da (D), is deduced from the positions of the minima above the plasma freque ncy. The long Debye length (LBL) mode, which operates in the 7-168 kHz rang e, is a secondary mode of MIP that has been designed to probe cometary plas mas when lambda (D) is longer than 70 cm. In that case, the Rosetta spacecr aft presence cannot be neglected due to its conductive structures, the dime nsions of which are of order of the emitter-receiver distance. A numerical simulation of the LDL mode is then necessary. The discrete surface charge d istribution (DSCD) method, which is well adapted to the electric antenna pr oblems in a kinetic plasma, would be suitable for the Rosetta flyby measure ments. Here, all the conductive surfaces (spacecraft, solar panels and ante nnae) are compared with an alternating charge distribution that contributes to the LDL mode transfer function. The preliminary results show that in th e early parts of the Rosetta mission, the cometary plasma can reasonably be considered to be Maxwellian, homogeneous? isotropic, collisionless and unm agnetized in the range of do from 0.7 to 2.5 m. The numerical results al e compared with those obtained by ignoring the spacecraft influence. It appea rs that the resonance peak at the plasma frequency is sharpest and stronges t 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 x 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 bou ndary between the Wirtanen's cometary plasma and the Solar Wind in the prim e investigations of the Rosetta mission, and this will complete the measure ments made by the principal MIP mode in the non-magnetic cavity of the come t. (C) 2001 Elsevier Science Ltd. All rights reserved.