First Rosetta Observations of the Cometary Plasma at Churyumov-Gerasimenko with the Mutual Impedance Probe (RPC-MIP)

The ROSETTA spacecraft arrived in the vicinity of the comet in early August. As part of the Rosetta Plasma Consortium (RPC), the Mutual Impedance Probe (RPC-MIP) is designed to measure the bulk plasma properties in the comet environment. MIP is an active RF probe. It consists of two transmitters, which can operate separately as a monopole or coupled as a transmitting dipole, and a receiving dipole. The operating range of MIP is 7 kHz to 3.5 MHz that allows covering the plasma density range expected during the mission from solar wind to deep coma densities. The baseline distance between the transmitter and the receiver is 40-60 cm, which allows probing plasmas with Debye lengths up to 20-25 cm. For longer Debye lengths, MIP uses one of the two Langmuir Probes of the RPC-LAP instrument located at about 4 m from the MIP receiving dipole, which allows probing plasma with Debye lengths up to about 2 m. The MIP receiving dipole (baseline 1 m) can also be used in a passive mode to measure the electrical activity in the comet environment in the same frequency range. In the active mode, MIP measures the coupling complex impedance between the transmitting monopole (or dipole) and the receiving dipole. To model the frequency response of MIP, a surface charge distribution method is used that takes into account the charge distribution induced on the spacecraft structures by the transmitter. The spacecraft surface is approximated by discrete elements smaller in size than the plasma Debye length. In our model, it is assumed that each spacecraft elementary surface carries a uniform charge distribution. The electric field measured by the receiver is the sum of the contributions from the transmitter itself and of all the elementary surfaces that represent the spacecraft. The frequency of the transmitted current is varied in frequency steps. Assuming a transmitted current I(f) of constant amplitude, the potential difference V(f) between the two receivers provides the mutual impedance Z(f)=V(f)/I(f) of the instrument as a function of frequency. The first observations in active mode, as well as the background electric wave activity in the vicinity of the comet 67P/Churyumov-Gerasimenko are presented. Preliminary inferred plasma parameters, such as bulk electron density and temperature, are derived by using the model for the MIP frequency response.