ROSETTA MISSION MUTUAL IMPEDANCE PROBE MODELLING:THE SHORT AND LONG DEBYE LENGTH PLASMA CASES

J. GEISWILLER, J.G. TROTIGNON and C. BÉGHINCNRS, Laboratoire de Physique et Chimie de l’Environnement, 3A, Avenue de la Recherche

Scientifique, F-45071 Orléans Cedex 02, France

E. KOLESNIKOVA2

Department of Physics and Astronomy, University of Leicester, University Road, Leicester LEI 7RH,United Kingdom

Abstract. The Rosetta spacecraft (S/C), which is planned to meet comet 46P/Wirtanen in 2011,will carry a set of five wave and plasma instruments (i.e. the Rosetta Plasma Consortium). This isto measure the cometary plasma properties from the minimum value of activity of the comet to itsmaximum value at perihelion. The mutual impedance probe, MIP, is one of those (Trotignon et al.,1999) five.

The MIP sensor is a quasi-electrostatic emitting-receiving device derived from thequadrupolar probes of Storey et al. (1969). An alternative current, I (ω), of variablefrequency, ω, lying in the range of frequencies from 28 kHz to 3.5 MHz is injectedthrough the emitting electrode. Simultaneously, the difference in voltage, V (ω),is measured between two receiving electrodes located at 40 cm and 60 cm fromthe MIP transmitter. The ratio V (ω)/I (ω) is called the mutual impedance, Z(ω)

and depends on the plasma parameters. The electron density is deduced from themodulus of Z(ω), which peaks at the plasma frequency and is shown in Figure1. The local Debye length, λD, is deduced from the location of antiresonancesabove the plasma frequency which are formed by the beating of Landau waves. Thewave length of these waves is dependent on the electron temperature, the plasmafrequency and the emitting frequency. The MIP mode will study plasmas in thediamagnetic cavity of the comet whenever the Debye length will belong to therange from 3 mm to 20 cm.

The LDL mode will be used in long Debye length plasmas, i.e. in the primeinstants of the coma formation and the period of spacecraft crossings in the regionsbetween coma and solar wind. In this case, a spherical Langmuir probe located atabout 4 m from the MIP sensor is used as the emitting electrode. The difference involtage will then measured between the two MIP receiving electrodes in the rangeof frequencies from 7 kHz to 168 kHz.

A computer simulation of the corresponding frequency responses has been madein a collisionless, homogeneous, isotropic and kinetic plasma to precisely evaluatethe range of measurements of those modes. Here, we have not choosen to takeinto account either the presence of the ion-sheath around the sensor and the space-craft, or problems due to a non-maxwellian distribution of electrons which are

Astrophysics and Space Science 277: 317–318, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

318 J. GEISWILLER ET AL.

Figure 1. [left] Modelling of the expected normalized modulus of the MIP difference in voltagein a Maxwellian and isotropic plasma. The influence of the Debye length is well illustrated by thevariation of the antiresonances location when = ω/ωpe > 1. Here, the numerical frequencyresolution is of 0.1 ωpe below the plasma frequency and 0.01 ωpe above.Figure 2. [right] Range of measurements of the electron density and temperature expected in the MIPmode and the LDL mode. These theoritical values are deduced from the DSCD numerical simulationof the instruments in a Maxwellian plasma.

considered to be the object of future papers. We only focus on the influence of theS/C conductive structure in the LDL mode and that of the sensor structure in theMIP mode. In both cases the Discrete Surface Charge Distribution method (Béghinand Kolesnikova, 1998) is used which is basically based on the finite elementsmethod. Regarding the MIP mode, all the conductive structures of the sensor aremodelled as a distribution of point sources that are induced on the sensor surface bythe emitting monopole and its neighbouring charges. The surface potential of thesensor is calculated using the expression of the electrostatic potential induced by apoint source in a maxwellian plasma (Béghin, 1995). Regarding the LDL mode, theS/C presence is taken into account by a wire structure transparent for the plasma.Rigorously, this assumption is valid whenever the plasma Debye length is twicethe longest dimension of the S/C body, i.e. λD > 1.7 m.

The necessity to model very carefully the conductive structures which are in thevicinity of the MIP sensor must be pointed out. The LDL mode are indeed foundto be strongly dependent on the spacecraft whose dimensions are in the order ofthe emitting-receiving distance. The range of measurements of the MIP mode andthe LDL mode are summarized in Figure 2.

References

Béghin, C.: 1995, Radio Sci. 30, 307.Béghin, C. and Kolesnikova, E.: 1998, Radio Sci. 33, 503.Storey, L.R.O., Aubry, M.P. and Meyer, P.: 1969, in: J.O. Thomas and B.J. Landmark (eds.), Plasmas

Waves in space and in Laboratory, University Press Edinburgh 1, 303.Trotignon, J.G. et al.: 1999, MIP – RPC: the Mutual Impedance Probe in the Rosetta Plasma

Consortium. ESA SP-1165.