Quasi-thermal noise spectroscopy: Diagnostics in space media

Karine Issautier (1) , Nicole Meyer-Vernet (2) , Michel Moncuquet (3)

(1) Observatoire de Paris, LESIA, Place Janssen, 92195 Meudon cedex (France), 
    and E-mail: karine.issautier@obspm.fr
(2) As (1) above, but E-mail: Nicole.meyer@obspm.fr
(3) As (1) above, but E-mail:Michel.moncuquet@obspm.fr


Quasi-thermal noise spectroscopy used for in situ diagnostics in space plasmas is
presented. In a stable plasma, the thermal motion of the ambient particles
produces electrostatic fluctuations. The concept can be easily generalized to the
quasi-thermal conditions arising in space. This "quasi-thermal noise" is detected
by any sensitive receiver at the ports of an electric antenna immersed in a
plasma. It can be formally calculated as a function of both the particles
velocity distributions and the antenna geometry, and so, conversely, the
"spectroscopy" of this noise reveals the local plasma properties. The technique
provides mainly an accurate measurement of the electron density (a few %) because
it is based on the detection of a strong signal peak near the local plasma
frequency (which is close to a resonance for electrostatic waves). As explained
in [1], the overall shape and level reveals the thermal temperature whereas the
very fine structure of the peak yields the suprathermal electrons. For magnetized
plasmas, the wave spectrum is modified since the electron thermal motion excites
Bernstein waves. The modulus of the magnetic field is thus simply determined. The
method has the advantage of being relatively immune to spacecraft potential and
photoelectrons perturbations, since it senses a large plasma volume, and can be
used to cross-check other plasma sensors. The method was successfully applied in
cometary plasma tail with ICE spacecraft, and routinely used in magnetized or not
planetary environments as well as in the solar wind, in particular on board
Ulysses and Wind spacecraft. Finally, the technique will be implemented on
BepiColombo/MMO probe to measure the plasma parameters in Mercury environment. 

References
[1] K. Issautier, M. Moncuquet, N. Meyer-Vernet, S. Hoang, R. Manning, 
Quasi-thermal noise diagnostics in space plasmas, Astrophys. Space Sci, 277, 
pp. 309-311, 2001.

_______________
Presented at the 27th General Assembly of International Union of Radio 
Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.