A Short Review of Passive R.F. Electric Antennas as In Situ Detectors of Space Plasmas

We review the basic principles and recent or planned applications of passive, radio frequency electric antennas for in situ measurements of dusty plasmas in space. Electric antennas as passive wave detectors are reliable and versatile tools for such measurements, with the technique of Quasi Thermal Noise Spectroscopy and its generalization to dusty plasmas. The technique has been applied in the interplanetary medium, cometary plasma and dust tails, plasma environments of the Earth, Venus, Jupiter (including the Io plasma torus), Saturn (including the plasma torus and the E ring), with antennas of various shape aboard a number of spacecraft, including, most recently, Cassini. The technique is in the course of use on STEREO (NASA) in interplanetary magnetic clouds, is selected on MMO/Bepi-Colombo (JAXA-ESA) for in situ plasma measurements at Mercury, and planned on the Solar Orbiter (ESA) and other spacecraft projects. The diagnostic is based on the spectral analysis of the electric potential induced by the plasma particles as they pass by the antennas, and/or impact them or the spacecraft. The technique has a great advantage over usual electron detectors: its cross section for detection is much larger than the surface of the detector itself, ensuring a great sensitivity and a quasi-immunity to spacecraft perturbations. The spectral density induced by the passage of plasmas particles -with their dressing popularly known as Debye shielding, Langmuir waves, Bernstein waves and other members of the plasma menagerie - is easily calculated under stable conditions from the theory of the plasma quasi-thermal fluctuations. Around the plasma frequency, the spectrum is dominated by the QTN while at lower frequencies the spectrum is dominated by the shot noise produced by particles impacts on the antenna (in dipole mode) or on the spacecraft (in monopole mode). In this last case, the shot noise can be strongly enhanced by the impacts (and subsequent volatilization/ionization/recollection) of dust grains on the spacecraft, and a refined spectroscopy may provide both plasma and dust measurements.