Helmut Lammer ^* Daniel Schmid Martin Volwerk Fabian Weichbold Cyril Simon Wedlund Ali Varsani Magda Delva

• Space Research Institute, Austrian Academy of Sciences, Graz, Styria, Austria

Planetary exospheres are usually observed using transit spectroscopical methods such as the Lyman-α line, which is mainly limited by interstellar medium absorption, and airglow contamination from the geocorona when using low-orbit space telescopes, or by neutral and ion particle detectors and flight mass spectrometers. Here we discuss a complementary method that can be used for the characterization of exospheres based on the analysis of so-called ion cyclotron waves (ICWs), with magnetometer and plasma instruments. These ICWs are produced by pick-up ions from exospheric neutral atoms over a large space upstream of planetary bodies. The newborn exospheric ions generate an unstable secondary ion population in the solar wind plasma, where the interaction between the exospheric and solar wind ion populations can produce plasma waves from various instabilities. The observed wave power can be used to derive the corresponding pick-up ion and related neutral particle densities. Because the ion pick-up density is balanced by the ion production rate, one can reconstruct the exospheric neutral number density. Various exospheric particles (i.e., H, H 2 , D, He, etc.) can be distinguished by identifying their masses via their different gyrofrequencies even if they are very close to each other. Here we will discuss and analyze available ICW data as a tool for the reproduction of neutral atom profiles of extended exospheres such as Mercury, Venus, Mars, the Jovian satellites, and comets.