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Ion heating near the ion composition boundary at Venus

K. Szego, Z. Dobe, Z. Bebesi, L. Foldy, M. Fraenz, A. Opitz

Abstract


In this study we focus on plasma processes in the boundary layer above the ionopause of Venus. The first measurements which demonstrated the existence of such a boundary layer were those of the electron energy spectra obtained by the Pioneer Venus Orbiter Retarding Potential Analyser (ORPA). The measurements of the ASPERA-4 electron spectrometer on board Venus Express (VEX) confirmed the existence of such a layer. The upper end of the interaction layer, where planetary ions disappear, is called ion composition boundary (ICB). Due to the interaction of the two plasma populations near the ICB – the shocked solar wind and planetary ions – instabilities are excited. Significant collisionless momentum and energy exchange takes place because of wave-particle interaction, creating a highly turbulent layer. In earlier works it was proposed that modified two stream instabilities (MTSI) (see e.g. Dobe et al., 1999) might explain the 100-Hz waves observed by the electric field detector (OEFD) on board PVO in the dayside of Venus. The instability also heats the ions. PVO data covered only partially the energy range of the particles in question. Using the much better 3-D energy and spatial coverage of the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) instrument suite on board VEX, we compare here with data the charged particle heating due to the MTSI. The first data set is for less than average solar wind conditions, the second one is for a case when a strong solar storm hits Venus. After having summarised the properties of the modified two stream instability, and the ion heating and ion acceleration mechanism in the framework of a numerical hybrid model which retains electron inertia, we show that MTSI heating is effective up to a few hundred eV. We also discuss the limits of this approach.

 


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