Upstream Ion Cyclotron Waves at Venus and Mars

Abstract

The occurrence of waves generated by pick-up of planetary neutrals by the solar wind around unmagnetized planets is an important indicator for the composition and evolution of planetary atmospheres. For Venus and Mars, long-term observations of the upstream magnetic field are now available and proton cyclotron waves have been reported by several spacecraft. Observations of these left-hand polarized waves at the local proton cyclotron frequency in the spacecraft frame are reviewed for their specific properties, generation mechanisms and consequences for the planetary exosphere. Comparison of the reported observations leads to a similar general wave occurrence at both planets, at comparable locations with respect to the planet. However, the waves at Mars are observed more frequently and for long durations of several hours; the cyclotron wave properties are more pronounced, with larger amplitudes, stronger left-hand polarization and higher coherence than at Venus. The geometrical configuration of the interplanetary magnetic field with respect to the solar wind velocity and the relative density of upstream pick-up protons to the background plasma are important parameters for wave generation. At Venus, where the relative exospheric pick-up ion density is low, wave generation was found to mainly take place under stable and quasi-parallel conditions of the magnetic field and the solar wind velocity. This is in agreement with theory, which predicts fast wave growth from the ion/ion beam instability under quasi-parallel conditions already for low relative pick-up ion density. At Mars, where the relative exospheric pick-up ion density is higher, upstream wave generation may also take place under stable conditions when the solar wind velocity and magnetic field are quasi-perpendicular. At both planets, the altitudes where upstream proton cyclotron waves were observed (8 Venus and 11 Mars radii) are comparable in terms of the bow shock nose M. Delva ( )

Solar wind charge exchange X-ray emission from Mars

“…Indeed, according to Delva et al (2011), upstream proton cyclotron waves have been observed by Phobos-2 and MGS up to 11 R M . These waves are generated by pick-up of planetary neutrals by the solar wind, and therefore their occurrence at large distances may come from the presence of an extended hydrogen exosphere around the planet.…”

MAVEN observation of an obliquely propagating low‐frequency wave upstream of Mars

“…At Mars, upstream proton-cyclotron-like waves have been observed by a number of spacecraft including Phobos 2 and Mars Global Surveyor [Brain et al, 2002;Bertucci et al, 2005;Delva et al, 2011;Mazelle et al, 2004;Russell et al, 2006;Wei and Russell, 2006;Wei et al, 2011;Romanelli et al, 2013;Wei et al, 2014]. Proton-cyclotron-like waves are LF waves which are left-hand polarized in the spacecraft frame, have transverse power higher than compressional power, and have frequencies near the local proton gyrofrequency.…”

Properties of Plasma Waves Observed Upstream From Mars

“…A particular subset of low-frequency waves, with similar properties to the 30-s waves, though typically with slightly higher frequencies in the spacecraft frame, occurs frequently at Mars. These waves have left-handed circular polarization in the spacecraft frame, with a spacecraft frame frequency equal to the local proton cyclotron frequency (Brain et al, 2002;Delva et al, 2011;Romanelli et al, 2013;Russell et al, 1990Russell et al, , 2006Wei et al, 2014;Wei & Russell, 2006). Waves with similar properties occur in many locales, including Venus (Delva et al, 2008), comets (Mazelle & Neubauer, 1993), and the solar wind (Jian et al, 2014).…”