Strong interaction between Phobos and the solar wind

Details Strong interaction between Phobos and the solar wind Strong interaction between Phobos and the solar wind

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43b1431f&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P43B-1431

Physics

[5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [6230] Planetary Sciences: Solar System Objects / Martian Satellites

Scientific paper

During the Mars Express (MEX) closest approach to Phobos on July 23, 2008, the ASPERA-3/IMA (Ion Mass Analyser) sensor on board MEX carried out ion observations. The approach was in the upstream solar wind, and IMA detected unusual signatures of the proton fluxes close to Phobos apart from the commonly seen bow shock signatures. Because MEX has no magnetometer on board it is not possible to directly back trace the trajectories of the observed protons. Thus, it was not easy to confirm if those protons came from Phobos. However, after a careful analysis, we conclude that the origin of these protons is indeed Phobos. The reasons are: 1. The energy of the observed protons is slightly less than the solar wind proton energy, and the energy spectrum have a low-energy tail. The protons behave as backscattered solar wind protons which was reported by the Japanese Kaguya mission at the Moon. 2. We conducted test particle backtracing assuming that the protons originate from Phobos under various magnetic field conditions. A consistent solution for all independent observations was found. 3. We looked through all the IMA data observed in the undisturbed solar wind, and found that the strong signals were only observed during the Phobos flyby. These analyses indicate that Phobos is the most probable source of the observed protons during the flyby. Even though the generation mechanism is not fully understood, by taking Kaguya observation close to Moon as an analogy, the observed protons close to Phobos are most probably solar wind protons backscattered from Phobos. The process of backscattering of impinging keV particles has never been considered because most of the particles have been assumed to be absorbed at the very rough surface of the regolith. However, these investigations suggest that the backscattering of the solar wind protons are a general feature of the atmosphereless body covered by regolith, which would be applicable to Mercury, meteorite, and moons of giant planets.

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