A case study of proton precipitation at Mars: Mars Express observations and hybrid simulations
Catherine Dieval
Swedish Institute of Space Physics
Abstract:
Using data from the ASPERA-3 instrument (Analyzer of Space Plasma and Energetic Atoms) onboard Mars Express and hybrid simulations, the access of magnetosheath protons to the Martian atmosphere has been investigated. I will present present one orbit on the dayside of Mars close to the subsolar point with observations of significant proton fluxes. A fraction of the protons observed at an altitude of less than 400 km have energies of a few keV indicating a magnetosheath origin and trajectories indicating that they travel downward onto the atmosphere, which are taken as atmospheric precipitation. The measured energy and particle flux are 10^8-10^9 eV/cm2/s and 10^5-10^6 H^+/cm2/s, respectively. Under time stationary precipitation, the maximal horizontal scale of the precipitation regions was found to be thousands of kilometers along the orbit. Thus, the proton precipitation may affect the atmosphere globally. This was confirmed by our hybrid simulations which have shown that proton precipitation can occur in large regions below the induced magnetosphere boundary and that magnetosheath protons dominate over exospheric pick up protons produced by photoionization and charge exchange. The runs also confirm the influence of the convective electric field and the draping of the interplanetary magnetic field on the proton precipitation found in previous studies, and shows the influence of the energy of the particles on their spatial distribution at low altitudes. The H^+ precipitation is due to the small size of the Martian induced magnetosphere with respect to the proton gyroradius in the subsolar region.
Created 2010-11-26 16:34:10 by Mats Holmström Last changed 2010-11-29 12:44:15 by Mats Holmström
