Lecturer: Prof. Valery Shematovich (INASAN)
Date: 2011-02-24 10:30
Place: Aniara

Monte Carlo modeling of the protons and hydrogen atoms transport in the Martian upper atmosphere with induced magnetic field

V.I. Shematovich and D.V. Bisikalo,
Institute of Astronomy of the Russian Academy of Sciences,
48 Pyatnitskaya str., Moscow 119017,
Russian Federation

Abstract. The Mars Express ASPERA-3 measurements have showed that a significant portion of solar wind protons are reflected from the Mars atmosphere and form a significant H/ H+ backscattered flux. The information on the H/ H+ upward flux is needed to correctly describe the energy deposition of the solar wind plasma in the atmosphere. Thus, investigation of a mechanism that causes formation of the backscattered flux is a very relevant task. The energy deposition of precipitating protons starts in the upper atmosphere, therefore it is necessary to use the Boltzmann equations for high-energy protons and hydrogen atoms formed due to the charge exchange to describe this interaction. Numerical models, describing the penetration of protons into planetary atmospheres (including the Mars atmosphere) have been developed for several years. Nonetheless we developed the Monte Carlo model, since, as we guess, it has several advantages. The Direct Simulation Monte Carlo method has been used to solve the kinetic equation for H/ H+ penetration into the upper planetary atmosphere with CO2, N2 and O main constituents. The model takes into account the full set of physical processes including the momentum and energy transfer in the elastic and inelastic collisions; ionization of target atmospheric molecules/atoms; charge transfer and electron capture collisions. The model accounts for recent measurements or calculations of the total and differential cross sections. A key point of this model is the stochastic treatment of the scattering angle distribution in the elastic and inelastic collisions of H/ H+ with ambient atmospheric gas.
Estimations of the upward flux have been made for the Martian atmosphere during the solar minimum. We use a spectrum of the down moving protons at 500 km adopted from the Mars Express ASPERA-3 measurements in the energy range 300 eV ? 20 keV. The particle and energy fluxes of the downward moving protons were equal to 4.1×106 cm-2 s-1 and 1.37×10-2 ergs cm-2 s-1. It was found in the calculations with the Monte Carlo model, that 12% of particle flux and 9% of the energy flux of the precipitating protons were backscattered by the Martian upper atmosphere.
We also considered another important effect ? the influence of the induced magnetic field on the precipitation processes in the Martian upper atmosphere. The induced magnetic field has been measured by the Mars Global Surveyor (MGS) orbiter. It is evident that the presence of the horizontal component of the magnetic field induced by solar wind must lead to the increase of the H/ H+ upward flux because of the proton giro-motion. If we include in the model a 20 nT horizontal magnetic field in the altitude range of 85 km?500 km in the Martian upper atmosphere, it was found that up to the 40% - 50% of energy flux of the precipitating protons is backscattered depending on the velocity distribution of the precipitating protons. The main conclusion of the study is that the magnetic field plays a crucial role in the transport of charged particles and, therefore it determines the energy deposition of the solar wind in the upper atmosphere of Mars.

Created 2011-02-23 13:07:51 by Mats Holmström
Last changed 2011-02-23 14:36:31 by Mats Holmström