Abstract
Ground-based and space observations have revealed that the upper layers of planetary atmospheres contain both a thermal fraction of neutral atoms and molecules with the mean particle kinetic energy corresponding to the local gas temperature and a suprathermal (hot) fraction of neutral particles with the mean kinetic energy much higher than the local atmospheric temperature. Atmospheric photochemistry and solar wind/magnetospheric plasma inflow play an important role in the formation of suprathermal atoms and molecules in the rarefied atmospheric gas. Dissociative recombination, dissociation by ultraviolet photons and photoelectrons, and exothermic photochemical reactions are accompanied by the release of energy on the order of several eV; part of this energy can be stored as the internal excitation of the reaction products. If the production rate of these particles, which are typically suprathermal, is faster than thermalization rate in the elastic collisions, then a stable fraction of them is formed. The kinetics of suprathermal atoms and molecules in the rarefied atmospheric gas is described rigorously only at the microscopic level using the Boltzmann kinetic equation. Since the Boltzmann kinetic equation for suprathermal atoms is a complex integro-differential equation, instead of solving it directly, a stochastic modeling approach for kinetic systems usually is used.
Interest in understanding the role of suprathermal particles in the physics and chemistry of planetary atmospheres has increased significantly. In particular, hot particles produced in upper atmospheric layers have been shown to play an important role in the atmospheric chemistry. Specifically, they: (i) lead to local changes in the chemical composition, because the non-equilibrium rate coefficients of the chemical reactions (particularly with high activation energies) between suprathermal particles and an ambient atmospheric gas are much larger than those at thermal energies; (ii) produce nonthermal atmospheric emission features; (iii) form hot planetary coronae and enhance nonthermal atmospheric losses.
The stochastic modeling approach is presented. Such modeling had been used to investigate the formation, kinetics, and transport of suprathermal particles in the rarefied planetary atmospheres. This approach is based on the Direct Simulation Monte Carlo (DSMC) method. The current physical and mathematical models of suprathermal atom formation due to the atmospheric photochemistry and solar wind/magnetospheric plasma inflow are presented. These models are used to investigate the formation and kinetics of suprathermal heavy (carbon, nitrogen, and oxygen) atoms in the upper atmospheres of Venus, Earth, and Mars where they are formed in significant amounts due to the atmospheric photochemistry and plasma inflow.
Created 2010-10-07 02:37:49 by Mats Holmström Last changed 2010-10-13 13:16:20 by Mats Holmström
