Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p14b..07f&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P14B-07
Physics
[2152] Interplanetary Physics / Pickup Ions, [2780] Magnetospheric Physics / Solar Wind Interactions With Unmagnetized Bodies, [5704] Planetary Sciences: Fluid Planets / Atmospheres, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
This study quantifies several factors controlling the probability of a pickup oxygen ion to escape from the Mars upper atmosphere. Our Monte Carlo model releases billions of test particles through the electromagnetic fields of a magnetohydrodynamic model solution, monitoring their motion due to gravity and the Lorentz force throughout the simulation domain, which extends from the 200 km exobase altitude to 5 Martian radii from the center. We focus on the escape chances of the ions created at low altitudes on the dayside, where most ion production is located. The simulation results show that the electromagnetic fields are the dominant factor, surpassing the deceleration of gravity, in controlling ion transport and thus determine whether particles ultimately escape Mars or return to the planet. In a simulation case in which the strongest crustal fields face the Sun at nominal solar minimum conditions, on average only 45% of isotropically distributed new-born particles at ~400 km altitude are able to escape, even with a relatively high initial energy of ~10 eV. The percentage value slightly increases to 47%, if the velocity distribution is assumed to be half-isotropic without downward moving particles. The particle kinetic energy and the local time of the crustal fields are also important factors greatly influencing the escape probability. Furthermore, there is a distinct hemispheric asymmetry in the escape probability distribution, as defined by the upstream convection electric field direction. These findings imply that electromagnetic field effects are crucial in estimating Martian atmospheric loss for comparison with photochemical escape rates
Fang Xiao
Liemohn Michael W.
Luhmann Janet G.
Ma Yi
Nagy Andrew F.
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