Effects of Spatial Resolution on the Martian Pickup Ion Plume in Global
Martian Plasma Simulations
Abstract
A key to understanding the evolution of the Martian climate over its
history is the study of how the Martian atmosphere escapes to space.
Studying the near-Mars space environment allows us to better understand
atmospheric escape processes. One of these important processes is ion
escape, in which atmospheric particles that are primarily ionized by the
solar radiation above the exobase region can escape from the planet.
Various model results, as well as MAVEN observations, have shown several
important channels for ion escape in the Martian plasma environment. One
of these channels forms when pickup ions are accelerated away from the
planet by a motional electric field, creating a “plume” of escape
organized by the upstream solar wind electric field. Although plasma
models have predicted the existence of this plume before, only recently
have we been able to regularly identify it in observations. Relatively
little work has been done on how modeling choices influence the
morphology of the plume. Here we present a comparison of two BATS-R-US
multi-fluid MHD simulations, each with different spatial resolution, run
using input conditions taken from a single MAVEN orbit in which the
plume signature was clearly identified. Our analysis primarily focuses
on differences seen in the location and morphology of the ion plume.
While the two simulations match well at low altitudes, location
differences in the ion plume become clear at high altitudes. We also
analyze the effect of different spatial resolution on the simulated ion
escape rates. Detailed investigation of the plume region in these
simulations has also provided us with a better understanding of the
underlying physics that shape and act on the ion plume. We have analyzed
and identified regions where the v x B force accelerates ions while the
J x B force confines them. This in turn allows us to identify the
location of the plume. This study highlights the importance of choices
in spatial resolution when modeling features in the Martian
magnetosphere.