Modeling Proton Arcs and Ring Current-Ionosphere Coupling

V.K. Jordanova, T.J. Immel, M. Spasojevic, and M.F. Thomsen


Detached proton auroral arcs have been recently observed at Earth by the
instruments on IMAGE satellite. These events are characterized by IMAGE FUV
observations of subauroral arcs, at lower latitudes and separated from the main
auroral oval, and extending over several hours of local time in the afternoon
sector. The emissions have been related to the precipitation of 20-30 keV
protons measured by the FAST electrostatic analyzers. Plasmaspheric plumes were
simultaneously observed during many of these events by the EUV instrument on
IMAGE and the MPA analyzer on the geosynchronous Los Alamos spacecraft. In this
study we present simulations of the proton precipitation during several
subauroral arc events identified from IMAGE data sets. The inner magnetospheric
conditions were moderately disturbed during these periods. We employ our kinetic
ring current-atmosphere interactions model (RAM), which calculates the evolution
of H+, O+, and He+ ion distributions due to earthward transport, acceleration,
and loss. We use a high resolution convection model, where the strength of the
cross-tail electric field is inferred from the latitude of the equatorward edge
of the diffuse aurora. The ring current model is coupled with a neutral hydrogen
density model of the geocorona and a time-dependent plasmasphere model.
Measurements from the geosynchronous LANL satellites are used to simulate the
time-variant plasma inflow on the nightside and to compare with model results on
the dayside. The growth rate of electromagnetic ion cyclotron (EMIC) waves is
self-consistently calculated as the time progresses and global images of
precipitating ions are obtained. In good agreement with IMAGE observations, EMIC
waves are preferentially excited, and proton precipitation maximizes, within
regions of spatial overlap of energetic ring current protons and cold
plasmaspheric populations in the afternoon sector. This indicates that pitch
angle scattering by EMIC waves is a viable mechanism for particle precipitation
into the atmosphere and generation of detached proton auroral arcs.

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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006