Behavior of Oxygen Ions During Substorms  

Mei-Ching Fok, Thomas E. Moore, Samuel T. Jones, Dominique C. Delcourt, 
and Pontus C. Brandt


In the magnetosphere oxygen ions have many unique features that are not found in
protons. O + only come from the ionosphere, making them perfect tracers for
studying the ionospheric responses to the solar wind driver. The non-adiabatic
property of these heavy ions also leads them to exhibit more dramatic responses
to disturbances and reconfiguration in the magnetosphere. O + were found to be
more abundant in solar maximum and during magnetic storms. Before the launch of
the Imager for Magnetopauseto-Aurora Global Exploration (IMAGE) mission, the
precise timing of O + enhancement during substorms was not known. However,
observations from the High Energy Neutral Atom (HENA) imager on IMAGE have
revealed the instant buildup of energetic (50Ð200 keV) O + right after substorm
onset. The enhancements start at high altitudes and 5Ð10 minutes later extends
to low altitudes. We have performed theoretical simulations in order to
understand the physical processes responsible to the O + enhancements during
substorms. The Lyon-Fedder-Mobarry (LFM) MHD model is used to specify the
dynamic magnetic and electric fields during a substorm triggered by a steady
southward IMF condition. Hundred thousands of test oxygen ions are released from
the auroral zone throughout the event. The subsequent trajectories of these ions
are calculated by the particle code of Delcourt. We then compute the density and
pressure contributed by these ions by carefully assigning each particle a phase
space density. More than an order of magnitude increase in pressure is found in
the plasma sheet after the substorm onset and the peak pressure is traveling
earthward and westward as dipolarization continues. The complete distributions
of O + in the inner magnetosphere are then calculated by the Comprehensive Ring
Current Model with boundary pressure, density and potential given by the
LFM-Delcourt model. We will present our simulation results and the comparison
with the HENA data. Mechanism for O +  acceleration will also be discussed.

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Presentation, 2nd Annual Asia Oceanic Geosciences Society Meeting, Singapore 
20-24 June 2005