Ion Precipitation from the Ring Current: Local Time and Storm Phase
Dependence, and Importance for Interpreting One and Two Vantage Point
Energetic Neutral Atom Image Inversions

D.G. Mitchell (JHU/APL, Laurel, MD 20723), P.C. Brandt, E.C. Roelof, and 
R. Demajistre

Ion loss from the storm-time ring current is dominated by four primary
mechanisms. One is "flow-through" loss, by which the ion number and/or energy
content of the ring current is reduced when the source plasma sheet population
decreases, such that convection loss on open ion drift paths exceeds the ion
source of convection into the ring current from the tail.

A second is energy loss of ions (primarily on closed drift paths) through
Coulomb interaction with cold plasma. A third is charge-exchange loss, whereby
ions charge exchange with cold gas atoms in the exosphere, and radiate to
space as energetic neutral atoms (ENA). The fourth is precipitation loss and
ENA emission as ions in the loss cone interact with the atmosphere at the
foot-points of field lines. This last loss process is particularly difficult
to predict, and depends strongly on mechanisms that scatter ions into the loss
cone (otherwise, the loss cone would quickly empty and no additional losses
would occur by this mechanism).

Some of the ions that enter the loss cone do not precipitate into the
atmosphere, but instead radiate to space as ENA, often after multiple
charge-exchange interactions. In fact, when the loss cone is populated, the
ENA emission from the entire ring current is often dominated by emission from
the interaction of ions with the oxygen exobase. In this paper, we explore the
dependence of this emission on local time as a function of energy and storm
phase. We also discuss the complexity this emission introduces in the process
of inversion of single vantage point and two vantage point ENA images to
obtain the source ion distributions.

___________
Presentation at the Yosemite Conference of Inner Magnetospheric Interactions,
3 - 6 February 2004, Yosemite, California, USA