Contribution of ion flow-out and charge exchange processes to the decay of the
storm-time ring current

K. Keika, M. Nose, K. Takahashi, S. Ohtani, P.C. Brandt, D.G. Mitchell, S.P.
Christon, R.W. McEntire


The decay of the ring current during the storm recovery phase can be attributed
mainly to loss of energetic ions. Its mechanisms, however, are still open
problems. Although ion flow-out and charge exchange processes are the most
probable causes of such loss, it is yet to be identified how much these
processes contribute to the loss. In this study we evaluate the contribution of
each process to the decay of the ring current, using energetic ion data acquired
by the EPIC instrument onboard Geotail and energetic neutral atom (ENA) data
acquired by the HENA imager onboard IMAGE. The HENA imager observes ENAs
(>=~10~keV) that are produced when energetic ring current ions experience
charge exchange collisions with cold neutral atoms and molecules. We can
calculate the rate of energy loss due to the charge exchange process. The
Geotail spacecraft is frequently located in the magnetosheath and upstream of
the Earth's bow shock, where the Geotail/EPIC instrument observes energetic ions
flowing out from the magnetosphere. Because the instrument provides charge state
information of 9 - 210 keV/e ions, it can distinguish the leaking ions from
solar wind ions accelerated at the Earth's bow shock. From this data we can
calculate the energy density of leaking ions. In order to evaluate total energy
loss via the ion flow-out process, we investigate if it is possible from HENA
images to determine the leakage area. The area can also be estimated from
statistical studies on energetic ions outside the magnetosphere. We will present
two interesting storms (September 23, 2001 and April 22, 2001). During the
recovery phase of these storms, Geotail/EPIC observed energetic protons and
singly-charged oxygen ions outside the magnetosphere on the dusk side, and IMAGE
was located in a vantage point in the northern hemisphere. We will estimate
contributions of the two processes to the decay of the storm-time ring current.

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Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 
8-12 December 2003