Contribution of charge exchange at high altitudes to ring current decay:
IMAGE/HENA  observation

K. Keika, M. Nose, P.C. Brandt, S. Ohtani, K. Takahashi, and D.G. Mitchell


Several mechanisms have been proposed to explain the decay of the storm-time
ring current. Although charge exchange is believed to be one of the most
probable causes of the decay, there have been few observations that have been
able to quantitatively estimate its contribution. In this paper, we evaluate the
contribution of charge exchange process to the ring current decay, using
energetic neutral atom (ENA) data (> 10 keV) obtained by the HENA imager onboard
the IMAGE satellite. We focus on charge exchange collisions at high altitudes
(i.e., at larger radial distance within ring current L-shells) in particular.
The HENA imager detects ENAs which are generated when ring-current energetic
ions lose their charges through collisions with neutral atom and molecules of
the upper atmosphere and exosphere. The energy of a detected neutral atom is
considered to be equal to the energy lost by a ring current ion. The rate of
energy loss through charge exchange within each line-of-sight of pixels can be
expressed as:
dE/dt = 4 pi int EdE s^2 ds d Omega cdot sigma_{10} n_H j_{ION}
where sigma_{10} is the charge exchange cross section, n_H is the density of
geocorona, j_{ION} is the flux of ring current ions, s is the distance from the
satellite to the position of ENA production, d\Omega is a solid angle of each
line-of-sight, and E is energy of ENAs, if we assume isotropic pitch angle
distribution. Since s needs to be determined for estimating the energy loss, we
assume that all ENAs were generated at the spherical shell with a radial
distance of 8 R_E. We calculated the energy loss rate for the recovery phase of
three storms (April 22, 2001, September 23, 2001, and October 21, 2001), using
both hydrogen (> 10 keV) and oxygen (> 50 keV) data. The IMAGE satellite was
located above the North Pole (MLAT > 80 degrees) and near its apogee (radial
distance > 7 R_E) during those intervals. The calculated loss rate  was less
than 1/10 of the decay rate of ring current ions estimated from Burton's formula
(Burton et al., p.4202, JGR, 1975)  for all events. The above assumption
regarding the position of ENA production likely gives an overestimate, because
many parts of the detected ENAs were probably produced around the magnetic
equator. Therefore, our result suggests that charge exchange at radial distances
greater than 3~R_E hardly contributes to the decay of the ring current. We will
report dependence of the energy loss rate on the recovery rate of storms, using
more realistic assumptions. Energy loss at low altitudes (i.e., around
footpoints of ring current L-shells) will be also discussed. Our first
assumption (i.e., isotropic pitch angle distribution) is not considered valid at
low altitudes at high latitudes, where ion distribution could be strongly
anisotropic as a result of ions lost into the atmosphere.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004