Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm33b..03k&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM33B-03
Physics
2720 Energetic Particles, Trapped, 2730 Magnetosphere: Inner, 2731 Magnetosphere: Outer, 2778 Ring Current, 2788 Storms And Substorms
Scientific paper
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 = 4π ∫ E dE ∫ ∫ s2 ds dΩ ṡ σ 10 nH jION , where σ 10 is the charge exchange cross section, nH is the density of geocorona, jION is the flux of ring current ions, s is the distance from the satellite to the position of ENA production, dΩ 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 RE. 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 RE) 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~RE 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.
C:son Brandt Pontus
Keika Kunihiro
Mitchell Donald G.
Nose' M.
Ohtani Shin
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