Storm-substorm relationship: Variations of the hydrogen and oxygen energetic 
neutral atom intensities during storm-time substorms

S. Ohtani, P. C. Brandt, D. G. Mitchell, H. Singer, M. Nose, G. D. Reeves, 
and S. B. Mende


The present study observationally addresses the role of the magnetospheric
substorm in the storm-time ring current intensification. The intensity of
energetic neutral atom (ENA) emission, which is measured by the high-energy
neutral atom (HENA) imager onboard the IMAGE satellite, is carefully used as a
guide for inferring the change of the ring current intensity. First, a storm
event of August 2000 is examined in detail with a focus on a substorm that
occurred at the start of the storm recovery phase (as defined by Sym-H). During
the expansion phase of this substorm, the Sym-H index recovered (increased) as
the geosynchronous magnetic field dipolarized. At the same time the low-energy
(27Ð60 keV) hydrogen, high-energy (60Ð119 keV) hydrogen, and total oxygen (<160
keV for this event) ENA intensities increased, suggesting that the ring current
intensified. The apparent recovery of Sym-H can therefore be attributed to the
reduction of the tail current rather than the decay of the ring current. The
substorm-related change of the ENA intensity is examined statistically by
conducting a superposed epoch analysis, for which the onset of geosynchronous
dipolarization is adopted as a reference time. The result reveals that the ENA
intensity tends to decrease before substorm onsets and to increase after onsets,
and so does the Sym-H index although its pre-onset decrease is less clear than
the post-onset increase. It is therefore suggested that in the course of
substorms, the change of the ring current intensity is opposite to what is
expected from the change of the Sym-H index. The decay and intensification of
the ring current can be attributed to substorm-related changes of the near-Earth
magnetic field and convection. Another important result is that the response of
the ENA intensity to substorms strongly depends on species and energy range. The
variation of the low-energy hydrogen ENA intensity is not clearly organized by
the substorm onset, and its relative change is less than 10%. The high-energy
hydrogen intensity decreases by about 20% during the growth phase and then
recovers to the initial level leaving no significant net increase. In contrast,
the increase in the oxygen ENA intensity during the expansion phase
overcompensates for the preceding reduction, resulting in a net increase of 20%.
The net enhancement of the oxygen ENA intensity suggests the importance of
non-adiabatic acceleration associated with near-Earth dipolarization. The
timescale of dipolarization is comparable to the oxygen gyroperiod, and
therefore the oxygen ions may be accelerated preferably by the associated
inductive electric field. It is inferred that the substorm-related energization
of the oxygen ions makes an important contribution to the storm-time ring
current intensification.

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Journal of Geophysical Research, 110, A07219, doi:10.1029/2004JA010954.