Global IMAGE/HENA observations of the ring current: Examples of rapid response
to IMF and ring current-plasmasphere interaction

P.C. Brandt, D.G. Mitchell, Y. Ebihara, B.R. Sandel, E.C. Roelof, J.L. Burch, 
R. Demajistre


Energetic neutral atom (ENA) images in the energy range 16Ð50 keV obtained by
the High-Energy Neutral Atom (HENA) imager on board the IMAGE satellite are
presented from the period 23-26 May 2000. Two events during this period are
investigated: (1) A rapid decrease on 23 May of the entire dayside ring current
fluxes and (2) an unexpected local time distribution of the ring current during
the storm commencing on 24 May. In the first event, ENA fluxes from the dayside
ring current decrease rapidly for 10 min. The start of the decrease is
associated with a sharp, southward turning of the interplanetary magnetic field
(IMF) and a steep increase of the cross polar cap potential. Simulated
equatorial ion fluxes are able to reproduce the event using a kinetic ion drift
model. The observations are explained in terms of rapidly changing drift paths
during a reconfiguration of the convectional electric field in the
magnetosphere. The reconfiguration timescale (~10 min) is too short to modify
the pitch angle distribution (PAD). The results show that the solar wind
electric field over the dayside magnetosphere penetrates to the inner
magnetosphere within a few minutes. In the second event an enhanced ring current
develops during the storm on 24 May. Below 27 keV at L < 4 the ring current
extends from midnight clockwise through noon toward the dawn sector. At energies
above 27 keV the images show an enhancement in the ENA emissions coming from the
noon region and a lack of ENA emissions from the duskside. Apart from the main
phase of the storm the convection through the magnetosphere was steady
throughout the 3-day recovery phase. Images in extreme ultraviolet (EUV) of the
plasmasphere reveal a spatial relation between the ENA emissions and a
plasmaspheric tail. Data from the Polar satellite together with preliminary ENA
simulations suggest that the lack of ENA emissions is an effect of ion PADs
peaked around 90 degrees. We discuss possible explanations for the modification
of the PADs, such as adiabatic drifts, charge exchange, and wave-particle
interaction.

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Journal of Geophysical Research, 107, A11, 1359, doi:10.1029/2001JA000084, 2002