IMAGE/high-energy energetic neutral atom: Global energetic neutral atom imaging of the plasma sheet and ring current during substorms

Energetic neutral atom (ENA) images in the 10-60 keV range show the substorm dynamics of the ion distribution out to 14 R-E in the plasma sheet. The images were obtained during two substorms in the mainphase of the 4 October 2000 storm by the high-energy energetic neutral atom (HENA) imager on board the IMAGE satellite. During the two substorm sequences the edge of the most tailward ENA emissions appears to move earthward in association with dipolarization observed at geosynchronous distance by the GOES satellite. At the time of geosynchronous dipolarization, the auroral onset was observed by the far ultra violet (FUV) imager onboard IMAGE. Preliminary equatorial ion distributions derived from the individual ENA images in the substorm sequences have been obtained through a constrained linear inversion technique. The results show that the ion fluxes in the >8 R-E plasma sheet suddenly decrease around dipolarization during approximately 30 min. About 12-20 min after the start of the plasma sheet flux decrease, ion injections are seen by the geosynchronous satellites. The peak of the ring current flux is pushed in from geosynchronous to L=4 over the substorm sequences. Two exospheric models are used in the inversion. We find that more realistic plasma sheet fluxes are obtained with an exospheric model that contains enhanced exospheric hydrogen densities on the nightside. Implications for earthward propagation of the plasma sheet ion distribution during dipolarization are discussed.