Global ENA observations of the storm mainphase ring current: Implications for skewed electric fields in the inner magnetosphere

The local time distribution of the ring current in the 27119 keV range during several geomagnetic storm main phases have been investigated. The data was obtained by the high energy neutral atom (HENA) imager onboard IMAGE. Global proton distributions are derived from the observed energetic neutral atom (ENA) images using a linear inversion technique. During storms with low IMF B-y the peak of the proton distribution is around 01 MLT. For storms with high IMF B-y the peak can rotate to dawn. The rotation angle depends on solar wind velocity and interplanetary magnetic field (IMF) B-y, but less on IMF B-z. We discuss how this morphology implies the existence of strong and skewed equatorial electric fields in the inner magnetosphere. Our results are consistent with in-situ ring current measurements, radar observations and with kinetic models that self-consistently calculate the electric field produced by the closure of the partial ring current.