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

Abstract

Energetic neutral atom (ENA) images in the energy range 16–50 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°. We discuss possible explanations for the modification of the PADs, such as adiabatic drifts, charge exchange, and wave‐particle interaction.