Energetic neutral atom response to solar wind dynamic pressure enhancements

We have investigated the response of the ring current to solar wind dynamic pressure (P-dyn) enhancement impacts on the magnetosphere by using energetic neutral atom (ENA) images obtained by the High-Energy Neutral Atom (HENA) imager on board the IMAGE spacecraft. In this work we present several events by distinguishing between pressure events where only the pure compression effects exist (without substorm effects) and those where the substorm is triggered by the Pdyn enhancement, and we find notable differences between these two types of events. First, for the pure compression events, we present four events where the Pdyn increases by similar to 100% to similar to 450% under northward interplanetary magnetic field (IMF) conditions. The Pdyn enhancement results in weak-to-modest ENA emission increases in both the hydrogen and oxygen channels: The total ENA emission rate increases by similar to 25% to similar to 40% relative to averages over the 1-hour period prior to the Pdyn enhancement impact. This ENA enhancement is due to ions adiabatically energized by the compression. The increased ENA emission rate drops as Pdyn decreases, implying that the ENA responses are directly caused by the adiabatic compression and decompression process. Also, the pure compression events lead to overall global, quasi-simultaneous increases of ENA in contrast to pure substorm-induced ENA enhancements that are initially localized in the nightside region followed by the drift effect. Next we present two events where the Pdyn enhancement was similar to 100% and >300%, respectively, under strongly southward IMF conditions, IMF B-z <= -10 nT, and a substorm was triggered. For both events the hydrogen and oxygen ENA emission increase is far more significant than that for the pure compression events, more than 100% even when the low-altitude contribution was excluded. This is primarily owing to energetic ions generated by the triggered substorm, although the compression effect itself occurs as well. Also, the ENA enhancement appears to be not as global in magnetic local time (MLT) as for the pure compression events, because of the predominant substorm effect that starts near midnight and spreads in MLT.