Multi-Point Themis Observations of Multiple Transient Ion Foreshock Phenomena

Abstract:

Recent simulations and observations have shown that when solar wind rotational discontinuities (RDs) approaching Earth’s bow shock interact with ions backstreaming in the foreshock they can result in a new phenomenon, foreshock bubbles (FBs). FBs result in very intense pressure perturbations upstream of and inside Earth’s magnetosphere and are therefore important for solar wind-magnetosphere coupling. To examine the time evolution of FBs and compare FBs with hot flow anomalies (HFAs), we examined multi-point THEMIS observations of both FBs and HFAs in which 4 or more spacecraft all observed the same events. Such observations allow us to disambiguate between spatial and temporal evolution of the FBs and HFAs. Our results show that FBs form further upstream from the bow shock and then expand with a higher speed to a much larger size compared to HFAs. We discuss how these results are consistent with simulations. Particle acceleration caused by the bouncing motion of particles between the earthward moving shock upstream of the FBs and the Earth’s bow shock as revealed in simulations is also discussed based on our observations of ion distribution functions.