Multipoint observations of the structure and evolution of foreshock bubbles and their relation to hot flow anomalies

When backstreaming foreshock ions pass through approaching solar wind discontinuities, they may get concentrated upstream of the discontinuities and form foreshock bubbles (FBs). Because FBs result in very intense global pressure variations upstream of and inside Earth's magnetosphere, they are important for solar wind-magnetosphere coupling. Information about these recently discovered phenomena is limited, however. To elucidate FB spatial structure, evolution, expansion, and formation conditions, we use multipoint Time History of Events and Macroscale Interactions during Substorms observations in which three or more spacecraft observed the same events. From single-case studies of two foreshock bubbles and one hot flow anomaly (HFA), we demonstrate how we determine FB spatial structure, evolution, and expansion and propose a model to explain these properties. We discuss the different conditions leading to formation of FBs and HFAs. Multiple case studies of six FBs, five HFAs, and one spontaneous HFA show that FBs typically expand faster than HFAs and their expansion speed is likely determined by the solar wind speed.