Hot flow anomalies near the Earth's bow shock

The discovery in 1984/85 of short intervals (a few minutes) near to the Earth's bow shock of very hot plasma flowing roughly perpendicular to the nominal solar wind direction sparked some intrigue as to the underlying cause(es) and local process(es). Typical characteristics of these events, now known as Hot Flow Anomalies (HFAs), include one or both edges of compressed/mildly shocked solar wind, a low and turbulent magnetic field in the extremely hot interior, a flow direction which is organized by local time, and a gross change in interplanetary magnetic field orientation from before to after the events. These latter two properties suggest that HFAs arise from the interaction of an interplanetary current sheet with the bow shock, as subsequently investigated theoretically. A crucial role is played by the dynamics of reflected protons in that the orientation of the magnetic field can lead to either a deficit of particles near the current sheet or a focussing along the current sheet into the upstream region.