THEMIS observations of a Hot Flow Anomaly at the Earth's bow shock and its impact on the magnetosphere

Hot Flow Anomalies are generated at the bow shock by otherwise unremarkable discontinuities in the solar wind magnetic field (measured at 1AU). They drastically reduce the solar wind ram pressure on minute timescales, and are thought to be responsible for a variety of magnetospheric signatures. Despite their intrinsic interest as a plasma physics phenomenon, and their impact on space weather prediction, a number of questions remain concerning thermalization mechanisms, occurrence rates, and their overall impact on the magnetosphere. Here, we present THEMIS observations of a Hot Flow Anomaly (HFA) at the Earth's bow shock on 4 July 2007 at 10:26UT. At this time, the five THEMIS spacecraft were in a configuration unique to the first dayside season; three spacecraft were separated by a few hundred kilometers with the other two 1Re up and downstream. At the time of observation, THEMIS A was upstream of the bow shock, with the other spacecraft in the magnetosheath; this is a rare example where the event was observed on both sides of the bow shock. We show that THEMIS A observed classical HFA signatures; we use field and particle data to study the orientation and motion of the underlying discontinuity and examine in new detail the plasma thermalization mechanisms inside the HFA. Data from the other THEMIS spacecraft are used to study the evolution of the HFA in the magnetosheath. We also calculate the impact of the HFA on the magnetopause, and present THEMIS ground based data, complemented by other datasets, showing its impact on the magnetosphere as a whole.