Hot Flow Anomalies: Explosions at the Earth's Bow Shock

A series of 7 Hot Flow Anomaly Events (HFAs) has been observed by the THEMIS C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on August 19, 2008. Both young (no shocks at edges, two distinct ion populations) and mature (strong shocks at edges, a single hot ion population) HFAs have been observed. Further upstream, THEMIS B observed 4 proto-HFAs (density and magnetic field strength depletions, plasma heating but no flow deflections) which later developed into HFAs observed by THEMIS C. We present evidence indicating that electromagnetic right-hand resonant ion beam instabilities heat ions inside HFAs. Observations of small amplitude perturbations (\triangle B/B<50%) consistent with the resonant ion beam instability in a proto-HFA, 30s electromagnetic waves (\triangle B/B ∼1) in a young HFA, and magnetic pulsations in a mature HFA (\triangle B/B ∼4) indicate that they are at early, middle and late (nonlinear) stages of the electromagnetic right-hand resonant ion beam instabilities. Both young and mature HFAs are associated with strong electromagnetic waves near the lower hybrid frequency (0.1-1 Hz). The lower hybrid waves are the likely source of the electron heating inside HFAs. THEMIS B observations of four proto-HFAs which later developed into HFAs observed by THEMIS C indicate that these four HFAs might extend beyond 14 R_E upstream from the bow shock, while the other 3 HFAs may extend between 5 and 14 R_E upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the TD with a quasi-parallel bow shock configuration rather than lying centered on the driving IMF discontinuity.