Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events

Abstract

A series of seven hot flow anomaly (HFA) events has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on 19 August 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 four 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 (ΔB/B < 50%) consistent with the resonant ion beam instability in a proto‐HFA, 30 s electromagnetic waves (ΔB/B ∼ 1) in a young HFA, and magnetic pulsations in a mature HFA (ΔB/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 RE upstream from the bow shock, while the other three HFAs may extend between 5 and 14 RE upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the tangential discontinuity with a quasi‐parallel bow shock configuration rather than lying centered on the driving interplanetary magnetic field discontinuity.