First observations of foreshock bubbles upstream of Earth's bow shock: Characteristics and comparisons to HFAs

Abstract

Using multipoint in situ observations upstream of Earth's bow shock from the THEMIS mission, we present the first observations of foreshock bubbles (FBs) and compare them to observations of hot flow anomalies (HFAs). FBs are recently conceptualized kinetic phenomena that can form under the commonplace condition of a rotational discontinuity in the interplanetary magnetic field interacting with backstreaming energetic ions in Earth's quasi‐parallel foreshock. FBs may have remained elusive until now due to their many observational similarities to HFAs and the lack of coordinated multipoint measurements. Here we introduce identification criteria for distinguishing between HFAs and FBs using in situ observations, and use them to analyze five example events that occurred on Bastille Day (14 July) and 11–12 August 2008. Three of these events satisfy the criteria for FBs and are inconsistent with multiple criteria for HFAs. The remaining two events are consistent with the traditional picture of HFAs. Furthermore, FBs involve two converging shocks, and using these events, we demonstrate their effectiveness at particle acceleration. Considering that their formation conditions are not extraordinary, FBs may be ubiquitous at collisionless, quasi‐parallel shocks in a variety of astrophysical settings.