Spontaneous hot flow anomalies at quasi-parallel shocks: 1. Observations

We present Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of a Spontaneous Hot Flow Anomaly (SHFA) upstream from the prenoon bow shock at 0431 UT on 12 August 2007. Although the SHFA exhibited the greatly heated and deflected solar wind plasmas used to identify hot flow anomalies (HFAs), it did not result from the standard mechanism invoked for the formation of HFAs, namely the interaction of an interplanetary magnetic field (IMF) discontinuity with the bow shock. We employ THEMIS A, B, C, and D observations to describe the evolution of the event from a proto-SHFA exhibiting regions of depressed magnetic field strength and density but little evidence for plasma heating or flow deflection, to a well-developed SHFA further downstream. These observations show that SHFA can be generated without the presence of an IMF discontinuity and are therefore a new category of HFAs.