Kinetic Dynamics of Early and Mature Phase Hot Flow Anomalies Observed Upstream of Earth’s Bow Shock

Abstract:

This article will present new information about the “early” and “mature” phase hot flow anomalies (HFA) observed in the solar wind (SW) upstream of Earth’s bow shock. HFAs are nonlinear structures with density depleted regions populated by hot plasmas with temperatures as high as ten million degrees (Schwartz et al., 1985; Thomsen et al., 1986). The particle pressure exceeds the magnetic pressure and the structure expands at super Alfvénic speeds steepening the edges into shock waves. Thomsen et al. (1990) reported HFAs can be in an “early” or “mature” phase. The early phase HFA includes the SW and reflected particles and the mature phase a single hot population possibly produced by counter streaming instability that merges the incoming SW with the reflected particles. Examination of the velocity space distributions indicates that both types of HFAs are populated by the SW and several plasma components from different sources, including the reflected and gyrating particles. Each plasma component can vary and evolve independently of the others but the SW beam remains stable in both types of HFAs. Bulk parameters calculated by averaging over these multiple plasma distributions introduce artifacts yielding results that do not describe accurately the physics. What causes the gyrating and reflecting populations to evolve in space and time have not been identified but they probably involve SW fluctuations that alter the dynamics of the current sheet connection to the bow shock. A kinetic characterization of HFA is presented using the velocity space features. The physics of HFAs is related to formation of collisionless shock waves and understanding the mechanisms remain a challenge to theorists and modelers.