THEMIS satellite observations of hot flow anomalies at Earth's bow shock

Abstract

Abstract. Hot flow anomalies (HFAs) at Earth's bow shock were identified in Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellite data from 2007 to 2009. The events were classified as young or mature and also as regular or spontaneous hot flow anomalies (SHFAs). The dataset has 17 young SHFAs, 49 mature SHFAs, 15 young HFAs, and 55 mature HFAs. They span a wide range of magnetic local times (MLTs) from approximately 7 to 16.5 MLT. The largest ratio of solar wind to HFA core density occurred near dusk and at larger distances from the bow shock. In this study, HFAs and SHFAs were observed up to 6.3 R E and 6.1 R E (Earth radii), respectively, upstream from the model bow shock. HFA-SHFA occurrence decreases with distance upstream from the bow shock. HFAs of the highest event core ion temperatures were not seen at the flanks. The ratio of HFA ion temperature increase to HFA electron temperature increase is highest around 12 MLT and slightly duskward. For SHFAs, (T ihfa /T isw )/(T ehfa /T esw ) generally increased with distance from the bow shock. Both mature and young HFAs are more prevalent when there is an approximately radial interplanetary magnetic field. HFAs occur most preferentially for solar wind speeds from 550 to 600 km s −1 . The correlation coefficient between the HFA increase in thermal energy density from solar wind values and the decrease in kinetic energy density from solar wind values is 0.62. SHFAs and HFAs do not show major differences in this study.

Jets Downstream of Collisionless Shocks

“…The authors associated these jets with hot flow anomalies (see Sect. 5.3), which interestingly have been found to occur at much lower rates of a few per day (Schwartz et al 2000;Chu et al 2017).…”

“…The impact rates can be compared to occurrence rates of other transients. According to THEMIS statistics, hot flow anomalies (HFAs) occur about once every 2 hours and foreshock bubbles (FBs) only about once per day under favorable, high solar wind speed conditions Chu et al 2017). found foreshock cavitons to be detected at a rate of ∼2 per day.…”

Statistical study of particle acceleration in the core of foreshock transients

“…After this, we calculate the density, mean thermal energy, mean kinetic energy, shown in Figures 1f-1h, and other products. Inside the core, the density of the non-solar wind component increased, because to form an HFA, foreshock ions need to be concentrated by a driver discontinuity [e.g., Liu et al, 2016b], and some cold solar wind ions can be thermalized in the core and mix with the non-solar wind ions [Wang et al, 2013;Chu et al, 2017]. In Figures 1g and 1h, we see that inside the core, the non-solar wind ion mean thermal energy increased while the mean kinetic energy decreased.…”

Multispacecraft Observations of Tailward Propagation of Transient Foreshock Perturbations to Midtail Magnetosheath

“…Foreshock is the region upstream of the bow shock that is magnetically connected to the bow shock when the interplanetary magnetic field (IMF) is quasi‐parallel to the bow shock normal. There are many different types of transient (a time scale of a few minutes) ion foreshock perturbations, and some of them can be generated by the kinetic interaction of an IMF directional discontinuity (DD) with the quasi‐parallel bow shock, including hot flow anomalies (HFAs; e.g., Chu et al, ; Lucek et al, ; Omidi & Sibeck, ; Schwartz et al, ; Thomsen et al, ; Zhang et al, ), foreshock bubbles (FBs; e.g., Liu et al, ; Omidi et al, ; Turner et al, ), and foreshock cavities. While all of these three types consist of a low‐density core with enhancements of energetic particles (thus higher temperature), HFAs and FBs also include significant deflected flows away from the upstream solar wind direction.…”