Active current sheets and candidate hot flow anomalies upstream of Mercury's bow shock

Abstract

Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities interacting with collisionless bow shocks. They are typically formed when the normal component of the motional (convective) electric field points toward the embedded current sheet on at least one of its sides. The core region of an HFA contains hot and highly deflected ion flows and rather low and turbulent magnetic field. In this paper, we report observations of possible HFA-like events at Mercury identified over a course of two planetary years. Using data from the orbital phase of the MESSENGER mission, we identify a representative ensemble of active current sheets magnetically connected to Mercury's bow shock. We show that some of these events exhibit magnetic and particle signatures of HFAs similar to those observed at other planets, and present their key physical characteristics. Our analysis suggests that Mercury's bow shock does not only mediate the flow of supersonic solar wind plasma but also provides conditions for local particle acceleration and heating as predicted by previous numerical simulations. Together with earlier observations of HFA activity at Earth, Venus, Mars, and Saturn, our results confirm that hot flow anomalies could be a common property of planetary bow shocks and show that the characteristic size of these events is controlled by the bow shock standoff distance and/or local solar wind conditions.

Hot flow anomaly observed at Jupiter's bow shock

“…The HFA occurrence rate at Saturn has been given as once per 15 days, however, that is considered a lower bound [Masters et al, 2009]. Uritsky et al [2014] observed that the size of HFAs from Mercury, Venus, Earth, Mars, and Saturn tended to increase with the size of the planetary bow shock. What makes the HFA observed at Jupiter different from those observed at other planets is the immense size of the structure.…”

“…HFAs have not only been observed at Earth but also at Mercury [Uritsky et al, 2014], Venus [Collinson et al, 2012], Mars [Øieroset et al, 2001;Collinson et al, 2015], and Saturn [Masters et al, 2009]. HFAs have not only been observed at Earth but also at Mercury [Uritsky et al, 2014], Venus [Collinson et al, 2012], Mars [Øieroset et al, 2001;Collinson et al, 2015], and Saturn [Masters et al, 2009].…”

“…The typical and extreme HFA sizes and bow shock standoff distances for the planets other than Jupiter come fromUritsky et al [2014]. The typical HFA sizes are indicated by the diamonds, and the largest sizes are indicated by the bar.…”

Ion Kinetics in a Hot Flow Anomaly: MMS Observations

“…HFAs can have a significant impact on the magnetopause (Sibeck et al, 2000(Sibeck et al, , 1999, producing disturbances throughout the magnetosphere to the ground (Eastwood et al, 2008(Eastwood et al, , 2011Hartinger et al, 2013;Zhao et al, 2017a). They have been observed at the bow shocks of Mercury (Uritsky et al, 2014), Venus (Collinson et al, 2014(Collinson et al, , 2015, Mars (Collinson et al, 2012), Jupiter (Valek et al, 2017), and Saturn (Masters et al, 2008(Masters et al, , 2009. They are frequent (>3 per day, Facskó et al, 2008Facskó et al, , 2009Schwartz et al, 2000;∼7 can be observed within a 12-hr interval, Zhang et al, 2010).…”

Propagation characteristics of young hot flow anomalies near the bow shock: Cluster observations

“…Thomas et al [1991] reported that the HFAs were generated by the interaction of the bow shock and tangential discontinuities upstream from the Earth's bow shock. Besides the Earth's bow shock, HFAs have also been observed at Saturn's bow shock [Masters et al, 2008[Masters et al, , 2009, at Venus [Slavin et al, 2009;Collinson et al, 2012Collinson et al, , 2014, at Mars [Øieroset et al, 2001], and at Mercury [Uritsky et al, 2014]. The hot tenuous hole is formed when the motional electric field is pointing toward the discontinuity [Thomas et al, 1991].…”