Global Asymmetries of Hot Flow Anomalies

A three‐dimensional global hybrid simulation is conducted for the interaction of the Earth's bow shock with an interplanetary directional tangential discontinuity (TD) whose normal lies in the equatorial plane. Although the convection electric field points into the TD from its trailing side both north and south of the equator, the interaction only generates a hot flow anomaly (HFA) in the southern hemisphere. HFA generation results from bow shock‐reflected ions energized by the inward‐pointing electric field on one side of the TD and inward ion gyration on the other side. The latter only occurs south of the equator due to the global bow shock geometry. The global HFA asymmetry is driven by a reversal in the north‐south component of reflected ion velocities in the two hemispheres, such that ions gyrate into (away from) the TD in the southern (northern) hemisphere. Our results indicate HFAs must generally exhibit north‐south and dawn‐dusk asymmetries. Plain Language Summary Interplanetary magnetic field (IMF) discontinuities frequently occur in the solar wind. When interplanetary tangential discontinuities (TDs) interact with the terrestrial bow shock, localized large‐amplitude transient structures known as hot flow anomalies (HFAs) can be generated and cause significant perturbations in the magnetosphere, even though the pristine TD exhibits no plasma variations. Previous studies based on satellite observations and global hybrid simulations suggest that HFAs occur when convection electric fields point toward the TD on at least one side, a condition under which the bow shock ions are likely to be injected and accumulated in the TD. The previous global simulations, however, were based on 2‐D assumptions. In this paper, we conduct a 3‐D global hybrid simulation for the interaction of the bow shock with an interplanetary directional TD, in which the IMF is symmetric about the equator on both sides of the TD. Although the convection electric field points into the TD from one side of the discontinuity at locations both north and south of the equator, the interaction only generates an HFA in the southern hemisphere. By examining ion velocity distributions, a mechanism is proposed for the global asymmetry of the HFA. Our simulation indicates that HFAs must generally exhibit north‐south and dawn‐dusk asymmetries. Key Points Global hybrid model predicts that hot flow anomalies (HFAs) generally exhibit north‐south and dawn‐dusk global asymmetries due to the 3‐D bow sh