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Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are two types of transient phenomena characterized by flow deflected and hot cores bounded by one or two compressional boundaries in the foreshock. Using conjunction observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we present an MHD HFA with a core filled with magnetosheath material around the bow shock and a typical kinetic FB associated with foreshock ions upstream of the bow shock, occurring simultaneously under the same solar wind/interplanetary magnetic field (IMF) conditions. The displacements of the bow shock moving back and forth along the sun-earth line are observed. Electron energy shows enhancements from ∼50 keV in the FB to ∼100 keV in the HFA core, suggesting additional acceleration process across the bow shock within the transient structure. The magnetosheath response of an HFA core-like structure with particle heating and electron acceleration is observed by the Magnetospheric Multiscale (MMS) mission. Ultralow frequency waves in the magnetosphere modulating cold ion energy are identified by THEMIS, driven by these transient structures. Our study improves our understanding of foreshock transients and suggests that single spacecraft observations are insufficient to reveal the whole picture of foreshock transients, leading to an underestimation of their impacts (e.g., particle acceleration energy and spatial scale of disturbances). 

Abstract In the ion foreshock, there are many foreshock transients driven by back streaming foreshock ions. When the foreshock ions interact with tangential discontinuities (TDs), hot flow anomalies form if the foreshock ion‐driven current decreases field strength at TDs, but the opposite situation has been paid little attention. Using 2.5‐D local hybrid simulations, we show that a compressional boundary with enhanced field strength and density can form. We examine how the foreshock ions interact with TDs under various magnetic field geometries to drive currents that lead to compressional boundaries. The current driven by the foreshock ions should peak on its initial side of a TD so that the enhanced field strength at the TD in turn increases this current by keeping more foreshock ions on their initial side. Which side the current peaks can be determined by whether the foreshock ions initially cross the TD and/or how their velocity is projected into the local perpendicular direction. Additionally, the foreshock ion‐driven currents from two sides could compete, and whether a compressional boundary forms is determined by the net current profile. Because such compressive structures in the foreshock can drive magneto sheath jets and cause many geoeffects, it is necessary to fully understand their formation. 

Abstract Hot flow anomalies are ion kinetic phenomena that play an important role in geoeffects and particle acceleration. They form due to the currents driven by demagnetized foreshock ions around a tangential discontinuity (TD). To understand the profile of such currents around a TD with foreshock ions on both sides, we use 2.5‐D local hybrid simulations of TDs, interacting with a planar shock with various shock geometries. We find that the electric field direction relative to the TD plane provides information about how the foreshock ion‐driven currents affect the magnetic field around the TD. For TDs embedded in the quasi‐parallel shock on both sides, the foreshock ions from one side of TD can cross it determining the current profile on the other side. In contrast, for TDs embedded in the quasi‐perpendicular shock, sheath‐leaked ions enter the TD and determine the current profile. We find that the foreshock ultra‐low frequency waves can periodically modulate how foreshock ions interact with the TD and thus the current profile. Studying the effects of various magnetic field configurations allows us to build a more comprehensive model of hot flow anomalie formation. 

Abstract The ion foreshock, filled with backstreaming foreshock ions, is very dynamic with many transient structures that disturb the bow shock and the magnetosphere‐ionosphere system. It has been shown that foreshock ions can be generated through either solar wind reflection at the bow shock or leakage from the magnetosheath. While solar wind reflection is widely believed to be the dominant generation process, our investigation using Time History of Events and Macroscale Interactions during Substorms mission observations reveals that the relative importance of magnetosheath leakage has been underestimated. We show from case studies that when the magnetosheath ions exhibit field‐aligned anisotropy, a large fraction of them attains sufficient field‐aligned speed to escape upstream, resulting in very high foreshock ion density. The observed foreshock ion density, velocity, phase space density, and distribution function shape are consistent with such an escape or leakage process. Our results suggest that magnetosheath leakage could be a significant contributor to the formation of the ion foreshock. Further characterization of the magnetosheath leakage process is a critical step toward building predictive models of the ion foreshock, a necessary step to better forecast foreshock‐driven space weather effects. 

Abstract Shock waves are sites of intense plasma heating and charged particle acceleration. In collisionless solar wind plasmas, such acceleration is attributed to shock drift or Fermi acceleration but also to wave–particle resonant interactions. We examine the latter for the case of electrons interacting with one of the most commonly observed wave modes in shock environments, the whistler mode. Such waves are particularly intense in dynamic, localized regions upstream of shocks, arising from the kinetic interaction of the shock with solar wind discontinuities. These regions, known as foreshock transients, are also sites of significant electron acceleration by mechanisms not fully understood. Using in situ observations of such transients in the Earth’s foreshock, we demonstrate that intense whistler-mode waves can resonate nonlinearly with >25 eV solar wind electrons and accelerate them to ∼100–500 eV. This acceleration is mostly effective for the 50–250 eV energy range, where the accelerated electron population exhibits a characteristic butterfly pitch-angle distribution consistent with theoretical predictions. Such nonlinear resonant acceleration is very fast, implying that this mechanism may be important for injecting suprathermal electrons of solar wind origin into the shock region, where they can undergo further, efficient shock-drift acceleration to even higher energies. 

Abstract Thermalization and heating of plasma flows at shocks result in unstable charged-particle distributions that generate a wide range of electromagnetic waves. These waves, in turn, can further accelerate and scatter energetic particles. Thus, the properties of the waves and their implication for wave−particle interactions are critically important for modeling energetic particle dynamics in shock environments. Whistler-mode waves, excited by the electron heat flux or a temperature anisotropy, arise naturally near shocks and foreshock transients. As a result, they can often interact with suprathermal electrons. The low background magnetic field typical at the core of such transients and the large wave amplitudes may cause such interactions to enter the nonlinear regime. In this study, we present a statistical characterization of whistler-mode waves at foreshock transients around Earth’s bow shock, as they are observed under a wide range of upstream conditions. We find that a significant portion of them are sufficiently intense and coherent (narrowband) to warrant nonlinear treatment. Copious observations of background magnetic field gradients and intense whistler wave amplitudes suggest that phase trapping, a very effective mechanism for electron acceleration in inhomogeneous plasmas, may be the cause. We discuss the implications of our findings for electron acceleration in planetary and astrophysical shock environments. 

Mesoscale (on the scales of a few minutes and a few R E ) magnetosheath and magnetopause perturbations driven by foreshock transients have been observed in the flank magnetotail. In this paper, we present the 3D global hybrid simulation results to show qualitatively the 3D structure of the flank magnetopause distortion caused by foreshock transients and its impacts on the tail magnetosphere and the ionosphere. Foreshock transient perturbations consist of a low-density core and high-density edge(s), thus, after they propagate into the magnetosheath, they result in magnetosheath pressure perturbations that distort magnetopause. The magnetopause is distorted locally outward (inward) in response to the dip (peak) of the magnetosheath pressure perturbations. As the magnetosheath perturbations propagate tailward, they continue to distort the flank magnetopause. This qualitative explains the transient appearance of the magnetosphere observed in the flank magnetosheath associated with foreshock transients. The 3D structure of the magnetosheath perturbations and the shape of the distorted magnetopause keep evolving as they propagate tailward. The transient distortion of the magnetopause generates compressional magnetic field perturbations within the magnetosphere. The magnetopause distortion also alters currents around the magnetopause, generating field-aligned currents (FACs) flowing in and out of the ionosphere. As the magnetopause distortion propagates tailward, it results in localized enhancements of FACs in the ionosphere that propagate anti-sunward. This qualitatively explains the observed anti-sunward propagation of the ground magnetic field perturbations associated with foreshock transients.