Search for: All records

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 Recent observations show very near‐Earth reconnection (∼8–13R_E) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30R_E). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future. 

Abstract The ion foreshock is very dynamic, characterized by various transient structures that can perturb the bow shock and influence the magnetosphere‐ionosphere system. One important driver of foreshock transients is solar wind directional discontinuities (DDs) that demagnetize foreshock ions leading to a local current. If this current decreases the field strength at the DD, a hot flow anomaly (HFA) can form. Recent hybrid simulations found that when the current increases the field strength at the DD, a compressional structure forms with enhanced density and field strength opposite to HFAs. Using MMS and THEMIS observations, we confirm this situation. We demonstrate that the current geometry driven by the foreshock ions plays a critical role in the formation. The initial gyrophase of foreshock ions, due to their specular reflection, determines whether they can cross the DD. When many of the foreshock ions cannot cross the DD and the local current they drive increases the field strength at the DD, the enhanced field strength inhibits more foreshock ions from crossing the DD, further enhancing the local current. This feedback loop promotes the growth of the compressional structure. Such foreshock ion‐driven compressional structures can result in dynamic pressure enhancements in the magnetosheath, leading to magnetosheath jets. Our study enables prediction of the location and formation probability of such compressional structures and their potential geoeffectiveness. 

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 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 The ion foreshock is highly dynamic, disturbing the bow shock and the magnetosphere‐ionosphere system. To forecast foreshock‐driven space weather effects, it is necessary to model foreshock ions as a function of upstream shock parameters. Case studies in the accompanying paper show that magnetosheath ions sometimes exhibit strong field‐aligned asymmetry toward the upstream direction, which may be responsible for enhancing magnetosheath leakage and therefore foreshock ion density. To understand the conditions leading to such asymmetry and the potential for enhanced leakage, we perform case studies and a statistical study of magnetosheath and foreshock region data surrounding ∼500 Time History of Events and Macroscale Interactions during Substorms mission bow shock crossings. We quantify the asymmetry using the heat flux along the field‐aligned direction. We show that the strong field‐aligned heat flux persists across the entire magnetosheath from the magnetopause to the bow shock. Ion distribution functions reveal that the strong heat flux is caused by a secondary thermal population. We find that stronger asymmetry events exhibit heat flux preferentially toward the upstream direction near the bow shock and occur under larger IMF strength and larger solar wind dynamic pressure and/or energy flux. Additionally, we show that near the bow shock, magnetosheath leakage is a significant contributor to foreshock ions, and through enhancing the leakage the magnetosheath ion asymmetry can modulate the foreshock ion velocity and density. Our results imply that likely due to field line draping and compression against the magnetopause that leads to a directional mirror force, modeling the foreshock ions necessitates a more global accounting of downstream conditions. 

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 A variety of magnetosphere‐ionosphere current systems and waves have been linked to geomagnetic disturbance (GMD) and geomagnetically induced currents (GIC). However, since many location‐specific factors control GMD and GIC intensity, it is often unclear what mechanisms generate the largest GMD and GIC in different locations. We address this challenge through analysis of multi‐satellite measurements and globally distributed magnetometer and GIC measurements. We find embedded within the magnetic cloud of the 23–24 April 2023 coronal mass ejection (CME) storm there was a global scale density pulse lasting for 10–20 min with compression ratio of . It caused substantial dayside displacements of the bow shock and magnetopause, changes of and , respectively, which in turn caused large amplitude GMD in the magnetosphere and on the ground across a wide local time range. At the time this global GMD was observed, GIC measured in New Zealand, Finland, Canada, and the United States were observed. The GIC were comparable (within factors of 2–2.5) to the largest ever recorded during 14 year monitoring intervals in New Zealand and Finland and represented 2‐year maxima in the United States during a period with several Kp7 geomagnetic storms. Additionally, the GIC measurements in the USA and other mid‐latitude locations exhibited wave‐like fluctuations with 1–2 min period. This work suggests that large density pulses in CME should be considered an important driver of large amplitude, global GMD and among the largest GIC at mid‐latitude locations, and that sampling intervals are required to capture these GMD/GIC. 

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 When a solar wind discontinuity interacts with foreshock ions, foreshock transients such as hot flow anomalies and foreshock bubbles can form. These create significant dynamic pressure perturbations disturbing the bow shock, magnetopause, and magnetosphere‐ionosphere system. However, presently these phenomena are not predictable. In the accompanying paper, we derived analytical equations of foreshock ion partial gyration around a discontinuity and the resultant current density. In this study, we utilize the derived current density strength to model the energy conversion from the foreshock ions, which drives the outward motion or expansion of the solar wind plasma away from the discontinuity. We show that the model expansion speeds match those from local hybrid simulations for varying foreshock ion parameters. Using MMS, we conduct a statistical study showing that the model expansion speeds are moderately correlated with the magnetic field strength variations and the dynamic pressure decreases around discontinuities with correlation coefficients larger than 0.5. We use conjunctions between ARTEMIS and MMS to show that the model expansion speeds are typically large for those already‐formed foreshock transients. Our results show that our model can be reasonably successful in predicting significant dynamic pressure disturbances caused by foreshock ion‐discontinuity interactions. We discuss ways to improve the model in the future.