Foreshock transients such as hot flow anomalies (HFAs) are frequently observed in the dayside foreshock. They can disturb the local bow shock, magnetopause, and consequently the magnetosphere‐ionosphere system through dynamic pressure perturbations. Recent multipoint observations found that such perturbations can even propagate from the dayside to the midtail. However, whether the drivers of such perturbations, foreshock transients, persist in the midtail foreshock has not been observed. Thus, it is unclear whether the observed nightside magnetosheath/magnetopause perturbations are traveling waves or continuously driven by a propagating foreshock transient. Using two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft, we report direct observational evidence of foreshock transients in the midtail foreshock. We present a case study showing an elongated mature HFA propagating with its driver discontinuity from TH‐C (X ~ −43 RE) to TH‐B (X ~ −48 RE). Our results confirm that foreshock transients disturb not only the dayside bow shock but also the nightside bow shock while propagating tailward.
Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth’s radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions.
more » « less- NSF-PAR ID:
- 10370129
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Space Science Reviews
- Volume:
- 218
- Issue:
- 5
- ISSN:
- 0038-6308
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Foreshock transients can result in significant dynamic pressure perturbations downstream, causing the magnetopause to move locally outward and inward. These near‐magnetopause phenomena in turn generate magnetospheric field‐aligned currents (FACs). FACs driven by solar wind impulses are commonly found to be due to flow vortices, but it remains unclear whether the FACs driven by those localized foreshock transients are contributed by flow vortices or pressure gradients. We report on a fortuitous conjunction between the Magnetospheric Multiscale (MMS) mission, which was observing a foreshock transient at the flank of the bow shock, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, immediately downstream of MMS, which was observing magnetopause disturbances arising from that transient. Using observations from the three THEMIS spacecraft to calculate local current density perturbations within the outward motion region of the magnetosphere, we find that flow vortices play a dominant role in generating the current there; the contribution from pressure gradients is one order of magnitude smaller. Using a global hybrid simulation that reproduces the observed foreshock transient perturbations, we traced the simulated FACs generated by the transient's interaction with the magnetopause. We find that in the outward magnetopause motion region the simulated FACs are driven by flow vortices, in agreement with THEMIS observations. Deeper inside the magnetosphere, the faster convection of bipolar flow vortices than the local magnetospheric flow leads to reversal of the simulated FACs. Our results improve our understanding of how foreshock transients disturb and energize the magnetosphere‐ionosphere system.
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Abstract In the dayside foreshock, many foreshock transients have been observed and simulated. Because of their strong dynamic pressure perturbations, foreshock transients can disturb the local bow shock, magnetosheath, magnetopause, and thus the magnetosphere‐ionosphere system. They can also accelerate particles contributing to shock acceleration. Recent observations and simulations showed that foreshock transients also exist in the midtail foreshock, which can continuously disturb the nightside bow shock, magnetosheath, and magnetopause while propagating tailward for tens of minutes. To further understand the characteristics of midtail foreshock transients, we studied them statistically using Acceleration Reconnection Turbulence & Electrodynamics of Moon’s Interaction with the Sun observations. We selected 111 events that have dynamic pressure decrease along the local bow shock normal by more than 50%. We show that the dynamic pressure decrease is contributed by both density decrease and speed decrease. Around 90% of the events have electron temperature increase by more than 10% with a temperature change ratio proportional to the solar wind speed. Midtail foreshock transients more likely occur at the dawnside than the duskside. They are more significant closer to the bow shock and rather stable along the tailward direction. They have similar formation conditions compared to the dayside foreshock transients, except the ones related to the bow shock geometry. Our study indicates that the characteristics of foreshock transients based on dayside observations need to be generalized. Our study also implies that foreshock transients can exist for tens of minutes (even longer for larger planar shocks), continuously disturbing the local shock and accelerating/heating particles.
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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.
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