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Abstract 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.
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Impact of Foreshock Transients on the Flank Magnetopause and Magnetosphere and the Ionosphere
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.
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- Award ID(s):
- 1941012
- PAR ID:
- 10301175
- Date Published:
- Journal Name:
- Frontiers in Astronomy and Space Sciences
- Volume:
- 8
- ISSN:
- 2296-987X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fspas.2021.751244
