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Magnetosheath jets with enhanced dynamic pressure are common in the Earth's magnetosheath. They can impact the magnetopause, causing deformation of the magnetopause. Here we investigate the 3‐D structure of magnetosheath jets using a realistic‐scale, 3‐D global hybrid simulation. The magnetosheath has an overall honeycomb‐like 3‐D structure, where the magnetosheath jets with increased dynamic pressure surround the regions of decreased dynamic pressure resembling honeycomb cells. The magnetosheath jets downstream of the bow shock region with θBn ≲ 20° (where θBn is the angle between the upstream magnetic field and the shock normal) propagate approximately along the normal direction of the magnetopause, while those downstream of the bow shock region with θBn ≳ 20° propagate almost tangential to the magnetopause. Therefore, some magnetosheath jets formed at the quasi‐parallel shock region can propagate to the magnetosheath downstream of the quasi‐perpendicular shock region. Plain Language Summary Magnetosheath jets are high‐speed transient structures frequently observed in the magnetosheath, and they can impact and dent the magnetopause. However, their three‐dimensional (3‐D) structure is still under debt despite decade‐long research. By performing high‐resolution, 3‐D numerical simulation, we reveal that the magnetosheath has an overall honeycomb‐like 3‐D structure where the jets surround regions with lower plasma velocity resembling honeycomb cells. Key Points Magnetosheath jets are studied by a realistic‐scale, 3‐D global hybrid simulation under a radial interplanetary magnetic field (IMF) The magnetosheath has a honeycomb‐like 3D structure where regions of increased dynamic pressure surround those of decreased dynamic pressure The magnetosheath jets formed at the quasi‐parallel shock can propagate to the magnetosheath downstream of the quasi‐perpendicular shock

When interplanetary magnetic field discontinuities interact with planetary bow shocks, hot flow anomalies (HFAs) form in the solar wind and can extend into the magnetosheath. Here we reconstruct the three‐dimensional geometry of an HFA bounded by two jet regions in the terrestrial magnetosheath. Using a previously established conceptual model of HFA evolution together with in situ measurements in the magnetosheath and pristine solar wind, we derive the structure's geometrical characteristics and show that its normal aligns with the discontinuity normal. It spans most of the dayside magnetosheath. Ground magnetometer data corroborate the reconstruction, revealing both the scale of the disturbance and its dusk‐to‐dawn propagation. Notably, one bounding jet reaches 11 RE${R}_{\\mathrm{E}}$in width, significantly larger than the sizes of typical magnetosheath jets reported in the literature.

Magnetopause position is controlled mainly by the solar wind dynamic pressure and north‐south interplanetary magnetic field component and these quantities are included in different empirical magnetopause models. We have collected about 50,000 of dayside magnetopause crossings observed by THEMIS in course of 2007–2019 and compared the observed magnetopause position with model prediction. The difference between observed and predicted magnetopause radial distance, Robs − Rmod is used for quantifying the model‐observation agreement. Its median values are well predicted for cases up to Robs ≈ 12 RE for all models but higher positive deviations are found for larger magnetopause distances, mainly under a nearly radial field and low dynamic pressure. The analysis reveals their connection with transient magnetopause displacements caused by strong sunward flows in the magnetosheath. We discuss the possible origin of the observed magnetosheath flow switching in terms of the interaction of magnetosheath jets with the magnetopause. Plain Language Summary Comparison of the observed magnetopause crossings with prediction of the magnetopause models reveals that under nearly radial interplanetary magnetic field can be the magnetopause observed several Earth radii farther from the Earth than the models predict. Comprehensive examination of several cases characterized by suitable spacecraft locations leads to the conclusion that the source of such magnetopause displacements is connected with the reformation of nearly parallel bow shock resulting in a strong antisunward jet in the magnetosheath. The jet creates a dip in the magnetopause surface that reverses its direction. The sunward flow in the magnetosheath pulls the magnetopause also sunward. Since these effects are transient in their nature, they cannot be captured by statistical magnetopause models. Key Points The magnetopause is often observed several RE upstream its nominal position under nearly radial IMF Extreme magnetopause displacements are accompanied with strong antisunward magnetosheath jets Reversal of the jet direction is associated with the magnetopause outward displacement

Plasma flows with enhanced dynamic pressure, known as magnetosheath jets, are often found downstream of collisionless shocks. As they propagate through the magnetosheath, they interact with the surrounding plasma, shaping its properties, and potentially becoming geoeffective upon reaching the magnetopause. In recent years (since 2016), new research has produced vital results that have significantly enhanced our understanding on many aspects of jets. In this review, we summarise and discuss these findings. Spacecraft and ground-based observations, as well as global and local simulations, have contributed greatly to our understanding of the causes and effects of magnetosheath jets. First, we discuss recent findings on jet occurrence and formation, including in other planetary environments. New insights into jet properties and evolution are then examined using observations and simulations. Finally, we review the impact of jets upon interaction with the magnetopause and subsequent consequences for the magnetosphere-ionosphere system. We conclude with an outlook and assessment on future challenges. This includes an overview on future space missions that may prove crucial in tackling the outstanding open questions on jets in the terrestrial magnetosheath as well as other planetary and shock environments.

Geoeffective magnetosheath plasma jets (those that interact with the magnetopause) are an important area of research and technology, since they affect the “space-weather” around the Earth. We identified such large-scale magnetosheath plasma jets with a duration of >30 s using plasma and magnetic data acquired from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) multi-spacecraft experiment during the years 2007 to 2009. We present a statistical survey of 554 of such geoeffective jets and elaborate on four mechanisms for the generation of these jets as the upstream solar wind structures of tangential discontinuities (TDs), rotational discontinuities (RDs), the quasi-radial interplanetary magnetic field (rIMF) and the collapsing foreshock (CFS) interrupting the rIMF intervals. We found that 69% of the jets are generated due to the interaction between interplanetary discontinuities (TD: 24%, RD: 25%, CFS: 20%) with the bow shock. Slow and weak jets due to the rIMF contributed to 31% of these jets. The CFS and rIMF were found to be similar in their characteristics. TDs and RDs contributed to most of the fast and powerful jets, with large spatial scales, which might be attributed to transient effects in the travelling foreshock.

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1 , 2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3 – 5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7 – 11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth’s turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling. Observations of electron-scale current sheets in Earth’s turbulent magnetosheath reveal electron reconnection without ion coupling, contrary to expectations from the standard model of magnetic reconnection.

The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.

Shocks are one of nature’s most powerful particle accelerators and have been connected to relativistic electron acceleration and cosmic rays. Upstream shock observations include wave generation, wave-particle interactions and magnetic compressive structures, while at the shock and downstream, particle acceleration, magnetic reconnection and plasma jets can be observed. Here, using Magnetospheric Multiscale (MMS) we show in-situ evidence of high-speed downstream flows (jets) generated at the Earth’s bow shock as a direct consequence of shock reformation. Jets are observed downstream due to a combined effect of upstream plasma wave evolution and an ongoing reformation cycle of the bow shock. This generation process can also be applicable to planetary and astrophysical plasmas where collisionless shocks are commonly found. Several mechanisms exist for formation of jets observed in Earth’s magnetosheath. Here, the authors show evidence of high-speed downstream flows generated at the Earth’s bow shock as a direct consequence of shock reformation, which is different than the proposed mechanisms.

This study presents comprehensive observations of intense high‐speed jets (HSJs) and their global impacts on the inner magnetosphere and ionosphere, using multi‐satellite and ground‐based observations. Cluster‐4, located near the bow shock, observed signatures associated with foreshock transients generated by a solar wind directional discontinuity. Downstream of the bow shock, THEMIS‐A, positioned post‐noon (∼12.8 MLT) in the magnetosheath, detected a sunward plasma flow prior to crossing into the magnetosphere. Nearly simultaneously, THEMIS‐E, inside the magnetosphere at ∼13.4 MLT, suddenly crossed into the magnetosheath and observed intense earthward HSJs. The strong compression of the magnetopause current sheet forced GOES‐13 to temporarily enter the magnetosheath, while SuperDARN radars registered enhanced poleward ionospheric convection and magnetometers detected signatures of westward currents. These sequential observations provide a rare, integrated demonstration of how an upstream foreshock disturbance transfers energy and momentum throughout the coupled magnetosphere‐ionosphere system.

The magnetopause is the boundary between the Earth's magnetosphere and the solar wind. Magnetosheath high‐speed jets can impact the magnetopause, causing local indentation and subsequently rebound. However, the comprehensive response of the magnetopause to the impact of a high‐speed jet remains unclear. In this study, we establish that the full spatiotemporal response pattern of the magnetopause to the impact of an isolated magnetosheath high‐speed jet can be characterized as an “Indentation‐Rebounce‐Relaxation” sequence from a statistical view. Based on the pressure balance, we estimate the spatial and temporal scales of the entire response process to range from 0.5 to 3.2 Earth radii and 0.9–4.7 min, respectively. Furthermore, we find that the interaction between the magnetopause and the high‐speed jet during the rebounce phase distorts the magnetopause, subsequently generating pairs of field‐aligned currents. These generated field‐aligned currents may flow to the ionosphere, potentially contributing to magnetosphere‐ionosphere coupling. Plain Language Summary The Earth's magnetopause, separating the Earth's magnetosphere from the solar wind, is usually distorted by upstream magnetosheath pressure perturbations. One of these is the magnetosheath high‐speed jets, which are localized dynamic pressure enhancements that can induce the local deformation of the magnetopause. Previous studies have found that magnetosheath high‐speed jets can interact with the magnetopause, causing local indentation and subsequent rebound. However, due to the limitations of in‐situ spacecraft observations, the complete response of the magnetopause to the impact of a high‐speed jet remains unknown. Using Magnetospheric Multiscale satellite data, we establish a comprehensive response pattern of the magnetopause to an HSJ for the first time based on the statistical analysis of multiple cases, which is described as a sequence of “Indentation‐Rebounce‐Relaxation”. This study will help us better understand the solar wind‐magnetosphere coupling process. Key Points The spatiotemporal response of the magnetopause to an isolated HSJ can be sequenced as ‘Indentation‐Rebounce‐Relaxation’ The estimated spatial and temporal scales of the magnetopause response range from 0.5 to 3.2 Earth radii and 0.9–4.7 min, respectively The high‐speed jet induces deformation of the magnetopause, subsequently generating pairs of field‐aligned currents