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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.

This paper reports on the standing whistler waves upstream of Mercury's quasi‐perpendicular bow shock. Using MESSENGER's magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (a) a constant phase with respect to the shock, (b) propagation along the normal direction to the shock surface, and (c) rapid damping over a few wave periods. We inferred the speed of Mercury's bow shock as ∼26 km/s and a shock width of 1.87 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of the bow shock. Our results further support the theory that these waves are generated by the current in the shock. Plain Language Summary The strength of planetary bow shocks varies with the planet's heliocentric distance from the Sun. Studying the bow shocks of other planets is important for extending our understanding of collisionless‐shock physics. In the solar system, the bow shocks of Mercury are unique as they are produced by low Mach numbers and low plasma beta solar wind blowing over a small magnetized body that is 1–2 orders smaller than Earth. The standing whistler waves upstream of the bow shock of Mercury were determined through statistical analyses. Similar to the observations at Earth, these waves were rapidly damping with a proportion of the wave periods; however, the damping distance at the spacecraft frame was considerably shorter at only a few kilometers upstream in the small‐scale bow shock of Mercury. The high occurrence rate of standing whistler waves suggests that Mercury's bow shock is a natural plasma laboratory, which can be used to further investigate low MA planetary shocks during the upcoming BepiColombo mission. Key Points First survey of standing whistler waves upstream of Mercury's bow shock Standing whistler waves are common at Mercury during interplanetary coronal mass ejections Our results support the theory that current in shock generates standing whistler waves

Hot flow anomalies (HFAs) at Earth's bow shock were identified in Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellite data from 2007 to 2009. The events were classified as young or mature and also as regular or spontaneous hot flow anomalies (SHFAs). The dataset has 17 young SHFAs, 49 mature SHFAs, 15 young HFAs, and 55 mature HFAs. They span a wide range of magnetic local times (MLTs) from approximately 7 to 16.5MLT. The largest ratio of solar wind to HFA core density occurred near dusk and at larger distances from the bow shock. In this study, HFAs and SHFAs were observed up to 6.3 RE and 6.1 RE (Earth radii), respectively, upstream from the model bow shock. HFA-SHFA occurrence decreases with distance upstream from the bow shock. HFAs of the highest event core ion temperatures were not seen at the flanks. The ratio of HFA ion temperature increase to HFA electron temperature increase is highest around 12MLT and slightly duskward. For SHFAs, (Tihfa=Tisw)/(Tehfa=Tesw) generally increased with distance from the bow shock. Both mature and young HFAs are more prevalent when there is an approximately radial interplanetary magnetic field. HFAs occur most preferentially for solar wind speeds from 550 to 600 km s-1. The correlation coefficient between the HFA increase in thermal energy density from solar wind values and the decrease in kinetic energy density from solar wind values is 0.62. SHFAs and HFAs do not show major differences in this study.

It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than ~6 RE transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.

The present study examines the interaction of solar wind discontinuities with the Earth's bow shock, using multipoint observations in the magnetosheath by Time History of Events and Macroscale Interactions During Substorms (THEMIS), Cluster, and Double Star TC1. We focus on the deformation and evolution of two discontinuities observed on 21 June 2007, one of which involves a density increase and a magnetic field decrease, while the other is accompanied by a density decrease and a magnetic field increase. In the magnetosheath, the discontinuities are deformed into a concave shape; that is, the normal is inclined toward dusk (dawn) on the dawnside (duskside). The density‐increase (‐decrease) discontinuity is being compressed (expanded) as it propagates in the magnetosheath. We conclude that the compression (expansion) is due to antisunward (sunward) motion of the bow shock which is initiated or enhanced by the impact of the discontinuity on the bow shock. The steepening of Bz reversal followed by an overshoot of the total magnetic field, which appears at the trailing edge of the density‐decrease discontinuity, is also discussed.

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.

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

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.

Collisionless shocks can exhibit non‐stationary behavior even under steady upstream conditions, forming a complex transition region. Ion phase‐space holes, linked to shock self‐reformation and surface ripples, are a signature of this non‐stationarity. We statistically analyze their occurrence using 521 crossings of Earth's quasi‐perpendicular bow shock. Phase‐space holes appear in 65% of cases, though the actual rate may be higher as the holes may not be resolved during fast shock crossings. The occurrence rate peaks at 70% for shocks with Alfvén Mach numbers MA>7${M}_{A} > 7$ . These findings suggest that Earth's quasi‐perpendicular bow shock is predominantly non‐stationary.

A global magnetohydrodynamic model predicts the response of the magnetosphere to the passage of a foreshock transient. We simulate the transient as an antisunward‐ and dawnward‐moving slab of hot tenuous solar wind plasma and weak magnetic field strengths on magnetic field lines connected to the bow shock. The slab elicits large‐amplitude outward bow shock motion with a stronger jump in plasma and magnetic field parameters on the trailing than the leading edges of this motion. The outward bulge in the bow shock bounds a magnetosheath region containing a hot tenuous plasma with weakened magnetic field strengths and flows deflected away from the Sun‐earth line. The magnetopause bulges outward into this magnetosheath region to distances beyond the nominal bow shock. Despite the large amplitude magnetopause motion, perturbations at geosynchronous orbit are miniscule. Model predictions compare well to the observed characteristics of foreshock transients and their effects on the magnetosphere. Plain Language Summary The arrival of a dawnward or duskward‐moving slab of very hot and tenuous solar wind plasma creates large amplitude but localized outward bulges in the bow shock wave that stands upstream from the Earth's magnetosphere. The magnetopause, or outermost boundary of the magnetospheric magnetic field, protrudes several Earth radii outward to fill the cavity created by these bulges. Both the bow shock bulge and the magnetopause protrusion move dawnward or duskward with the slab along the locus of points connecting it to the bow shock. Despite the large amplitude bow shock and magnetopause motion, the passage of a slab produces only modest signatures at dayside geosynchronous orbit. Key Points A global magnetohydrodynamic model simulates the interaction of a low density slab of solar wind plasma and weak radial magnetic fields with the bow shock The interaction generates a hot tenuous transient foreshock event with weak magnetic fields bounded by shocks A large amplitude wave on the magnetopause that extends far upstream into this transient event elicits no strong geosynchronous signature