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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

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

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.

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.

Magnetosonic (MS) waves are dominant plasma waves causing severe Martian ionospheric erosion. They are generally considered to originate upstream of Martian bow shock with frequencies near the upstream proton gyrofrequency. However, whether MS waves can be locally excited lacks theoretical analysis. Here we present an event of MS waves with frequencies above and closely related to the local proton gyrofrequency in the Martian ionosphere. Concurrently, ring beam hot proton distributions are observed due to the penetration of magnetosheath protons. By employing the observed plasma and magnetic field data, the calculated linear growth rates for MS waves agree well with the observed wave power spectra, demonstrating that they can be locally excited by unstable ring beam hot protons at Mars. Our results could be of great help in understanding the excitation of MS waves in a heavy ion‐rich environment around unmagnetized planets. Plain Language Summary Magnetosonic (MS) wave is one of the most important plasma waves contributing to the Martian atmospheric loss. They are generally considered to originate upstream of the Martian bow shock. Recent observations at Mars have shown that some MS waves with frequencies near the local proton gyrofrequency were accompanied by ring/shell‐like hot proton distributions. However, whether these waves can be locally excited by such protons lacks the support of the theoretical analysis. In this letter, on the basis of linear instability analysis, we show that MS waves with frequencies well above the local proton gyrofrequency can be locally excited by unstable ring beam hot protons in the Martian ionosphere. These results advance our knowledge of the MS wave excitation in a heavy ion‐rich environment around planets without an intrinsic magnetic field. Key Points Magnetosonic waves above the local proton gyrofrequency are observed in the Martian ionosphere Ring beam hot proton distribution associated with magnetosonic waves is formed by the penetration of solar wind protons simultaneously Magnetosonic waves are locally generated by the ring beam hot protons

Mirror-mode (MM) structures, characterized by intermittent sharp depressions or enhancements of magnetic magnitude, are key plasma-instability-driven phenomena and commonly observed in high-β and temperature-anisotropic space plasmas. Although MM structures have been well studied in Earth’s magnetosheath, their properties in Jupiter’s magnetosheath remain poorly understood due to the limited spatial and temporal coverage of previous missions. Here, by using Juno observations, we present the first comprehensive statistical analysis of MM structures in Jupiter’s magnetosheath. We investigate their spatial distribution and occurrence rate across a broad range of latitudes, thereby clarifying their global spatial characteristics. In addition, we examine the electron pitch-angle distributions associated with MM events to explore particle behavior and possible wave–particle interactions within these structures. These results advance our understanding of the formation and evolution of MM structures in giant planetary magnetosheaths and their related electron dynamics in Jupiter’s magnetosheath.

We present the analysis of 2,152 electrostatic solitary waves observed aboard the Magnetospheric Multiscale in the Earth's magnetosheath. The electric field of the solitary waves is predominantly bipolar and parallel to the local magnetic field. In contrast to previous reports, we reveal similar occurrence rates of solitary waves of positive and negative polarity of the electrostatic potential. Both types of solitary waves have spatial half‐widths of 10–150 m or 1–15 Debye lengths, amplitudes of the electrostatic potential of 0.01–1.5 V or 0.01%–1% of local electron temperature, and plasma frame speeds within ion thermal speed. We argue that the solitary waves are electron and ion holes produced separately in space or time by local processes, whose nature is, however, still elusive. We speculate that the solitary waves can mediate the energy between thermal electrons and ions in the Earth's magnetosheath and discuss other applications of the presented results. Plain Language Summary Spacecraft measurements in the Earth's magnetosheath showed that electric field fluctuations at electron scales are predominantly electrostatic and consist, in particular, of bipolar solitary waves, whose properties, nature, and origin have not been addressed statistically. This letter presents a statistical analysis of solitary waves collected in three magnetosheath intervals. In contrast to previous reports, we reveal similar occurrence rates of solitary waves of positive and negative polarity. We determine solitary wave properties and interpret these structures in terms of electron and ion holes produced by local processes, whose nature remains elusive. The plasma frame speeds of the solitary waves are within ion thermal speed, which makes them potentially efficient in mediating the energy between thermal electrons and ions and contributing to plasma heating processes. Since similar solitary waves have been observed in the solar wind and magnetosheaths of other planets, where electric field measurements are much more limited, the presented results enhance our capabilities of interpreting spacecraft measurements in other space plasma environments. Key Points Solitary waves of positive and negative polarity can occur equally frequently in the magnetosheath Both types of solitary waves are Debye‐scale structures with speeds within ion thermal speed and well below electron thermal speed The solitary waves are electron and ion holes produced by local processes operating separately in space or time

Electromagnetic ion cyclotron (EMIC) waves and mirror modes (MMs), both driven by ion temperature anisotropy, are commonly observed in planetary magnetosheaths. Conventional explanations for their co‐occurrence are largely based on linear instability theory in proton–electron plasmas, which requires comparable growth rates for the EMIC and MM instabilities. Magnetosheath plasmas, however, contain a fraction of heavy ions, and how such composition affects the coexistence of EMIC waves and MMs has been less explored. Using kinetic hybrid simulations with typical magnetosheath parameters, we show that although the presence of heavy ions suppresses the initial linear EMIC instability, EMIC waves arise as MMs develop. The evolving MMs generate flat‐top proton velocity distributions with enhanced resonant populations, which in turn excite EMIC waves. These results extend the conventional coexistence scenario of MMs and EMIC waves and reveal a new pathway for energy transfer among MMs, EMIC waves, and particles in magnetosheath plasmas.

Electromagnetic ion cyclotron (EMIC) and mirror-mode (MM) waves are widely observed in terrestrial and other planetary magnetosheaths. Even though linear theory supports that EMIC and MM waves may grow at comparable rates under suitable plasma conditions, their coexistence is rarely reported within magnetosheaths, primarily due to their similar free energy source from anisotropic plasmas. Using MAVEN spacecraft data, we present the first direct in situ observations of mixed MM and EMIC waves in the Martian magnetosheath. Our observations reveal the concurrent presence of EMIC waves and MM waves, both generated in the Martian magnetosheath. Unlike in the Earth’s magnetosheath, where strong plasma compressions at the quasi-perpendicular bow shock can drive the growth of EMIC or MM waves, our results suggest that, in the Martian magnetosheath, the substantial ion anisotropy to generate EMIC and MM waves is provided by both upstream bow shock compressions and ubiquitous ion pickup processes of newborn ions. This study offers new insights into the role of ion pickup processes in the excitation and growth of EMIC and MM waves within planetary magnetosheaths, particularly in magnetosheaths where both shock heating and newborn ion pickup processes provide the prevailing anisotropic plasma environment, as seen in the magnetosheaths of Venus, Mars, and comets.

Saturn’s magnetosheath provides a natural laboratory for studying the generation and evolution of low-frequency plasma waves that play a key role in regulating temperature anisotropy and mediating wave–particle interactions. Using magnetic field and plasma measurements from the Cassini spacecraft, we investigate the occurrence of mirror mode waves (MMWs) and electromagnetic ion cyclotron (EMIC) waves in relation to the large-scale motion of Saturn’s magnetosheath boundaries, namely, the bow shock and magnetopause. A systematic classification of spacecraft crossings reveals that although MMWs are pervasive across all dynamic conditions, their morphology transitions between peak-like and dip-like structures depending on the direction of boundary motion. EMIC waves occur preferentially during outward motion of the bow shock, particularly on the dawnside, where recent upstream pressure reductions may have driven boundary expansion and formed localized low-beta regions favorable for cyclotron instability. These findings highlight the importance of time-dependent boundary dynamics in shaping the wave environment of planetary magnetosheaths, with broader implications for energy dissipation and plasma stability in rapidly rotating planetary magnetospheres and other astrophysical plasmas with evolving boundaries.