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Solar wind directional discontinuities can generate transient mesoscale structures such as foreshock bubbles and hot flow anomalies (HFAs) upstream of Earth's bow shock. These structures can have a global impact on near‐Earth space, so understanding their formation conditions is essential. We investigate foreshock transient generation at a rotational discontinuity using a global 2D hybrid‐Vlasov simulation. As expected, a foreshock bubble forms on the sunward side of the discontinuity. Later, when the discontinuity reaches the shock, new structures identified as HFAs develop, despite the initial discontinuity not being favorable to HFA formation. We demonstrate that the foreshock bubble provides the necessary conditions for their generation. We then investigate the evolution of the transient structures and the large‐scale bow shock deformation they induce. Our results provide new insights on the formation and evolution of foreshock transients and their impact on the shock.

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 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.

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.

We investigate the spatiotemporal evolution of foreshocks preceding the 2025 Mw 8.8 Kamchatka earthquake by applying single‐station analysis to waveforms recorded at the nearest seismic station. Our results reveal a southwestward migration of foreshocks toward the mainshock epicenter through multiple stages, with a migration speed of ∼7.0 km/day, comparable to that of slow slip events along subduction zones, suggesting that aseismic slip may accompany the foreshock activity. The foreshocks occurred near the southern edge of the reduced kinematic locking of plate interface, where the relatively young and warmer slab is subducting. Repeated migrating transients likely acted as efficient stress transfers, potentially associated with slow slip, progressively loading the nucleation area and promoting rupture initiation, while also contributing to the progressive weakening of the locked megathrust. Together, these observations support a model in which migrating foreshocks reflects progressive stress redistribution that prepares the fault for catastrophic failure.

At Mars, the MAVEN spacecraft has made observations of Hot Flow Anomalies (HFAs) in the foreshock. Due to the bow shock's proximity to the planet, it is theorized that HFAs contribute to atmospheric escape at Mars through the excavation of ionospheric ions. A case study investigates one HFA observation, with parameters suggesting a novel mechanism for planetary ion extraction. The event is further characterized by elevated number densities of O+${\\mathrm{O}}^{+}$and O2+${\\mathrm{O}}_{2}^{+}$ions observed prior to the current sheet crossing. Statistical study is conducted on a set of 91 events, facilitating the estimation of HFA frequency at Mars to be 1/day. The estimated ion escape of the event is approximately 9%$\\%$of the typical ion escape rate under nominal conditions at Mars. Calculation of further events reveals escape rates between 1 and 9%$\\%$of the nominal conditions. This represents a modest contribution to the overall escape, highlighting a potentially underexplored pathway.

Laboratory experiments and theoretical models suggest that earthquakes are preceded by extended nucleation phases, perhaps by slow but accelerating slip. However, such nucleation phases are hard to observe before natural earthquakes. Here we identify clustered foreshock sequences that could be nucleation signatures. We develop a coherence‐based power metric to detect foreshock sequences along the San Jacinto fault zone (SJFZ) and then track the temporal evolution of foreshocks' moment‐rate power. The results show that a small but significant fraction of M ≥ 2.5 earthquakes (19 out of 681) are preceded by 5 to 20‐s‐long clustered foreshock sequences, which may reflect extended nucleation phases. The sequences preferentially occur near the base of the seismogenic zone, which likely contains frictionally heterogeneous patches of varying sizes. We identify a build‐up of 2‐8‐Hz moment‐rate power during the sequences and consider some interpretations: that the growing power reflects accelerating aseismic slip or growing cascades of ruptures.

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.

In this study, we identified 51 dayside diffuse auroral patches and examined their two‐dimensional evolutions by using the Time History of Events and Macroscale Interactions during Substorms probes and the ground‐based all‐sky imager at the South Pole. Two typical events show diffuse auroral patches associated with upstream dynamic pressure enhancements of the bow shock and magnetospheric compressions, followed by their east–west propagations. The statistical results suggest that most conjunction events were associated with foreshock activities, while the remaining events were associated with dynamic pressure enhancements in the pristine solar wind. These azimuthal motions can be either eastward or westward, with initial locations at ∼12–13 and ∼9–10 Magnetic Local Time, respectively, exhibiting a dawn‐dusk asymmetry. Additionally, poleward motions were found in all events. Larger dynamic pressure enhancements correspond to faster poleward motions and could push the initial diffuse auroral brightening toward lower latitudes. These characteristics of their poleward motions were consistent with the Tamao path.

Foreshocks, though well‐documented phenomena preceding many large earthquakes, have limited forecasting utility due to their non‐pervasive occurrence and non‐distinctive characteristics. Using California as an example, we investigate how seismic monitoring capability, particularly the completeness magnitude (Mc${M}_{c}$ ), influences the inferred proportion of mainshocks with foreshocks (Pf${P}_{f}$ ). We test four foreshock identification methods, namely the fixed‐window, nearest neighbor clustering, empirical statistical (ES) methods and the epidemic‐type aftershock sequence (ETAS) model. The fixed‐window method shows Pf${P}_{f}$decreasing with higher Mc${M}_{c}$due to the misclassification of background events as foreshocks. In contrast, clustering and ES methods yield relatively stable Pf${P}_{f}$across different Mc${M}_{c}$values. The ETAS model suggests that many foreshocks in California are associated with aseismic driving processes, but the identification of the processes diminishes at high Mc${M}_{c}$ . These results show that improved seismic monitoring capability does not significantly increase Pf${P}_{f}$but is crucial for distinguishing processes driving foreshocks. Plain Language Summary Foreshocks are seismic events that sometimes occur before large earthquakes. However, they are not always present and do not have clear distinguishing features, limiting their usefulness for earthquake forecasting. We examine how the earthquake monitoring capability affects the observed proportion of large earthquakes that have foreshocks. Using seismic data from California, we apply four foreshock identification methods: the fixed‐window method, nearest neighbor clustering, empirical statistical (ES) methods, and the epidemic‐type aftershock sequence (ETAS) model. Our results show that the fixed‐window method leads to less observations of large earthquakes with foreshocks when the monitoring capability is worse. In contrast, clustering and ES methods provide more stable proportions of large earthquakes with foreshocks even when the monitoring capability varies. And the ETAS model suggests that many foreshocks in Califorina are associated with aseismic processes. However, poor monitoring capability limits the ability to distinguish between foreshocks driven by aseismic processes and those triggered by cascading seismic failure through stress transfer. These findings indicate that while enhanced seismic monitoring does not necessarily lead to a higher proportion of identified foreshocks, it is essential for understanding the underlying physical mechanisms driving foreshock activity. Key Points We assess the impact of magnitude of completeness (Mc${M}_{c}$ ) on multiple foreshock identification methods in California Foreshock dependence on Mc${M}_{c}$is weak in some methods because the magnitude difference between mainshock and largest foreshock is small Foreshocks driven by aseismic processes are numerous in California, but their detectability diminishes at high Mc${M}_{c}$