Oleksandr Goncharov

The bow shock current (BSC) flows on the bow shock due to the compression of the interplanetary magnetic field (IMF) across the shock. Although previous studies have shown that the BSC can connect to magnetospheric current systems via the magnetosheath, it is still not known how this happens. For southward IMF, there is both numerical and observational evidence of BSC closure through the equatorial magnetosheath. For east‐west IMF (IMF dominated by the ‐component), the closure has instead been suggested to take place at higher latitudes, but this has yet to be investigated. In this paper we present the first hybrid‐kinetic simulation results in support of BSC closure through the high‐latitude magnetosheath, for east‐west IMF. Our simulations indicate large‐scale current closure away from (toward) the magnetosphere in the northern (southern) hemisphere, both in the dayside and nightside magnetosheath, for IMF . This is consistent with the north‐to‐south flowing BSC under such conditions. Moreover, our results suggest that the BSC primarily crosses the magnetosheath behind the quasi‐perpendicular bow shock.

The bow shock current (BSC) flows on the bow shock due to the compression of the interplanetary magnetic field (IMF) across the shock. Although previous studies have shown that the BSC can connect to magnetospheric current systems via the magnetosheath, it is still not known how this happens. For southward IMF, there is both numerical and observational evidence of BSC closure through the equatorial magnetosheath. For east-west IMF (IMF dominated by the By-component), the closure has instead been suggested to take place at higher latitudes, but this has yet to be investigated. In this paper we present the first hybrid-kinetic simulation results in support of BSC closure through the high-latitude magnetosheath, for east-west IMF. Our simulations indicate large-scale current closure away from (towards) the magnetosphere in the northern (southern) hemisphere, both in the dayside and nightside magnetosheath, for IMF By > 0. This is consistent with the north-to-south flowing BSC under such conditions. Moreover, our results suggest that the BSC primarily crosses the magnetosheath behind the quasi-perpendicular bow shock.

Plasma structures with enhanced dynamic pressure know as jets are frequently observed in the magnetosheath. We examine a possible connection of these jets with structures reported in the foreshock like SLAMS or plasmoids. A statistical study of the relation of the upstream conditions and the occurrence rates of magnetosheath (MSH) jets and foreshock structures revealed that foreshock structures prefer the low interplanetary magnetic field and high Mach number whereas MSH jets are more frequently observed under extremely high solar wind density. The case studies of simultaneous observations in the foreshock and the magnetosheath have shown that the presence of foreshock structures is not a necessary condition for observations of strong MSH jets in the downstream and vice versa. We suggest that the source of a significant part of MSH jets is the bow shock reformation initiated by the changes of magnetic field orientation just in front of the bow shock.

Plasma structures with the enhanced dynamic pressure, density, and/or bulk speed, commonly referred to as magnetosheath (MSH) jets, can be detected downstream of both quasi-perpendicular and quasi-parallel bow shocks. Although their presence in the MSH is well established, their true three-dimensional morphology, internal structure, and characteristic scales remain under active debate. Recent simulation results, suggesting filamentary interconnected jet structures, contrast with the simplified “pancake” or cylindrical geometries often inferred from single-spacecraft observations, highlighting the need of multi-point studies.We present case studies of complex structure of MSH jets using coordinated measurements of THEMIS, MMS, and other missions. We focus on their spatial structure and temporal evolution as they propagate through the MSH, with attention to multi-spacecraft signatures in plasma and magnetic field parameters. Rather than drawing general conclusions, this work aims to illustrate the capabilities and limitations of multi-spacecraft observations for determination of jet morphology and evolution. We try to place individual events into the broader context of ongoing discussions. Particular attention is given to the interaction of the MSH jet with the magnetopause and its role in (localized) boundary dynamics.

Plasma structures with enhanced dynamic pressure, density or speed are often observed in Earth’s magnetosheath. These structures, known as jets and fast plasmoids, can be registered in the magnetosheath, downstream of both the quasi-perpendicular and quasi-parallel bow shocks Using measurements by the Magnetospheric Multiscale (MMS) spacecraft, Goncharov et al., (2020) showed similarities in the plasma properties of the jets and fast plasmoids. On the other hand, they pointed out that the different magnetic fields inside the structures suggest that the formation mechanisms are not the same. Previous studies established that foreshock structures can be a source of the jets (Raptis et al., 2022). Xirogiannopoulou et al. (2024) found that the subsolar foreshock contains several types of structures with enhanced density or/and magnetic field magnitude, like plasmoids, SLAMS and mixed structures. Following these results, we use multi-spacecraft data collected by THEMIS, Cluster, Magnetospheric Multiscale Spacecraft (MMS) and OMNI missions, and present analytical multi-spacecraft statistical and case studies on the connection between the activity around the bow shock. Based on our comparative analysis, we discuss features of jet-like structures, and their relation to the different phenomena in the foreshock.

Plasma waves play a central role in the solar wind–magnetosphere interaction, especially in the foreshock and magnetosheath, where reflected particles, shock processes, and turbulence shape their properties. While these regions have been widely studied, the way foreshock waves evolve as they encounter the bow shock and how their downstream signatures relate to the upstream conditions remain poorly understood. Using multi-spacecraft observations from the MMS and/or THEMIS missions, we examine wave activity in the foreshock and characterize their key properties and put our findings in context with previous statistical results. We also present a statistical analysis of wave activity in the magnetosheath and compare it with the simultaneous measurements in both regions to explore how wave signatures change across the bow shock. This approach provides a more complete picture of how plasma waves vary across regions and offers new insight into their evolution in the near-Earth environment.

Plasma structures with enhanced dynamic pressure know as jets are frequently observed in the magnetosheath. We examine a possible connection of these jets with structures reported in the foreshock like SLAMS or plasmoids. A statistical study of the relation of the upstream conditions and the occurrence rates of magnetosheath (MSH) jets and foreshock structures revealed that foreshock structures prefer the low interplanetary magnetic field and high Mach number whereas MSH jets are more frequently observed under extremely high solar wind density. The case studies of simultaneous observations in the foreshock and the magnetosheath have shown that the presence of foreshock structures is not a necessary condition for observations of strong MSH jets in the downstream and vice versa. We suggest that the source of a significant part of MSH jets is the bow shock reformation initiated by the changes of magnetic field orientation just in front of the bow shock.

The bow shock current (BSC) plays an important role in supplying the magnetosphere with solar wind energy, in particular during times of low solar wind magnetosonic Mach numbers. Since the magnetic pile‐up in the magnetosheath has to be maintained, the BSC cannot close locally, but must instead connect to magnetospheric current systems. However, the details of this closure remain poorly understood. For east–west interplanetary magnetic field (IMF) it has been hypothesized that the BSC partly closes to the high‐latitude ionosphere, as field‐aligned currents (FACs) on open field lines, but there is still no statistical evidence of this. In order to investigate this hypothesis, we use 9 years of Defense Meteorological Satellite Program (DMSP) data to construct normalized FAC maps of the northern hemisphere polar cap. We sort them according to different IMF clock angles, IMF magnitudes and magnetosonic Mach numbers. By separating opposite polarity FACs, we show that, on average, a unipolar FAC exists in the dayside polar cap when the IMF , regardless of the sign of the IMF . This current flows out of (into) the ionosphere in the northern hemisphere for IMF and is thus of the correct polarity to connect to the north–south component of the BSC. Moreover, it is strongest when the BSC flows predominantly in the north–south direction. These results constitute the first statistical evidence in support of at least a partial closure of the BSC to the ionosphere during non‐zero IMF .

Plasma structures with an enhanced dynamic pressure, known as jets are often observed in the Earth’s magnetosheath. These structures are more often detected downstream of the quasi-parallel bow shock, i.e., behind the foreshock. This region is dominated by waves and reflected particles which interact with each other and create different transients. Xirogiannopoulou et al. (2024) found that the subsolar foreshock contains several types of structures with enhanced density or/and magnetic field magnitude - plasmoids, SLAMS and mixed structures. Many previous studies established that some of these foreshock structures can be a source of magnetosheath jets (Raptis et al., 2022). Following these results, we use data collected by the cross-calibrated THEMIS spacecraft and present multi-spacecraft case studies of the connection between the foreshock and magnetosheath structures. According to our observations, we suggest that the generation of magnetosheath jets is associated with some additional mechanism from the ones we know (ex. BS ripples) that is more complicated or the knowledge we have is incomplete.

Transient enhancements in the dynamic pressure, density or speed are often observed in the Earth’s magnetosheath. They are known as jets and/or plasmoids and can be registered downstream both quasi-perpendicular and quasi-parallel bow shocks (BS). They travel from the BS through the magnetosheath and disturb the ambient plasma. Using measurements by the Magnetospheric Multiscale (MMS) spacecraft, Goncharov et al. (2020) showed similarities in the plasma properties of the jets and fast plasmoids. However, they pointed out that the different magnetic fields inside the structures suggest that the formation mechanisms are different. On the other hand, the parameters of structures registered close to the BS/magnetopause or in the sub-solar/flank magnetosheath differ. Based on our comparative analysis, we discuss features of jet-like structures, their properties, occurrence, evolution, and relation to the upstream/local parameters.