Eva Krämer

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.

In the cometary plasma environment, the solar wind interacts with the expanding coma of heavy cometary ions. At high gas production rates, it can have plasma environment characteristics similar to the environment of Mars, with a bow shock and a cometosheath. Magnetosheath jets are transient enhancements in the dynamic pressure of the magnetosheath plasma. They can be generated at a disturbed bow shock surface and have been confirmed at Mars, Earth, and possibly Jupiter and Mercury. We have adapted commonly used methods to find magnetosheath jets to the Rosetta data at 67P/Churyumov–Gerasimenko and find candidate events throughout the postperihelion mission. We show that the identified density enhancements have similarities to magnetosheath jets. The events are of typical duration and density enhancement, and occur at a time when the bow shock is thought to be close to the spacecraft. We found a cometary ion velocity enhancement, although no conclusion on the solar wind velocity can be reached due to lack of data. These jet-like features are embedded in the heavy-ion population. We hypothesize that jets are formed in the solar wind ion population in the cometosheath, and then, as the two plasmas interact, are transferred to the cometary ion population. While we cannot rule out other phenomena that could cause these jet-like enhancements, they bear a remarkable resemblance to magnetosheath jets at Mars and Earth. If they are indeed jets, then this is the first time jets have been shown to exist in a plasma environment dominated by heavy ions.

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.

The magnetopause marks the boundary where the pressure of the magnetosheath balances the magnetic pressure of Earth’s magnetic field. Localized increases in magnetosheath pressure can therefore displace the magnetopause. One known plasma structure that exhibits such a local pressure increase is magnetosheath jets defined as transient dynamic pressure enhancements. These jets are particularly prominent downstream of the quasi-parallel bow shock, where they frequently interact with the magnetopause. To investigate the spatial and temporal characteristics of these interactions, we employ the 3D hybrid-kinetic plasma model Amitis with a steady plasma inflow. Our results show that jets downstream of the quasi-parallel bow shock can generate magnetopause surface waves that propagate flankward. Interestingly, while jets appear to have a similar velocity as the surface waves and continuously drive them, the jets’ apparent flankward motion does not align with the ion bulk velocity measured within the jets. This discrepancy may suggest ongoing jet generation at the bow shock, potentially linked to foreshock compressive structures.

As comets journey through the solar system, ices on the surface sublimate and the released neutral gas is ionized. Thus, the comet nucleus is surrounded by a cloud of heavy ions and electrons that interact with the solar wind. Their environment therefore is a unique laboratory to study plasma pick-up processes and multi-ion plasmas. In turn, remote observations of comets with imaging telescopes on ground and in space provide information about the solar wind properties at the comet. Comets are therefore laboratory and measurement at the same time. Studying the impact of solar wind structures like corotating interaction regions and interplanetary coronal mass ejections can therefore inform our knowledge of the processes in a collisional plasma. Active comets have a highly disturbed bow shock that can give rise to a number of cometosheath structures, one of which are magnetospheric jets: regions of enhanced dynamic pressure in the magnetosheath. They are usually associated to the region behind a quasi-parallel shock and have been well documented at Earth.However, in recent years there have been efforts to identify these structures in other magnetosheaths as well. For example, recently it has been shown that they exist in the Martian magnetosheath. As the cometary plasma environment has very similar characteristics as the Martian one, it stands to reason that these structures also exist at Comets.We present a study that uses Rosetta magnetic field, density and ion measurements to identify possible jet structures.We find that they are ubiquitous especially in the plasma environment where Rosetta is probably in the cometosheath. Their occurrence matches what was found for similar events at Mars. The magnetic field can correlate or anti-correlate with the density enhancement, just like it is observed at Earth and Mars. Although velocity data is limited, at least some of the events show a velocity increase in the cometary ion population. This could be the first detection of a magnetosheath structure in the non-solar wind component of the plasma.

The magnetopause marks the boundary where the pressure of the magnetosheath balances the magnetic pressure of Earth's magnetic field. Localized increases in magnetosheath pressure can therefore displace the magnetopause. One known plasma structure that exhibits such a local pressure increase is magnetosheath jets defined as transient dynamic pressure enhancements. These jets are particularly prominent downstream of the quasi-parallel bow shock, where they frequently interact with the magnetopause. To investigate the spatial and temporal characteristics of these interactions, we employ the 3D hybrid-kinetic plasma model Amitis with a steady plasma inflow. Our results show that jets downstream of the quasi-parallel bow shock can generate magnetopause surface waves that propagate flankward. Interestingly, while jets appear to have a similar velocity as the surface waves and continuously drive them, the jets apparent flankward motion does not align with the ion bulk velocity measured within the jets. This discrepancy may suggest ongoing jet generation at the bow shock, potentially linked to foreshock compressive structures. Additionally, we find that jets might also contribute to triggering the development of the Kelvin-Helmholtz instability, leading to the formation of vortices that extend further tailward.

Magnetosheath jets, plasma structures with enhanced dynamic pressure, are frequently observed in the terrestrial magnetosheath. However, their mass, momentum, and energy content are still unknown. We utilize Amitis, a 3D hybrid‐kinetic plasma simulation, to study the mass, momentum, and energy content of jets in the subsolar magnetosheath. We also analyze the kinetic, thermal, and electromagnetic energy flux associated with jets. Jets comprise up to 21% of the quasi‐parallel magnetosheath and can carry up to half of the kinetic energy. Furthermore, jets convert kinetic energy to thermal energy. Our hybrid simulations also suggest that while jets can form downstream of the quasi‐perpendicular shock, their volume and energy content are much small compared to jets downstream of the quasi‐parallel bow shock. We conclude that magnetosheath jets play a vital role in heating up the magnetosheath and significantly influence the dynamics of the quasi‐parallel magnetosheath.

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 .

Magnetosheath jets, transient plasma structures of enhanced dynamic pressure, have been observed to trigger ultra‐low frequency (ULF) waves in the magnetosphere. These ULF waves contribute to energy transport in the magnetosphere‐ionosphere system. Therefore, there is a need to estimate the energy input into the ionosphere due to jet‐triggered ULF waves. In this study, we combine measurements from Magnetospheric Multiscale, ground‐based magnetometers, the EISCAT radar on Svalbard, and SuperDARN to estimate the Joule heating in the ionosphere resulting from jet impacts at the magnetopause. Focusing on three jets observed on 2016‐01‐07 we were able to calculate the Joule heating for two jets. We found an average Joule heating rate of mW/m² which is on par with other processes such as field line resonances. However, due to the short duration and spatial confinement of the jet‐induced ULF waves, the average energy input was only J. This suggests that the energy deposition of jet‐triggered ULF waves is small compared to other magnetospheric processes, and thus does not significantly impact the average energy budget of the magnetosphere.

Shock-generated transients, such as hot flow anomalies (HFAs), upstream of planetary bow shocks, play a critical role in electron acceleration. Using multimission data from NASA’s Magnetospheric Multiscale and ESA’s Cluster missions, we demonstrate the transmission of HFAs through Earth’s quasi-parallel bow shock, accelerating electrons to relativistic energies in the process. Energetic electrons initially accelerated upstream are shown to remain broadly confined within the transmitted transient structures downstream, where they get further energized due to the elevated compression levels potentially by betatron acceleration. Additionally, high-speed jets form at the compressive edges of HFAs, exhibiting a significant increase in dynamic pressure and potentially contributing to further localized compression. Our findings emphasize the efficiency of quasi-parallel shocks in driving particle acceleration far beyond the immediate shock transition region, expanding the acceleration region to a larger spatial domain. Finally, this study underscores the importance of a multiscale observational approach in understanding the convoluted processes behind collisionless shock physics and their broader implications.