Yuqi Liu

Aims . Core and suprathermal electrons play a key role in the Venusian ionosphere and in plasma processes relevant to ion escape. We investigated their density, temperature, and pressure contributions during the Parker Solar Probe Venus gravity assists VGA3, VGA4, and VGA7. Methods . We combined Parker Solar Probe/SPAN-E electron velocity distribution function fitting with PSP/FIELDS quasi-thermal noise spectroscopy. Quasi-thermal noise fitting was used to derive the total electron density and temperature, while electron velocity distribution function fitting above 20 eV was used to constrain the suprathermal electron component. The core electron properties were inferred from density and pressure balance between the total and suprathermal populations. Results . The median total electron densities were about 74, 61, and 420 electrons per cubic centimeter for VGA3, VGA4, and VGA7, respectively, whereas the corresponding suprathermal densities were only about 2.5, 2.1, and 2.3 electrons per cubic centimeter. The inferred core electron temperature was generally below 1 eV, while the suprathermal temperature reached median values of about 18, 21, and 41 eV. Conclusions . The nightside Venusian electron population was dominated in number density by cold core electrons, whereas suprathermal electrons contributed substantially to the electron pressure. The combined analysis constrained the electron pressure structure relevant to ambipolar electric fields and ion escape.

The interaction between the solar wind and the Martian upper atmosphere involves mass loading by newly created pickup ions (PUIs). Accordingly, these PUIs can modify the solar wind flow upstream of the Martian bow shock through momentum exchange. While the relevant solar wind deceleration and a deflection opposite to the solar wind convective electric field direction have been identified in previous studies, the present study reveals an extra solar wind deflection transverse to both the solar wind flow and the convective electric field by statistically analyzing the Solar Wind Ion Analyzer and magnetometer data of Mars Atmosphere and Volatile EvolutioN between 2015 and 2019. This transverse deflection demonstrates opposite shifts under two groups of interplanetary magnetic field orientations, with mode values of approximately +2.3 and −2.1 km s ⁻¹ . Both Student’s t-test and the Mann–Whitney U-test confirm the statistical significance of the transverse deflection reversal. An estimate based on momentum conservation between the solar wind and PUIs indicates that the observed transverse deflection is consistent with the expectation from PUI mass loading. This study provides the first direct observational evidence of the transverse solar wind deflection upstream of the Martian bow shock and demonstrates that the impact of PUI mass loading on the solar wind velocity is inherently three-dimensional, involving simultaneous a deceleration along the solar wind flow direction, a deflection opposite to the convective electric field direction, as well as an extra deflection perpendicular to both the solar wind flow and the convective electric field.

This study investigates upstream wave generation at quasi-parallel shocks using large-scale one-dimensional hybrid simulations. The results reveal a spatial evolution of ultra-low-frequency waves from the far upstream region to the shock front. In the far upstream region, the waves are quasi-monochromatic, incompressible, and exhibit left-hand polarization in the downstream frame. Closer to the shock front, these waves transition into shorter-wavelength compressional fluctuations characterized by both left- and right-hand polarizations. Linear kinetic instability analysis based on simulated upstream proton velocity distributions confirms that the observed waves are primarily excited by local instabilities driven by backstreaming proton populations throughout the foreshock region. Near the shock front, the wave activity arises from a combination of locally excited waves from hot diffuse ions and upstream-generated waves convected downstream, jointly producing the chaotic polarization and compressional signatures observed. The parallel shock simulation also captures an early splitting of backstreaming protons into two beams. Trajectory analysis identifies the low-energy beam as an artifact resulting from proton reflection prior to full shock formation, underscoring the need for adequate simulation time to achieve a more realistic foreshock. A comparison of simulations of shock normal angles ranging from 0° to 40° indicates that while the overall pattern of wave evolution remains consistent across the quasi-parallel regime, increasing shock normal angle enhances wave compressibility and induces a polarization transition from circular to elliptical in the far upstream region.

Context . The escape of oxygen ions from Mars has played a crucial role in the planet’s long-term atmospheric evolution and habitability. The crustal magnetic fields influence ion escape, but the exact role remains debated. Previous studies have presented contrasting conclusions, suggesting that the crustal fields may either suppress or enhance oxygen ion escape. To date, the extent and mechanisms of this influence remain insufficiently understood. Aims . This study aims to investigate the influence of the Martian crustal magnetic fields on the oxygen ion escape at Mars. Methods . Several groups of 3D global hybrid simulations of Mars-solar wind interaction were performed, with the escaping oxygen ion trajectories traced. The results from the simulations with or without the crustal fields and under different interplanetary magnetic field conditions were then compared. Results . The simulation results show that the presence of crustal fields enhances the ionospheric oxygen ion escape, while the exospheric oxygen ion escape rate remains largely unaffected. The crustal magnetic fields alter the local electric and magnetic environments and, subsequently, modify the local oxygen ion density and flow direction in the ionosphere. First, the steep magnetic inclination and large magnetic strength in crustal field regions increase the density of low-altitude ionospheric oxygen ions and facilitate their outward transport, thereby promoting ion escape. Second, the crustal fields modify the local electric field structure, which also affects ion acceleration and escape. When strong crustal fields are located on the dayside, their obstruction of the upstream plasma flow weakens the dayside radial electric field at low altitudes in the southern hemisphere. The weakened electric field tends to assist or reduce ion escape, depending on whether it points toward or away from Mars, respectively. In any case, the influence of the magnetic field topology change (the steep magnetic inclination and large magnetic strength) in crustal field regions dominates the effect of weakened electric field, resulting in a higher escape rate than that without the crustal fields. Additionally, when strong crustal fields are on the nightside, the dayside moderate crustal fields still enhance the local density and outward transport of ionospheric oxygen ions, while their impact on the local electric field remains limited. The net effect is enhanced ion escape over the +E hemisphere where the solar wind motional electric field points away from Mars.

Proton cyclotron waves (PCWs) upstream of Mars are thought to be associated with the instabilities of pickup ions. The instabilities of pickup ions of ring distributions have larger maximum growth rates compared with beam distributions. However, observations revealed a notably‐reduced occurrence rate of PCWs for pickup ions of ring distributions. Linear instability analysis and a corresponding two‐dimensional particle‐in‐cell (PIC) simulation are performed to investigate the instabilities of pickup ion ring distributions upstream of Mars. Linear instability analysis indicates that a pickup ion ring distribution is unstable to the ion cyclotron, ion Bernstein, and mirror instabilities. The corresponding PIC simulation confirms the linear analysis results and further demonstrates that the pickup ions are scattered toward an isotropic shell distribution by the waves excited. Interestingly, the saturation energy of the waves is much lower than that driven by a corresponding pickup ion beam distribution, and mirror waves eventually dominate the system.

Three-dimensional global hybrid simulations are performed to explore how the interplanetary magnetic field (IMF), the Martian crustal fields, and planetary pickup ions affect the twisting of the Martian magnetotail. The results agree with previous studies that the crustal magnetic fields cause the Martian magnetotail to twist counterclockwise or clockwise depending on the sign of the IMF Y-component in the Mars solar orbital coordinates. However, the twist is more pronounced when the crustal fields are on the nightside, contradicting the early explanation that the crustal fields affect the twist through dayside magnetic reconnection between the crustal fields and the draped IMF. Additionally, planetary pickup ions also contribute to the twist because their mass loading slows down the plasma flow and leads to the bending of the magnetic field lines in the magnetotail. It is demonstrated that the twist inside Mars’ shadow in the near magnetotail region (at X = −1.5 R _M, where R _M is Mars’ radius) is mainly attributable to the crustal fields, while the influence of planetary pickup ions starts to dominate outside Mars’ shadow and in regions further away from Mars.

Using the high‐time‐resolution data from the Magnetospheric Multiscale mission, precursor waves upstream of foreshock transient (FT) shocks are statistically investigated using the four‐spacecraft timing method. The wave frequencies and wave vectors determined in the plasma rest frame (PRF) are shown to follow the cold plasma dispersion relation for whistler waves. Combining with the feature of the right‐hand polarization in the PRF, the precursors are identified as whistler‐mode waves around the lower hybrid frequency. The occurrence of whistler precursors is independent of the Alfvén Mach number and the FT shock normal angle. More importantly, all the whistler precursors have group velocities pointing upstream in the shock frame, suggesting the dispersive radiation to be a possible generation mechanism. The study improves the understanding of not only the whistler precursors but also the overall FT shock dynamics.

This study investigates the propagation characteristics of Saturn's Narrowband (NB) emissions using a 3D ray‐tracing code incorporating Saturn's magnetic field and electron density parameters. The potential source regions and propagation zones of the L‐O mode, Z mode and whistler mode NB emissions are distinguished. The L‐O mode NB emissions, generated along local electron plasma frequency surfaces through mode conversion, exhibit straight‐line propagation but undergo reflections between the ionosphere, the plasma torus, and the magnetosheath. The slot region, characterized by a lower electron density distributed around the plasma torus boundary, significantly influences emission propagation, potentially leading to trapping and depolarization. The 5 kHz Z mode NB emissions propagate and fill the trapping region delineated by lower cut‐off and upper hybrid resonance frequencies. In contrast, the 20 kHz Z mode NB emissions are primarily confined near source regions at the north and south edges of the plasma torus, with the possibility of escaping under variable plasma conditions.