Mirror mode structures are born from a plasma instability driven by a large temperature anisotropy and appear downstream of planetary and interplanetary shocks, in their magnetosheath. As magnetic bottles imprisoning dense and hot plasma, they are usually observed downstream of their region of formation, where the anisotropy is large and free energy is available, implying that they are advected with the plasma flow to the detection region. At Earth and other planets, the quasi-perpendicular shock provides the plasma with the necessary heating along the perpendicular direction to the local magnetic field. At Mars, which boasts an extended exosphere, an additional source of temperature anisotropy exists, through unstable ring-beam velocity distributions, that is, through ions locally ionised and subsequently picked up by the local electric fields. We report here for the first time an example of near locally-generated mirror mode structures at Mars using the full plasma instrument suite on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We present three events in quasi-perpendicular and quasi-parallel shock conditions with a variable exosphere, discuss the locality of the mirror modes excited and show that, in addition to the classic quasi-perpendicular source of anisotropy, another source exists, that is, unstable pickup protons. The existence at Mars of this extra ion anisotropy-generating mechanism is reminiscent of comets.

During the first flyby of the BepiColombo composite spacecraft at Mercury in October 2021 ion spectrometers observed two intense spectral lines with energies between 10 and 70eV. The spectral lines persisted also at larger distances from Mercury and were ob- served again at lower intensity during cruise phase in March 2022 and at the second and third Mercury flyby as a single band. The ion composition indicates that water is the dominant gas source. The outgassing causes the composite spacecraft to charge up to a negative potential of up to -50V. The distribution and intensity of the lower energy signal depends on the intensity of low energy electron fluxes around the spacecraft which again depend on the magnetic field orientation. We interpret the observation as being caused by water outgassing from different source locations on the spacecraft being ion- ized in two different regions of the surrounding potential. The interpretation is confirmed by two dimensional particle-in-cell simulations.

Since its detection by Mariner 10, Helium has been a key focus in studies of Mercury’s exosphere. Recently, Weichbold et al. (2024) provided the first in-situ Helium measurements, inferring density from Ion Cyclotron Wave (ICW) events observed by the MESSENGER spacecraft. This approach enables, for the first time, a Helium density profile across a broad altitude range without relying on prior models. We present an ab-initio model for a steady-state, solar wind-driven Helium exosphere, which informed the interpretation of these ICW measurements. We discuss Helium release processes and evaluate whether meteorite impacts could account for specific instances of elevated Helium measurements. We developed a global, semi-analytical model based on a Helium-saturated regolith and an average Helium source flux of 2.5x10²³ He/s from solar wind ion implantation. We calculate the Helium flux distribution using an analytical lateral transport model and then generate local radial density profiles from a numerical (Monte Carlo) radial transport model. Additionally, we applied the radial transport model to estimate the scale and duration of large, sporadic Helium release events and assess the likelihood of detecting these events in-situ. The strong agreement between our model and the novel measurements confirms that the measurable Helium exosphere is dominated by thermally recycled particles. We show that elevated Helium measurements can result from the vaporization and release of Helium from large (1m) meteorite impacts, but it is statistically unlikely that more than one impact event is captured in the given set of measurements.

When the velocity shear between the two plasmas separated by Earth’s magnetopause is locally super-Alfvénic, the Kelvin-Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in-situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non-thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field-aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.

Remote observations by the Mariner10 and MESSENGER spacecraft have shown the existence of hydrogen in the exosphere of Mercury. However, to date the hydrogen number densities could only be estimated indirectly from exospheric models, based on the remotely observed Lyman-alpha radiances for atomic hydrogen (H), and the detection threshold of the Mariner10 occultation experiment for molecular hydrogen (H_2). Here we show the first on-site determined altitude-density profile of atomic H, derived from in-situ magnetic field observations by MESSENGER. The results reveal an extended H exosphere with densities that are ~1-2 orders of magnitude larger than previously predicted. Using an exospheric model that reproduces the H altitude-density profile, allows us to constrain the so far unknown H_2 density at the surface which is ~2-3 orders of magnitude smaller than previously assumed. These findings demonstrate the importance (1) of dissociation processes in Mercury’s exosphere and (2) of in-situ measurements giving complementary evidence of processes to remote observations, that will be realized in the near future by the BepiColombo mission.