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.

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.

We present general empirical analytical equations of bow shock structures historically used at Mars, and show how to estimate automatically the statistical position of the bow shock with respect to spacecraft data from 2D polar and 3D quadratic fits. Analytical expressions of bow shock normal in 2D and 3D are given for any point on the shock’s surface. This empirical technique is applicable to any planetary environment with a defined shock structure. Applied to the Martian environment and the NASA/MAVEN mission, the predicted bow shock location from ephemerides data is on average within 0.15 planetary radius Rp of the actual shock crossing as seen from magnetometer data. Using a simple predictor-corrector algorithm based on the absolute median deviation of the total magnetic field and the general form of quasi-perpendicular shock structures, this estimate is further refined to within a few minutes of the true crossing (≈0.05 Rp). With the refined algorithm, 14,929 bow shock crossings, predominantly quasi-perpendicular, are detected between 2014 and 2021. Analytical 2D conic and 3D quadratic surface fits, as well as standoff distances, are successively given for Martian years 32 to 35, for several (seasonal) solar longitude ranges and for two solar EUV flux levels. Although asymmetry in Y and Z Mars Solar Orbital coordinates is on average small, it is shown that for Mars years 32 and 35, Ls = [135-225] degrees and high solar flux, it can become particularly noticeable and is superimposed to the usual North-South asymmetry due to the presence of crustal magnetic fields.

We present here an in-depth analysis of one time interval when quasi-linear mirror mode structures were detected by magnetic field and plasma measurements as observed by the NASA/Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We employ ion and electron spectrometers in tandem to support the magnetic field measurements and confirm that the signatures are indeed mirror modes. Wedged against the magnetic pile-up boundary, the low-frequency signatures lasted on average ~10 s with corresponding sizes of the order of 15-30 upstream solar wind proton thermal gyroradii, or 10-20 proton gyroradii in the immediate wake of the quasi-perpendicular bow shock. Their peak-to-peak amplitudes were of the order of 30-35 nT with respect to the background field, and appeared as a mixture of dips and peaks, suggesting that they may have been at different stages in their evolution. Situated in a marginally stable plasma with β|| ~ 1, we hypothesise that these so-called magnetic bottles, containing a relatively higher energy and denser ion population with respect to the background plasma, were formed upstream of the spacecraft behind the quasi-perpendicular shock. These signatures are very reminiscent of magnetic bottles found at other unmagnetised objects such as Venus and comets, also interpreted as mirror modes. Our case study constitutes the first unambiguous detection of mirror modes around Mars, which had up until now only been surmised because of the lack of high-temporal resolution plasma measurements.