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In this series of papers, we present statistical maps of mirror-mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on extreme ultraviolet (EUV) solar flux levels (high and low) and, specific to Mars, on Mars Year (MY) as well as atmospheric seasons (four solar longitudes Ls). We first use magnetic-field-only criteria to detect these structures and present ways to mitigate ambiguities in their nature. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft from November 2014 to February 2021 (MY32–MY35), we detect events closely resembling MMs lasting in total more than 170 000 s, corresponding to about 0.1 % of MAVEN's total time spent in the Martian plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 1 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. Detection probabilities are higher on average in low-solar-EUV conditions, whereas high-solar-EUV conditions see an increase in detections within the magnetospheric tail. We tentatively link the former tendency to two combining effects: the favouring of ion cyclotron waves the closer to perihelion due to plasma beta effects and, possibly, the non-gyrotropy of pickup ion distributions. This study is the first of two on the magnetosheaths of Mars and Venus.

We present fast algorithms to automatically estimate the statistical position of the bow shock from spacecraft data, using existing analytical two-dimensional (2D) and three-dimensional (3D) models of the shock surface. We derive expressions of the standoff distances in 2D and 3D and of the normal to the bow shock at any given point on it. Two simple bow shock detection algorithms are constructed, one solely based on a geometrical predictor from existing models, the other using this predicted position to further refine it with the help of magnetometer data, an instrument flown on many planetary missions. Both empirical techniques are applicable to any planetary environment with a defined shock structure. Applied to the Martian environment and the NASA/MAVEN mission, the predicted shock position is on average within 0.15 planetary radius $R_p$ of the bow shock crossing. Using the predictor-corrector algorithm, this estimate is further refined to within a few minutes of the true crossing $\approx$0.05 $R_p$). Between 2014 and 2021, we detect 14,929 clear bow shock crossings, predominantly quasi-perpendicular. Thanks to 2D conic and 3D quadratic fits, we investigate the variability of the shock surface with respect to Mars Years (MY), solar longitude (Ls) and solar EUV flux levels. Although asymmetry in $Y$ and $Z$ Mars Solar Orbital coordinates is on average small, we show that for MY32 and MY35, Ls = [135-225$^\circ$] and high solar flux, it can become particularly noticeable, and is superimposed to the usual North-South asymmetry due in part to the presence of crustal magnetic fields., 55 pages, 10 figures, 5 tables

We present an in-depth analysis of a 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 last on average $\sim$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 are of the order of 30-35 nT with respect to the background field, and appear 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 $\beta_{||}\sim$1, we hypothesise that these so-called magnetic bottles, containing a relatively higher energy and denser ion population with respect to the background plasma, are 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 unmistakable identification and characterisation of mirror modes at Mars from the joint points of view of magnetic field, electron and ion measurements. Up until now, the lack of high-temporal resolution plasma measurements has prevented such an in-depth study., Comment: 37 pages, 11 figures, 1 table

We study the interaction of three solar wind structures, two stream interaction regions and one interplanetary coronal mass ejection, with Mars' plasma environment during 20-27 November 2007. This period corresponds to the solar minimum between the solar cycles 23 and 24 which was characterized by very low values of the solar wind density and dynamic pressure and low IMF magnitude. During that time the Mars-Express orbit was in the terminator plane, while the Earth, Sun, and Mars were almost aligned, so we use the ACE and STEREO probes as solar wind monitors in order to identify and characterize the structures that later hit Mars. We find that the passage of these structures caused strong variations of in the bow shock location (between 2.2 and 3.0~R$_M$), compression of the magnetospheric cavity (up to 45~\%) and an increased transterminator flow below 2~R$_M$ (by a factor of $\leq$8). This study shows that during times of low solar activity, modest space weather phenomena may cause large variations of plasma flow at Mars., Comment: Published in Journal of Geophysical Research

We use the multi-spacecraft capabilities of the Cluster and THEMIS missions to show that two types of foreshock may be detected in spacecraft data. One is the global foreshock that appears upstream of the Earth's quasi-parallel bow-shock under steady or variable interplanetary magnetic field. Another type is a traveling foreshock that is bounded by two rotational discontinuities in the interplanetary magnetic field and propagates along the bow-shock. Foreshock compressional boundaries are found at the edges of both types of foreshock. We show that isolated foreshock cavities are a subset of the traveling foreshock that form when two bounding rotational discontinuities are so close that the ultra-low frequency waves do not develop in the region between them. We also report observations of a spontaneous hot flow anomaly inside a traveling foreshock. This means that other phenomena, such as foreshock cavitons, may also exist inside this type of foreshock. In the second part of this work we present statistical properties of phenomena related to the foreshock, namely foreshock cavities, cavitons, spontaneous hot flow anomalies and foreshock compressional boundaries. We show that spontaneous hot flow anomalies are the most depleted transient structures in terms of the B-field and plasma density inside them and that the foreshock compressional boundaries and foreshock cavities are closely related structures.

[1] Recent global hybrid simulations (kinetic ions, fluid electrons) of the solar wind interaction with Earth's magnetosphere revealed the existence of a new structure at the edges of the foreshock, named foreshock compressional boundary (FCB). The FCB is associated with a strong compression of magnetic field magnitude and density followed by strong decreases of these two quantities. This structure represents a transition region that separates the highly disturbed foreshock plasma from the solar wind. Hybrid simulations have shown that the formation and strength of the FCB depend on the generation and nonlinear evolution of ultra-low frequency (ULF) waves which in turn have been produced by the interaction of foreshock backstreaming ions with the solar wind. In this work, we use Cluster magnetic field and plasma data to show that the FCB exists and is frequently observed in the foreshock region. We study FCB properties during 36 crossings for different interplanetary field geometries and solar wind speeds. The results presented here reveal that FCBs form for different solar wind conditions (i.e., cone angle, velocity), and are highly nonlinear structures with δB/Bo and δn/no amplitudes around 40% of the ambient values. We find that the FCB is sometimes a transition region between the pristine solar wind plasma and the foreshock plasma. At other times, it separates a region with large amplitude waves from regions with high frequency small amplitude waves. Preliminary analysis of ion distributions show that in some cases the FCB can coincide with the ULF wave boundary and in others with the intermediate ion boundary. Cluster data also show that the solar wind flow is decelerated and deviated when crossing the FCB.