Martian Shock Research Articles

The Drivers of the Martian Bow Shock Location: A Statistical Analysis of Mars Atmosphere and Volatile EvolutioN and Mars Express Observations.

The Martian interaction with the solar wind leads to the formation of a bow shock upstream of the planet. The shock dynamics appear complex, due to the combined influence of external and internal drivers. The extreme ultraviolet fluxes and magnetosonic Mach number are known major drivers of the shock location, while the influence of other possible drivers is less constrained or unknown such as crustal magnetic fields, solar wind dynamic pressure, or the Interplanetary Magnetic Field (IMF) intensity, and orientation. In this study, we compare the influence of the main drivers of the Martian shock location, based on several methods and published datasets from Mars Express (MEX) and Mars Atmosphere Volatile EvolutioN (MAVEN) missions. We include here the influence of the crustal fields, extreme ultraviolet fluxes, solar wind dynamic pressure, as well as (for MAVEN, thanks to magnetic field measurements) magnetosonic Mach number and Interplanetary Magnetic Field parameters (intensity and orientation angles). The bias due to the cross‐correlations among the possible drivers is investigated with a partial correlations analysis. Several model selection methods (Akaike Information Criterion and Least Absolute Shrinkage Selection Operator regression) are also used to rank the relative importance of the physical parameters. We conclude that the major drivers of the shock location are extreme ultraviolet fluxes and magnetosonic Mach number, while crustal fields and solar wind dynamic pressure are secondary drivers at a similar level. The IMF orientation also plays a significant role, with larger distances for perpendicular shocks rather than parallel shocks.

The Influence of Crustal Magnetic Fields on the Martian Bow Shock Location: A Statistical Analysis of MAVEN and Mars Express Observations

AbstractPrevious missions underlined the complex influence of the crustal magnetic fields on the Martian environment, including the plasma boundaries. Their influence on the bow shock is however poorly constrained, with most studies showing North/South differences attributed to the crustal fields, with various conclusions from little to strong variabilities. We analyze for the first time in detail the influence of crustal fields on the Martian shock location based on a multi‐mission analysis (MAVEN and MEX). We introduce the angular distance to the strongest crustal field region in the southern hemisphere that induces the largest influence (but not unique, with a minimum pressure threshold analyzed). Its impact is at large scale (>40–60° around), is modulated by the local time of the strongest source region (with no influence beyond terminator), and maximizes when the Interplanetary Magnetic field (IMF) is stable during the preceding hours. We introduce a technique, that is, partial correlations, to provide a coherent picture for both MAVEN/MEX due to existing cross correlations with Extreme UltraViolet (EUV). A composite parameter is proposed, that represents the combined influence of EUV, magnetosonic mach number (two major drivers) and crustal fields, the latter having an impact of hundreds of km. The influence of crustal fields on the shock appears seasonal and correlated with the Total Electronic Content, revealing a large scale coupling between the crustal fields, the ionosphere and the shock. The crustal field influence on the shock is thus significant and complex, with a coupling to both the ionosphere below and the IMF above.

The Structure of the Martian Quasi‐Perpendicular Supercritical Shock as Seen by MAVEN

AbstractThe Martian bow shock is a rich example of a supercritical, mass‐loaded collisionless shock that coexists with ultra‐low frequency upstream waves that are generated by the pick‐up of exospheric ions. The small size of the bow shock stand‐off distance (comparable with the solar wind ion convective gyroradius) raises questions about the nature of the particle acceleration and energy dissipation mechanism at work. The study of the Martian shock structure is crucial to understand its microphysics and is of special interest to understand the solar wind—planet interaction with a virtually unmagnetized body. We report on a complete identification and first characterization of the supercritical substructures of the Martian quasi‐perpendicular shock, under the assumption of a moving shock layer, using MAVEN magnetic field and solar wind plasma observations for two examples of shock crossings. We obtained substructures length‐scales comparable to those of the Terrestrial shock, with a narrow shock ramp of the order of a few electron inertial lengths. We also observed a well defined foot (smaller than the proton convected gyroradius) and overshoot that confirm the importance of ion dynamics for dissipative effects.

Martian shock and magnetic pile-up boundary positions and shapes determined from the Phobos 2 and Mars Global Surveyor data sets

A great many Martian bow shock and magnetic pile-up boundary crossings have been identified in the Phobos 2 and Mars Global Surveyor, MGS, data. From these observations the positions and shapes of the bow shock and magnetic pile-up boundary, MPB, have been derived and modelled, using curve-fitting techniques. The models thus derived separately from the Phobos 2 and MGS data sets do not differ drastically, despite the different time and space data coverages. The purpose of the paper is therefore to show the results obtained from the mixing of the Phobos 2 and MGS data bases and to compare the derived bow shock and MPB models with the ones obtained previously. The underlying objective was to see whether it was possible to determine improved bow shock and MPB models or not. The answer is definitely yes, and particularly for the MPB, thanks to the complementary nature of the observations. The boundaries crossed close to the subsolar direction or mostly far downstream by Phobos 2 indeed allow a better coverage of the Martian space environment to be considered. Nevertheless, in order to reduce the domination of the overabundant MGS data set and/or the crossings that are close to Mars ( x>−4 R M, i.e. x>−13 562 km) weighting factors have been introduced.

The bow shocks and upstream waves of Venus and Mars

Because they both lack measurable magnetic fields, Venus and Mars are often compared to each other in terms of their solar wind interaction. Upstream from each planet the most distant signs of this interaction occur at the bow shock, and in regions upstream from the shock where plasma waves are observed. In many respects the collisionless shocks at Venus and Mars are quite different. The Martian shock is located slightly farther from the planet (with respect to planetary size) and is more variable than the Venus shock. In addition, the position of the Martian shock is not observed to correlate strongly with solar cycle, unlike at Venus. These differences indicate that the solar wind obstacles at the two planets are somehow quite different. However, the characteristics of observed upstream waves at the two planets (and at other solar system bodies) suggest that similar processes are at work at both shocks, and that the size and shape of the shock do not play significant roles in wave generation or damping. This review compares the observations of the bow shocks and upstream waves at Venus and Mars, with reference to model predictions and observations at Mercury and Earth.

Observations of low‐frequency electromagnetic plasma waves upstream from the Martian shock

We have analyzed magnetic field data returned from Mars Global Surveyor (MGS) for signatures of electromagnetic plasma waves upstream from the Martian bow shock. We discuss two recurring wave features in the data. Left‐hand polarized waves (0.04–0.10 Hz) observed near the local proton gyrofrequency (PCWs) propagate at small to moderate angles to the magnetic field and have amplitudes that decrease with distance from Mars. They are concentrated in two locations upstream of the Martian shock. PCWs were reported from Phobos 2 observations and can be attributed to solar wind pickup of Mars' hydrogen exosphere. Higher‐frequency waves (0.4–2.3 Hz) are observed when MGS is magnetically connected to the Martian shock. These waves have not been reported at Mars before, but have been reported at many solar system bodies, and are attributed to whistler waves generated at the shock and propagating upstream. The sense of polarization (left‐handed or right‐handed) of the whistler waves observed in the spacecraft frame depends upon the angle between the magnetic field and the solar wind flow direction. The whistler waves at Mars follow the trends in frequency, amplitude, propagation angle, and eccentricity observed at other solar system bodies.

Electron plasma waves beyond the Mars' bow shock: The PWS/Phobos-2 observations

Abstract Electron plasma waves are commonly observed at or near the ambient solar wind plasma frequencies beyond planetary bow shocks. They are correlated with times when the interplanetary magnetic field is connected to the shock front, i.e., when entering the electron foreshock region in which suprathermal electrons accelerated at the bow shock are streaming back from it. Such electron plasma oscillations have been observed upstream of the Martian bow shock by the plasma wave system, PWS, on the Phobos 2 spacecraft. Phobos 2 has been operational in Martian orbits for almost two months. From January 29 to March 27, 1989, the Martian shock was crossed at least a hundred times and the upstream region extensively probed. The aim of this paper is to present the characteristics of the Martian electron plasma waves and to compare them with the Earth's and Venus'ones.

Characteristics of spectra from the martian bow shock and comparison with Venus, Earth, AMPTE, Jupiter, and Saturn

We present plasma wave spectra from crossings of the Martian bow shock by the Phobos 2 spacecraft and compare them with spectra from bow shocks at other planets and the AMPTE lithium release. We find that the shock spectra of the inner planets are very similar in shape, and their wave energy densities, when normalized to the upstream electron thermal energy density, are comparable. However, the Martian shock spectrum most resembles the spectrum from AMPTE, which may indicate either the presence of pickup particles or processes involving ions with gyroradii that are large compared with the size of the interaction region. The normalized wave amplitudes at Mars are midway between the high normalized amplitudes found at Jupiter and Saturn and the lower, normalized amplitudes at Earth. This is consistent with a trend that has recently been shown to occur out to Neptune's bow shock.

Application of the hypersonic analog to the standing shock of Mars

A magnetospheric boundary and its associated standing-shock wave for the earth are scaled down in size to apply to Mars. A suggestion by the Mariner-IV experimenters that their magnetometer may have detected a Martian shock at 0123 UT (earth) on July 15, 1965, is accepted. The scaling procedure used previously in the case of the Mariner-II flyby of Venus on December 14, 1962, as well as in the present case of Mars is repeated but with a different model. We propose a magnetospheric boundary for the earth, which is based on a maximum radius of 22 RE suggested by Imp-1 magnetometer data. We further suggest a low value (1.2) of the adiabatic exponent, γ. In addition we propose the use of the interplanetary magnetoacoustic Mach number, M∞*, which is a combination of both Alfven and ordinary gasdynamic Mach numbers. Our choice of values for these parameters is not unique since many such combinations exist which will show good agreement with observation. For example, the ratio of the Martian dipole moment to that for the earth is found to be 2.1 × 10−4 by our application of the hypersonic analog. This value compares very closely with 3 × 10−4 deduced as an upper bound by the Mariner-IV investigators. The present synthesis is presented as an independent calculation based on the above theoretical considerations, together with the experimenters' probable observation of a Martian shock wave.