Mars Bow Shock Research Articles

A Fast Fermi Acceleration at Mars Bow Shock

AbstractWe report, for the first time, strong evidences that a fast Fermi mechanism is taking place at the Mars bow shock. The MAVEN spacecraft observations from the Solar Wind Electron Analyzer instrument show electron flux spikes with energies up to ∼1.5 keV. These spikes are associated with sunward propagating electrons and appear when the interplanetary field line threading the spacecraft is connected near the Martian bow shock tangency point. The observed loss cone distribution is a salient feature of these backstreaming electrons as the phase space density peaks on a ring centered along the magnetic field direction. Moreover, the data show no evidence of any effect due to a hypothetical cross‐shock electric potential on the observed angular distributions. Although similar distributions are seen at the terrestrial bow shock, the quantitative analysis of the measurements strongly indicates that the electrons are produced at the shock foot and escape upstream before exploring the entire shock structure.

The Three‐Dimensional Bow Shock of Mars as Observed by MAVEN

AbstractThe Martian magnetosphere is a product of the interaction of Mars with the interplanetary magnetic field and the supersonic solar wind. The location of the bow shock has been previously modeled as conic sections using data from spacecraft such as Phobos 2, Mars Global Surveyor, and Mars Express. The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft arrived in orbit about Mars in November 2014 resulting in thousands of crossings to date. We identify over 1,000 bow shock crossings. We model the bow shock as a three‐dimensional surface accommodating asymmetry caused by crustal magnetic fields. By separating MAVEN's bow shock encounters based on solar condition, we also investigate the variability of the surface. We find that the shock surface varies in shape and location in response to changes in the solar radiation, the solar wind Mach number, dynamic pressure of the solar wind, and the relative local time location of the strong crustal magnetic fields (i.e., whether they are on the dayside or on the nightside).

Higher‐energy plasma ions found near the termination shock: Analyses of Voyager 2 data in the heliosheath and in the outer heliosphere

We have found in the Voyager 2 (V2) plasma science data in the heliosheath (HS) near the termination shock (TS) high‐energy ions (HEIs) in addition to the bulk plasma convective flow ions. The HEI detections temporally coincide with increased V2 plasma wave subsystem (PWS) activity in “event A” of Gurnett and Kurth (2008). Maxwellian fits to HEI detections indicate the HEIs are moving radially anti‐Sunward with a proton speed of 600 km/s, a density of 10−4 cm−3, and a thermal speed of 10 km/s. The heliosheath bulk convective protons have a speed of 204 km/s, a density of 0.0029 cm−3, and a thermal speed of 26.7 km/s. The HEI flux and ram pressure are approximately 10% and 30% of those of the bulk HS flow. Since the HEI speed is both close to twice the solar wind speed and independent of the heliosheath bulk plasma speed, the HEIs may be detections of pickup protons formed in the solar wind and convected through the TS. The HEIs also are reminiscent of the pickup protons upstream of the Mars bow shock where their energy also was independent of the bulk plasma speed and attributed to multiple reflections off the Mars bow shock. Gurnett and Kurth's (2008) event A enhanced PWS activity may be generated by a two‐stream instability from the interaction of these HEIs with the heliosheath bulk plasma ions. We present our findings, discuss their implications, and also present alternative interpretations.

Proton cyclotron waves at Mars and Venus

Proton cyclotron frequency waves are present upstream of the Mars bow shock and have been interpreted as due to the ionization of exospheric hydrogen and the subsequent acceleration of the proton by the solar wind electric field. These waves are left handed polarized on the spacecraft frame and propagate at a small angle to the magnetic field, presumably growing from the free energy of newly accelerated protons. A second consequence of this acceleration is the loss of the planet’s hydrogen exosphere. Proton cyclotron waves have not previously been reported for Venus. Herein, we report the occurrence of such waves in the Venus magnetosheath. However, we do not find clear examples of such waves in the solar wind upstream of the Venus shock. At Mars, the waves are found up to at least 12 planetary radii to the side of the planet. A process that could produce such an extended pattern of newly created ionization and resultant waves is a multi-step one; ionization near the planets, acceleration of ions by the solar wind electric field, reneutralization of ions allowing them to travel across field lines, and a second ionization far from the planet producing the waves.

On the smooth surface of the northern bow shock of Mars, based on Mars Global Surveyor's measurements

A very smooth and time-invariable bow shock of Mars is revealed using Mars Global Surveyor's data. The bow-shock position has been identified using magnetic and electron flux data obtained by the Magnetometer and Electron Reflectometer (MAG/ER) experiment aboard Mars Global Surveyor, in the time period between days 87 and 255 of 1998. From the magnetic field and the electron flux measurements, 148 bow-shock crossings were detected, concentrated mostly on the northern hemisphere of the planet. With these results, a 3D configuration of the bow shock is constructed and presented. The results show that part of the observed bow shock is a surprisingly smooth surface. It is possible that the bow shock is smooth only in the northern hemisphere, since the southern surface is characterized by local magnetic anomalies. Its real shape can only be revealed in a 3D representation in the planetary centered solar ecliptic coordinate system and questions the theoretically expected variation of the bow shock.

Upstream waves at Mars

Weak, ∼0.15 nT, narrow band emissions at the proton gyro frequency are observed by the Phobos magnetometer, MAGMA, upstream from the bow shock of Mars. These waves are left-hand elliptically polarized. They may be associated with the pick up of protons from the Martian hydrogen exosphere. Strong turbulence, similar to that observed at the terrestrial bow shock, is found on occasion in the upstream region when the IMF connects to the bow shock. On two occasions this turbulence occurred when the spacecraft crossed the orbit of Phobos. This coincidence raises the possibility that material in the orbits of Phobos interacts with the solar wind in such a way to either affect the direction of the IMF or to cause instabilities in the solar wind plasma. However, since on a third occasion these waves did not occur, these waves may be shock associated rather than Phobos associated.

Interplanetary magnetic field control of the Mars bow shock: Evidence for Venuslike interaction

The Mars bow shock location and shape have been determined by examining the PHOBOS spacecraft magnetometer data. Observations show that the position of the terminator bow shock varies with interplanetary magnetic field orientation in the same way as at Venus. The shock is farthest from Mars in the direction of the interplanetary electric field, consistent with the idea that mass loading plays an important role in the solar wind interaction with Mars. We also find that the shock cross section at the terminator plane is asymmetric and is controlled by the interplanetary magnetic field as expected from the asymmetric propagation velocity of the fast magnetosonic wave. Comparing with earlier mission data, we show that the Mars shock location varies with solar activity. The shock is farther from Mars during solar maximum. Thus the solar wind interaction with Mars appears to be Venuslike, with a magnetic moment too small to affect significantly the solar wind interaction.

The solar wind interaction with Mars: Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

Observations taken by Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 are used to model the shape, position, and variability of the Martian bow shock for the purpose of better understanding the interaction of this planet with the solar wind. Emphasis is placed upon comparisons with the results of similar analyses at Venus, the only planet known to have no significant intrinsic magnetic field. Excellent agreement is found between Mars bow shock models derived from the earlier Mariner‐Mars data set (24 crossings in 1964–1974) and the far more extensive observations recently returned by Phobos 2 (94 crossings in 1989). The best fit model to the aggregate data set locates the subsolar bow shock at a planetocentric distance of 1.56±0.04 RM. Mapped into the terminator plane, the average distance to the Martian bow shock is 2.66±0.05 RM. Compared with Venus, the bow wave at Mars is significantly more distant in the terminator plane, 2.7 RM versus 2.4 RV, and over twice as variable in location with a standard deviation of 0.49 RM versus 0.21 RV at Venus. The Mars 2, 3, and 5 and Phobos 2 data also contain a small number of very distant dayside shock crossings with inferred subsolar obstacle radii derived from gasdynamic modeling of 2000 to 4000 km. Such distant bow shock occurrences do not appear to take place at Venus and may be associated with the expansion of a small Martian magnetosphere under the influence of unusually low solar wind pressure. Finally, the altitude of the Venus bow shock has a strong solar cycle dependence believed to be due to the effect of solar EUV on the neutral atmosphere and mass loading. Comparison of the Phobos 2 shock observations near solar maximum (Rz = 141) with the Mariner‐Mars measurements taken much farther from solar maximum (Rz = 59) indicates that the Martian bow shock location is independent of solar cycle phase and, hence, solar EUV flux. These results are interpreted in terms of a hybrid solar wind interaction model in which this planet possesses a weak, but significant, intrinsic magnetic field.

Asymmetries in the location of the Venus and Mars bow shock

An examination of observations of the position of the terminator bow shock at Venus and Mars shows that the terminator bow shock varies with the angle between the local bow shock normal and the upstream magnetic field, θBN. The part of the shock on the quasi‐parallel side is closer to the planet than the part on the quasi‐perpendicular side, a result which had been suggested by an earlier computer simulation by Thomas and Winske [1990]. This bow shock asymmetry is observed to be larger at Mars than at Venus.

Wave activity in the neighborhood of the bowshock of Mars

Plasma wave activity in the neighborhood of the Martian bow shock were measured for the first time by the Soviet spacecraft Phobos‐2 in a wide frequency range from dc to 150 kHz. The wave activity varied in character as the spacecraft moved across different plasma regions: in the neigborhood of the Martian bow shock, inside the magnetosheath and in the tail region.In this paper we provide suggestions for the processes responsible for these plasma waves. The most interesting peculiarities of the wave activity around Mars is the sharp increse of wave intensity in the magnetosheath region. This increase is attributed to two different physical mechanisms. High frequency waves are excited at the shock front due to currents flowing along the front; these ion acoustic waves are convected inside by the solar wind. The low frequency waves (∼100 Hz) close to the inside boundary were, we believe, generated by heavy Martian ions diffusing through the planetopause into the magnetosheath.

Upstream waves at Mars: Phobos observations

The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high temporal resolution data from the MAGMA magnetometer. Strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft. On 2 occasions this turbulence occurred upon crossing the Phobos orbit. Also weak, ∼0.15 nT, waves are observed at the proton gyro frequency. These waves are left‐hand elliptically polarized and may be associated with the pick‐up of protons from the Mars hydrogen exosphere.