Statistical analysis of the movement of the Martian bow shock

Using data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission we investigate the flapping dynamics of the Martian bow shock (BS). While awaiting future dedicated two-spacecraft missions, we make use of the large number of single-spacecraft crossings from MAVEN to conduct a statistical study on observed multiple BS crossings.

The Martian bow shock has been studied extensively in the past, with a focus primarily on its formation, location, shape, and controlling factors. However, its dynamic motion, particularly flapping behavior, has received less attention - understandable given the constraints of single-spacecraft observations. From time series of magnetic field data, BS flapping, i.e. multiple crossings in a row, is observed in roughly 20% of all MAVEN orbits the first two years of the mission, which are investigated here. The multiple crossings are interpreted as a spatial change of the BS, moving in and out past the spacecraft. Preliminary analysis shows that the occurrence rate of the flapping is higher in the flank region compared to the ram side, but is otherwise evenly distributed around Mars. We find no preference for south or north hemisphere, and no dependence on the convective electric field direction. The median duration between two successive crossings is approximately 2 minutes. Estimates of the shock velocity from mass flux conservation laws during flapping events indicate that the BS moves faster on the dayside than on the flank. Flapping is more prevalent when the BS is quasi-perpendicular (75% of the cases) than when it is quasi-parallel (25% of cases). The closer to the planet the more quasi-parallel cases are found. The flapping does not seem to depend on the orbit-averaged solar wind dynamic pressure or magnetosonic Mach number values, as those parameters influence the BS on shorter time scales, as shown by Cheng et al., (2023).

These findings underscore the dynamic and complex nature of the Martian bow shock and enhance our understanding of its interaction with the solar wind. The results might have implications for energy transfer processes in weakly magnetized planetary systems and provide valuable context for comparative studies of bow shock dynamics across other planetary environments.