Instantaneous asymmetry of the Martian bow shock: A single- and dual-spacecraft study using MAVEN and Mars Express

Aims. We study the instantaneous asymmetry of the Martian bow shock during a change in the direction of the interplanetary magnetic field (IMF) and for steady-state conditions. Specifically, we study the asymmetry with regard to the convective electric field and to the crustal fields of Mars. 

Methods. Two methods were used: First, a single-spacecraft method in which a switch in hemisphere in the Mars solar-electric (MSE) coordinate system was studied during a change in the direction of the interplanetary magnetic field. Second, we used a dual-spacecraft method wherein near simultaneous bow shock crossings on opposite hemispheres were studied. The dual bow shock crossings were then compared to a bow shock model, and the difference in the distance to the model was used as a measure of asymmetry. 

Results. With the single-spacecraft method, an asymmetry with respect to the solar wind convective electric field, E_sw, was found, wherein the bow shock was farther from the planet in the Z_MSE <0 hemisphere, that is, the - E hemisphere. With the dual-spacecraft method, the mean of the magnitude of the asymmetries in the individual case was 0.13 R_M. However, the standard deviation was as high as the mean, and no significant asymmetry could be attributed either to the solar wind convective electric field or to the Martian crustal fields. A strong asymmetry without a clear correlation to these factors was found nonetheless. Possible causes of the measured asymmetry are discussed. 

Conclusions. The magnitude of the asymmetries in individual observations is larger than the average asymmetries. This indicates that the shape of the Martian bow shock is dynamic and influenced by fluctuations or wave phenomena.