Solar Wind Transverse Deflection Upstream of the Martian Bow Shock: Evidence from MAVEN Observations

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Abstract
The interaction between the solar wind and the Martian upper atmosphere involves mass loading by newly created pickup ions (PUIs). Accordingly, these PUIs can modify the solar wind flow upstream of the Martian bow shock through momentum exchange. While the relevant solar wind deceleration and a deflection opposite to the solar wind convective electric field direction have been identified in previous studies, the present study reveals an extra solar wind deflection transverse to both the solar wind flow and the convective electric field by statistically analyzing the Solar Wind Ion Analyzer and magnetometer data of Mars Atmosphere and Volatile EvolutioN between 2015 and 2019. This transverse deflection demonstrates opposite shifts under two groups of interplanetary magnetic field orientations, with mode values of approximately +2.3 and −2.1 km s −1 . Both Student’s t-test and the Mann–Whitney U-test confirm the statistical significance of the transverse deflection reversal. An estimate based on momentum conservation between the solar wind and PUIs indicates that the observed transverse deflection is consistent with the expectation from PUI mass loading. This study provides the first direct observational evidence of the transverse solar wind deflection upstream of the Martian bow shock and demonstrates that the impact of PUI mass loading on the solar wind velocity is inherently three-dimensional, involving simultaneous a deceleration along the solar wind flow direction, a deflection opposite to the convective electric field direction, as well as an extra deflection perpendicular to both the solar wind flow and the convective electric field.
Funding Information
  • MOST ∣ National Natural Science Foundation of China (42530302)

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