Statistical Properties of Solar Wind Upstream of Mars: MAVEN Observations

Using the data sets of Mars Atmosphere and Volatile EvolutioN and OMNI for the period 2014 October 10–2020 February 14 and the heliocentric distance of 1–1.66 au, we investigate the statistical properties of solar wind upstream of Mars for the first time. The key parameters, including interplanetary magnetic field (IMF), proton density (N), bulk velocity (∣V∣), and dynamic pressure (P _dyn), are surveyed with regard to variations of solar activity level and heliocentric distance. We find that the parameters ∣IMF∣, N, and P _dyn monotonously decrease with heliocentric distance. Both ∣IMF∣ and P _dyn are generally stronger at a higher solar activity level (F _10.7 ≥ 70 sfu), while such activity has little relevance to N. In contrast, ∣V∣ basically keeps a median of about 370 km s⁻¹ and is insensitive to the solar activity level and heliocentric distance. We also find that the IMF upstream of Mars at the higher solar activity level has a much smaller spiral angle in the inward sector; thus, IMF seems "straighter" than that in the outward sector, although that is not so for the inward sector of the upstream of Earth. Our statistical survey can be used as a reference for upstream solar wind of Mars at 1.4 ∼ 1.7 au, and could benefit the studies on solar wind as well as the Martian space environment.