The Incompressible Magnetohydrodynamic Energy Cascade Rate Upstream of Mars: Effects of the Total Energy and the Cross-helicity on Solar Wind Turbulence.

Solar wind turbulence is a dynamical phenomenon that evolves with heliocentric distance. Orbiting Mars since 2014 September, Mars Atmosphere and Volatile EvolutioN offers a unique opportunity to explore some of its main properties beyond ∼1.38 au. Here, we analyze solar wind turbulence upstream of Mars' bow shock, utilizing more than 5 years of magnetic field and plasma measurements. This analysis is based on two complementary methodologies: (1) the computation of magnetohydrodynamic (MHD) invariants characterizing incompressible fluctuations; (2) the estimation of the incompressible energy cascade rate at MHD scales (i.e., 〈 ε ^T 〉_MHD). Our results show the solar wind incompressible fluctuations are primarily in a magnetically dominated regime, with the component traveling away from the Sun having a higher median pseudoenergy. Moreover, turbulent fluctuations have a total energy per mass of up to ∼ 300 km² s⁻², a range smaller than reported at 1 au. For these conditions, we determine the probability distribution function of 〈 ε ^T 〉_MHD ranges mainly between ∼ −1 × 10⁻¹⁶ and ∼1 × 10⁻¹⁶ J m⁻³ s ⁻¹, with a median equal to −1.8 × 10⁻¹⁸ J m^{−3<subscript />}s ⁻¹, suggesting back transfer of energy. Our results also suggest that ∣〈 ε ^T 〉_MHD∣ is correlated with the total energy per mass of fluctuations and that the median of 〈 ε ^T 〉_MHD does not vary significantly with the cross-helicity. We find, however, that the medians of the inward and outward pseudoenergy cascade rates vary with the solar wind cross-helicity. Finally, we discuss these results and their implications for future studies that can provide further insight into the factors affecting the solar wind energy transfer rate. [ABSTRACT FROM AUTHOR]