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

Abstract

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 km ^2 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 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.

Keywords

• Interplanetary turbulence
• Solar wind
• Mars
• Magnetic fields
• Space plasmas