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

• Published in: The Astrophysical journal 2024-08, Vol.971 (1), p.10
• Main Authors: Romanelli, Norberto, Andrés, Nahuel, DiBraccio, Gina A., Verniero, Jaye L., Gruesbeck, Jacob R., Szabo, Adam, Espley, Jared R., Halekas, Jasper S.
• Format: Article
• Language: English
• Subjects: Charged particles; Distribution functions; Energy; Energy transfer; Fluctuations; Helicity; Interplanetary turbulence; Magnetic fields; Magnetic properties; Magnetohydrodynamic turbulence; Mars; Mars atmosphere; Planetary magnetic fields; Probability distribution; Probability distribution functions; Solar energy; Solar magnetic field; Solar wind; Solar wind turbulence; Space plasmas; Turbulence; Turbulent flow; Turbulent fluctuations; Upstream; Wind effects; Wind power
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad58b5

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.