Solar Wind Turbulence Spectra and Energy Injection Upstream of Mars

We statistically study the power spectral density (PSD) of the magnetic field turbulence in the upstream solar wind of the Martian bow shock by investigating the data from Tianwen-1 and MAVEN during November 13 and December 31 in 2021. Their spectral indices and break frequencies are automatically identified. According to the profiles of the PSDs, we find that they could be divided into three types A, B and C. Only less than a quarter of the events exhibit characteristics similar to the 1 AU PSDs (Type A). We observe the energy injection in more than one-third of the events (Type B), and find the disappearance of the dissipation range in over one third of the PSDs (Type C), which is likely due to the dissipation occurring at higher frequencies rather than proton cyclotron resonant frequencies.

We present an in-depth study of energy injection processes associated with Type-B spectra. Singular Value Decomposition analysis reveals that the gain regions are predominantly composed of compressive wave modes. Notably, a subset of these modes is identified as relatively pure, broadband ion cyclotron waves, a feature not recognized in prior statistical surveys of proton cyclotron waves. Statistical analysis of Type-B events observed by two spacecraft reveals spatial differences: events detected by MAVEN at the quasi-parallel bow shock nose are strongly influenced by the foreshock and correlate with reflected pickup ions. In contrast, concurrent events observed by Tianwen-1 on the flank show no clear connection to the foreshock or the ambient electric field direction, suggesting a potential link to upstream processes in the southern hemisphere.

The statistical study demonstrates the complicated turbulent environment of the solar wind upstream of the Martian bow shock.