Spectral Anisotropy at Ion Scales in the Solar Wind Upstream of the Martian Bow Shock

The spectral anisotropy of magnetic field fluctuations at ion scales in the solar wind upstream of the Martian bow shock remains poorly understood. In the solar wind near the Earth’s orbit, such anisotropy is known to be modulated by ion-scale waves, whereas in the Martian upstream, 1 Hz whistler waves are commonly observed and are expected to play a similar role. Using magnetic field data from Mars Atmosphere and Volatile EvolutioN, we first characterize the anisotropy of the spectral index at ion scales (i.e., spectral index as a function of the angle θ_VB between local mean magnetic field direction and solar wind velocity direction) and then evaluate the role of 1 Hz waves by applying a magnetic helicity filter to remove these wave signals. Before the wave removal, there exists a clear spectral bump near 1 Hz in the quasi-parallel direction, and the spectral index exhibits pronounced anisotropy. The average indices in the quasi-parallel and quasi-perpendicular directions are −4.18 ± 0.54 and −2.64 ± 0.44, respectively. After removing the 1 Hz waves, the spectrum in the quasi-parallel direction becomes flatter considerably with the index changing to −3.29 ± 0.48, while the spectrum in the quasi-perpendicular direction remains almost unchanged. The spectral flattening is most prominent within the θ_VB angular bin of 10^∘–20^∘, which coincides with the maximum occurrence rate of 1 Hz waves. This study provides new insights into wave–turbulence interaction near Mars and will improve our understanding of turbulent energy cascade and dissipation processes in a planetary upstream environment.