Statistical Properties of Plateau‐Like Turbulence Spectra in the Martian Magnetosheath: MAVEN Observations

Abstract

The Martian magnetosheath provides us with a natural laboratory to study plasma turbulence in the presence of pickup ions and locally generated instabilities. Unlike the typical magnetic‐field spectra with a single spectral scaling at magnetohydrodynamics (MHD) scales in Earth's magnetosheath, the magnetic‐field spectra in the Martian magnetosheath during 4 years of Mars Atmosphere and Volatile EvolutioN observations frequently present an additional spectral break‐point with a shallow slope at MHD scales which we define as a plateau‐like spectral feature. The average occurrence rate of plateau‐like magnetic‐field spectra is 56.6% of our measurement intervals. At moderate pick‐up angles, the occurrence rate increases to a maximum of ∼70.0%. Furthermore, we present a positive correlation with the local ion density and anti‐correlations with the local βi and the solar Extreme Ultra Violet irradiance. A similar occurrence rate in the quasi‐perpendicular and the quasi‐parallel magnetosheath (60.1% vs. 52.9%) indicates that the plateau‐like spectra are more likely formed locally than in the upstream solar wind. Our results suggest that energy injection from pickup ion driven micro‐instabilities, for example, in the form of proton cyclotron waves, has insufficient time to evolve into a fully developed cascade in such a confined space like the Martian magnetosheath.

Pickup Ion Modulation on Plateau-like Turbulence in the Martian Magnetosheath

“…For the case study, we reveal that the magnetic field spectrum exhibits flattened f −0.31 spectral scaling over the frequency range of 0.01-0.2 Hz, coinciding with the continuous presence of PCWs. This plateau-like spectral feature and the accompanying PUI properties align with our previous statistical result (Jiang et al 2023), which shows an average occurrence rate of the plateau-like feature of 56% and almost 70% at a pickup angle of around 30°.…”

“…When the frequency is greater than 0.2 Hz, at kinetic scales, the PSD displays a steeper power law of f −3.13 , which deviates from the typical f −2.8 scaling observed in Earth's magnetosheath (Li et al 2020). Our previous study has extensively examined such a plateau-like feature (Jiang et al 2023). The occurrence rate of plateau-like characteristics shows a comprehensive statistical correlation with PUI-related parameters, such as the pickup angle in velocity space and the ion β.…”

“…Based on the event list covering 4 yr (2015-2019) of MAVEN observations in the Martian magnetosheath given by Jiang et al (2023), we conduct a statistical analysis of the relationship between density ratios of pickup oxygen/hydrogen ions (n PUI,O /n PUI,H ) and the average transverse ratio of d d ^ near the proton cyclotron frequency, as well as the plateau spectral index of the magnetic field PSD. Similar to our case study, we used a comparable method to extract both pickup oxygen and hydrogen ions and obtain their relative ratio n PUI,O /n PUI,H .…”

“…A recent investigation revealed that the magnetic field spectrum within the Martian magnetosheath frequently exhibits an additional break at MHD scales, presenting a plateau-like spectral feature (Jiang et al 2023). This plateau-like spectrum is observed in 57% of the spectra and demonstrates a notable correlation with parameters associated with pickup ions (PUIs).…”