Characterization of turbulence in the Mars plasma environment with MAVEN observations

Suranga Ruhunusiri , J. S. Halekas , J. R. Espley , C. Mazelle (1, 2) , D. Brain , Y. Harada , G. A. Dibraccio , R. Livi , D. E. Larson , D. L. Mitchell , B. M. Jakosky , G. G. Howes

1 IRAP - Institut de recherche en astrophysique et planétologie
2 UT - Université de Toulouse

We characterize turbulence in the Mars plasma environment in a global scale for the first time by computing spectral indices for magnetic field fluctuations (slopes in the magnetic field power spectra) and determining how they vary with frequency and in different regions. In the magnetosheath, unlike in the solar wind, we find an absence of the inertial range which has a spectral index value equal to the Kolmogorov scaling value of -5/3. Instead, as observed in the magnetosheaths of other planets, we find that the spectral indices transition from low negative values close to -0.5 at low frequencies (< proton gyrofrequency) to values much lower than -5/3 at high frequencies (> proton gyrofrequency). This indicates that the pristine solar wind is modified at the Martian bow shock and that the fluctuations are dominated by locally generated fluctuations in the magnetosheath. The absence of spectral indices with the Kolmogorov scaling value indicates that the fluctuations in the magnetosheath do not have sufficient time to interact with one another leading to a fully developed energy cascade. Spectral index values near the Kolmogorov scaling value are observed for the low-frequency range near the magnetic pileup boundary, and this indicates the presence of fully developed energy cascade. In the wake, we find that the spectral indices have approximately the same values, typically near -2, for both the low- and high-frequency ranges. We observe seasonal variations of the spectral indices, mainly in the upstream region, which indicate the seasonal variations of the proton cyclotron waves.

Mots clés: Mars; MAVEN; turbulence; magnetosphere; plasma waves

Domaines: Planète et Univers [physics]

Suranga Ruhunusiri, J. S. Halekas, J. R. Espley, C. Mazelle, D. Brain, et al.. Characterization of turbulence in the Mars plasma environment with MAVEN observations. Journal of Geophysical Research Space Physics, 2017, 122, pp.656-674. ⟨10.1002/2016JA023456⟩. ⟨insu-03677089⟩