Evolution of the Earth’s Magnetosheath Turbulence: A Statistical Study Based on MMS Observations

Composed of shocked solar wind, the Earth's magnetosheath serves as a natural laboratory to study the transition of turbulence from low Alfvén Mach number, MA, to high MA. The simultaneous observations of magnetic field and plasma moments with unprecedented high temporal resolution provided by NASA's Magnetospheric Multiscale Mission (MMS) enable us to study the magnetosheath turbulence at both magnetohydrodynamics (MHD) and sub-ion scales. Based on 1841 burst-mode segments of MMS-1 from 2015 September to 2019 June, comprehensive patterns of the spatial evolution of magnetosheath turbulence are obtained: (1) from the subsolar region to the flanks, MA increases from <1 to >5. At MHD scales, the spectral indices of the magnetic-field and velocity spectra present a positive and negative correlation with MA. However, no obvious correlations between the spectral indices and MA are found at sub-ion scales. (2) From the bow shock to the magnetopause, the turbulent sonic Mach number, ${M}_{\mathrm{turb}}$, generally decreases from >0.4 to <0.1. All spectra steepen at MHD scales and flatten at sub-ion scales, representing positive/negative correlations with Mturb. The break frequency increases by 0.1 Hz when approaching the magnetopause for the magnetic-field and velocity spectra, while it remains at 0.3 Hz for the density spectra. (3) In spite of minor differences, similar results are found for the quasi-parallel and quasi-perpendicular magnetosheath. In addition, such spatial evolution of magnetosheath turbulence is found to be independent of the upstream solar wind conditions, e.g., the averaged Z-component of the interplanetary magnetic field and solar wind speed.