Magnetospheric Multiscale Observations of Turbulence in the Magnetosheath on Kinetic Scales

Abstract

Abstract Our previous studies have produced phenomenological models for turbulence in solar wind plasmas on large-(inertial) magnetohydrodynamic scales, based on observations by the Voyager, Ulysses, and THEMIS missions. Here we consider turbulence in the Earth’s magnetosheath, where timescales are often far shorter than those in the heliosheath, using observations from the currently operating Magnetospheric Multiscale (MMS) mission on much smaller kinetic scales. We employ a standard statistical analysis to obtain energy density spectra for the magnetic field strength and the ion speed at high time resolution. We find a clear breakpoint of the magnetic spectrum exponent from −0.8 to −5/2 near the ion gyrofrequency of 0.25 Hz. In fact, just behind the bow shock and near the magnetopause, the availability of the highest-resolution magnetic field observations enables us also to identify the expected spectral exponent of about −3, which is further followed by steeper spectra with the slopes from −7/2 to −11/2 (−16/3) in the kinetic regime above 20 Hz, possibly resulting from the kinetic Alfvén waves. Because the resolution of the ion plasma parameters is somewhat lower than that for the magnetic field, spectra for the ion velocity can only be resolved near the onset of kinetic scales. On the other hand, deep inside the magnetosheath, where only low-resolution data are available and we are still in the magnetohydrodynamic scale range, we recover the well-known −5/3 Kolmogorov’s spectrum. The obtained results on kinetic scales may be useful for better understanding the physical mechanisms governing turbulence.