Magnetospheric Multiscale Observations of Turbulence in the Magnetosheath on Kinetic Scales

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.

Higher‐Order Turbulence Statistics in the Earth's Magnetosheath and the Solar Wind Using Magnetospheric Multiscale Observations

“…A Kolmogorov power-law is observed in the inertial range of the solar wind, giving way to a steeper spectrum in the kinetic/dissipation range. In the magnetosheath we observe a direct transition from a shallow "1∕f "-type spectrum (Matthaeus & Goldstein, 1986) to a steep ∼ − 8∕3rd regime, without an intervening inertial range, confirming previous observations in the Earth's (Alexandrova, 2008;Czaykowska et al, 2001;Macek et al, 2018) as well as Saturn's (Hadid et al, 2015) magnetosheath. The reason for the apparent absence of a Kolmogorov spectrum in the magnetosheath is not clear.…”

“…The steeper −8/3rd spectrum appears to transition directly into the so‐called “1/ f ” regime (Matthaeus & Goldstein, ) without passing through an intermediate −5/3rd inertial range, in contrast to what is observed in the solar wind. This is consistent with the observations of Czaykowska et al () and Macek et al () (see also section and Alexandrova, ). We note that the steepening in the magnetosheath spectrum occurs near the ion gyroradius (∼133.7 km in our interval).…”

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

“…The Earth's magnetosheath is a highly turbulent region bounded by the bow shock and the magnetopause. For the magnetic energy spectra in the magnetosheath, some similarities with those in the solar wind have been shown in previous studies (e.g., Alexandrova 2008;Riazantseva et al 2017;Chhiber et al 2018), for example, the existence of the ∼f −1 scaling at large scales and the Kolmogorov spectral index −5/3 at MHD scales (Alexandrova et al 2008;Huang et al 2017;Chhiber et al 2018), a broad distribution of slopes, [−4, −2] with a peak near −2.8, at sub-ion scales (Czaykowska et al 2001;Alexandrova et al 2008;Šafránková et al 2013;Huang et al 2014;Chen & Boldyrev 2017;Zhu et al 2019), and steeper spectra at electron scales (Matteini et al 2017;Macek et al 2018). However, due to the existence of multiple origins of waves and instabilities (Fairfield 1976;Omidi et al 1994;Zimbardo et al 2010), magnetosheath turbulence is more complicated than the turbulence in the solar wind.…”

“…For density and velocity spectra, the Nyquist frequency of the MMS FPI burst-mode data is ∼3.3 Hz. However, high-frequency white noise (with a spectral slope of zero) is sometimes found in the PSD of density/velocity fluctuations, as shown in Figure 2 of Macek et al (2018). To reduce the potential interference of noise, the upper frequency of the sub-ion range is set to be 2.0 Hz after a one-by-one case check.…”

Magnetospheric MultiscaleMission Observations of Reconnecting Electric Fields in the Magnetotail on Kinetic Scales

“…This Letter focuses on the deviations from MHD, including Hall‐MHD, electron pressure, and inertial effects on both ion and electron scales as seen in the MMS data, which warrants further investigation. Following our previous study of turbulence within highly variable plasmas using MMS data (Macek et al, ), we analyze in greater detail the electric fields on sub‐ion scales in magnetotail regions near the X line, to compare the characteristics of reconnection processes when going from the ion to electron kinetic scales. This naturally leads to a description of space plasmas within kinetic theory, instead of the ideal MHD approach.…”