Ion-Scale Kinetic Alfvén Turbulence: MMS Measurements of the Alfvén Ratio in the Magnetosheath

Turbulence in the Earth's magnetosheath at ion kinetic scales is investigated with the magnetospheric multiscale spacecraft. Several possibilities in the wave paradigm have been invoked to explain plasma turbulence at ion kinetic scales such as kinetic Alfvén, slow, or magnetosonic waves. To differentiate between these different plasma waves is a challenging task, especially since some waves, in particular, kinetic slow waves and kinetic Alfvén waves, share some properties making the possibility to distinguishing between them very difficult. Using the excellent time resolution data set provided from both the fluxgate magnetometer and the Fast Plasma Instrument, the ratio of trace velocity fluctuations to the magnetic fluctuations (in Alfvén units), which is termed the Alfvén ratio, can be calculated down to ion kinetic scales. Comparison of the measured Alfvén ratio is performed with respect to the expectation from two-fluid magnetohydrodynamic theory for the kinetic slow wave and kinetic Alfvén wave. Moreover, the plasma data also allow normalized fluctuation amplitudes of density and magnetic field to be compared differentiating between magnetosonic-like and kinetic Alfvén-like turbulence. Using these two different ratios, we can rule out that the fluctuations at ion scales are dominated by magnetosonic-like fluctuations or kinetic slow-like fluctuations and show that they are consistent with kinetic Alfvén-like fluctuations. This suggests that in the wave paradigm, heating in the direction of the parallel magnetic field is predominantly by the Landau damping of the kinetic Alfvén wave.

Key Points
The Alfvén ratio is measured for the first time at ion kinetic scales in the magnetosheath
The Alfvén ratio shows the better agreement with the prediction for kinetic Alfvén rather than kinetic slow turbulence
Normalized ratio of the trace magnetic and electron density fluctuations rule out magnetosonic turbulence as the dominant process

Plain Language Summary
In this study, we have sought to characterize the nature of magnetic fluctuations in the Earth's magnetosheath by using a combination of velocity, density, and magnetic field measurements. We are able to determine that the fluctuations are dominated by kinetic Alfvén wave (KAW) fluctuations at these scales rather than kinetic slow or magnetosonic fluctuations. There have been several previous studies that have highlighted the importance of the KAW; however, theoretically, the wave shares a number of properties with the kinetic slow wave. One property that differs significantly is the Alfvén ratio (the ratio of velocity to magnetic fluctuations) where velocity fluctuations dominate for the kinetic slow wave and magnetic fluctuations dominate for the KAW. With the exceptional time resolution of both plasma and magnetic field data from NASA's Magnetospheric Multiscale mission, this ratio can be measured for the first time down to proton scales. The dominance of the KAW or KAW-like fluctuations is confirmed from the ratio since it decreases significantly at lengths close to that of the proton characteristic scales.