MMS observation of particle distributions in an Alfvén vortex

Alfvén vortex is a multi-scale nonlinear structure which is key ingredient to intermittency. Despite previous explorations mostly on the spatial properties, the plasma characteristics and the energy transfer process associated with the Alfvén vortex still remains unclear. Based on in-situ measurement from the Magnetospheric Multiscale (MMS) mission in the magnetosheath, we report for the first time, distinctive plasma features within an Alfvén vortex. This two-dimensional structure is identified as a quasi-monopole with a radius of around 10 proton gyroscales. Its perpendicular magnetic fluctuations are anti-correlated with the velocity fluctuations, suggesting the parallel current density and flow vorticity are anti-aligned. In different parts of the vortex (i.e., edge, middle, center), the ion and electron temperatures are found to be quite different and they behave in the reverse trend: the ion temperature variations are correlated with the parallel current, while the electron temperature variations are correlated with the parallel vorticity. Furthermore, the temperature anisotropies, together with the non-Maxwellian kinetic effects, exhibit strong enhancement at peaks of the vorticity/current within the vortex. Comparison between observations and numerical/theoretical results are made. In addition, the energy-conversion channels and the plasma compressibility associated with the Alfvén vortex are discussed. These results may help to understand the link between coherent vortex structures and kinetic processes, which determines the energy dissipation in the weakly-collisional space plasmas.