Magnetospheric Multiscale Observation of Kinetic Signatures in the Alfvén Vortex

Alfvén vortex is a multiscale nonlinear structure that contributes to the intermittency of turbulence. Despite previous explorations mostly on the spatial properties of the Alfvén vortex (i.e., scale, orientation, and motion), the plasma characteristics within the Alfvén vortex are unknown. Moreover, the connection between the plasma energization and the Alfvén vortex still remains unclear. Based on high-resolution in situ measurements from the Magnetospheric Multiscale mission, we report for the first time distinctive plasma features within an Alfvén vortex. This Alfvén vortex is identified as being a 2D (${k}_{\perp }\gg {k}_{\parallel }$) quasi-monopole with a radius of 10 proton gyroscales. Its magnetic fluctuations $\delta {B}_{\perp }$ are anti-correlated with velocity fluctuations $\delta {V}_{\perp }$, thus the parallel current density ${j}_{\parallel }$ and flow vorticity ${\omega }_{\parallel }$ are anti-aligned. In different part 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 ${j}_{\parallel }$, while the electron temperature variations are correlated with ${\omega }_{\parallel }$. Furthermore, the temperature anisotropies, together with the non-Maxwellian kinetic effects, exhibit strong enhancement at peaks of $| {\omega }_{\parallel }| (| {j}_{\parallel }| )$ within the vortex. Comparison between observations and numerical/theoretical results are made. In addition, the energy-conversion channels and the compressibility associated with the Alfvén vortex are discussed. These results may help to understand the link between coherent vortex structures and the kinetic processes, which determines how turbulence energy dissipates in the weakly collisional space plasmas.