Observation of an MHD Alfv\'en vortex in the slow solar wind

TL;DR

This paper reports the observation of an Alfvén vortex in the slow solar wind, identified through multi-spacecraft data analysis and modeling, providing insights into nonlinear structures at proton kinetic scales.

Contribution

It presents the first clear observational evidence of an Alfvén vortex in the solar wind, confirmed by multi-spacecraft measurements and vortex modeling.

Findings

Detection of an Alfvén vortex consistent with nonlinear MHD solutions.

Agreement between observed polarization and vortex model.

Vortex radius approximately 40 times the proton gyro-radius.

Abstract

In the solar wind, magnetic field power spectra usually show several power-laws. In this paper, magnetic field data from the Cluster mission during an undisturbed interval of slow solar wind is analyzed at 0.28Hz, near the spectral break point between the ion inertial and dissipation/dispersion ranges. Assuming Taylor's frozen-in condition, it corresponds to a proton kinetic scale of $kv_A/\Omega_p \sim 0.38$, where $v_A$ and $\Omega_p$ are the Alfv\'en speed and proton angular gyrofrequency, respectively. Data show that the Cluster spacecraft passed through a series of wavepackets. A strong isolated wavepacket is found to be in accordance with the four Cluster satellites crossing an Alfv\'en vortex, a nonlinear solution to the incompressible MHD equations. A strong agreement is seen between the data from four satellites and a model vortex with a radius of the order of $40$ times the local proton gyro-radii. The polarization at different spacecraft is compared and is found to agree with the vortex model, whereas it cannot be explained solely by the linear plane wave approach.