Fluid theory of coherent magnetic vortices in high-beta space plasmas

Abstract

In-situ observations in the Earth's and Saturn's magnetosheaths and in the solar wind reveal the presence of Alfvén vortices as intermittent structures in the range of scales from fluid lengths down to few ion lengths. The density and the magnetic field associated with them appear to be compressible for higher plasma betas. Until now, only incompressible Alfvén vortices have been known. Motivated by space plasma observations we develop a new model of magnetic vortices in high-beta plasmas with anisotropic temperature, possessing compressible density and magnetic field, whose typical size ranges from fluid to ion scales. At magneto-fluid scales we find novel non-propagating field-aligned cylindrical monopoles and inclined propagating dipoles. Their transverse magnetic and velocity fluctuations are aligned, but not identical, {and they exhibit density and compressible magnetic field fluctuations $δn$ and $δB_\Vert$ localized inside the vortex core. In the presence of thermal anisotropy and acoustic effects, they may be correlated or anti-correlated $δn/δB_\Vert={\rm constant}\gtrless 0$; fluctuations whose velocity along the magnetic field is below the ion thermal speed are always correlated.} At ion or kinetic scales (with the smallest radii $\sim c/ω_{pi}, ρ_{L i}$) {and in the absence of acoustic perturbations}, only dipolar Alfvén vortices survive with similar properties as those at fluid scales, except for their $δn/n_0$ that reaches the level of $δB_\Vert/B_0$. At kinetic scales we find also pressure balanced dipolar structures, possessing finite parallel electric field $E_\Vert$ and purely compressional magnetic field perturbation.