An evolving MHD vortex street model for quasi-periodic solar wind fluctuations

Abstract

We use magnetohydrodynamic (MHD) simulation to provide a dynamical basis for the “vortex street” model of the quasi-periodic meridional flow observed by Voyager 2 in the outer heliosphere. Various observations suggest that near the current sheet at solar minimum one can expect to find a vorticity distribution of two opposite shear layers with an antisymmetric staggered vorticity pattern due to structured high-speed wind surrounding low-speed equatorial flow. We show that this flow pattern leads to the formation of a highly stable vortex street through the nonlinear interaction of the two shear layers. Spatial profiles of various simulated parameters (velocity, density, meridional flow angle and the location of magnetic sector boundaries) and their relative locations in the quasi-steady vortex street are generally in good agreement with the observations. A strong, flow-aligned magnetic field, such as would occur in the inner heliosphere, inhibits the development of the street which would then be masked by the background interplanetary turbulence. The flow produced by the street induces a (relatively small) transport of plasma and magnetic flux as a result of the meridional flow away from the ecliptic region.