Quasi-periodic transverse plasma flow associated with an evolving MHD vortex street in the outer heliosphere

Abstract

We study a transverse (“meridional” in heliocentric coordinates) plasma flow induced by the evolution of a Karman vortex street using a Chebyshev-Fourier spectral algorithm to solve both the compressible Navier-Stokes and magnetohydrodynamic (MHD) equations. The evolving vortex street is formed by the nonlinear interaction of two vortex sheets initially in equilibrium, such as are naturally found either side of the heliospheric current sheet at solar minimum. We study spatial profiles of the total plasma velocity, the density, the meridional flow angle and the location of sector boundaries and find generally good agreement with Voyager 2 measurements of quasi-periodic transverse flow in the outer heliosphere. The pressure pulses associated with the meridional flows in the simulation are too small, although they are correctly located, and this may be due to the lack of any “warp” in the current sheet in this model. A strong flow-aligned magnetic field, such as would occur in the inner heliosphere, is shown to lead to weak effects that would be masked by the background interplanetary turbulence. We also study the plasma and magnetic transport resulting from the meridional flow, and find that deficits of magnetic quantities do occur near the ecliptic and that while the effect is relatively small, it is in general agreement with the most recent analysis of ‘flux deficit’ in the outer heliosphere.