North-south flows observed in the outer heliosphere at solar minimum: Vortex streets or corotating interaction regions?

During the last two solar minima in the distant heliosphere the equatorial heliospheric plasma velocity oscillated perpendicular to the ecliptic plane with an approximately 26-day period in the distant heliosphere. Two explan ations have been proposed. compressive interactions between streams and vel ocity shear interactions that produce a Karman vortex street. The latter in terpretation has been challenged on the basis that the velocity jumps are s upersonic, thereby suppressing the Kelvin-Helmholtz (K-H) instability. Here we examine this issue using a time-dependent compressible magnetohydrodyna mics code solved in spherical coordinates in the two-dimensional r - theta plane. We conclude that supersonic flow does suppress small-scale instabili ties and that the classic Karman vortex street cannot be excited. Both velo city shear layers and stream interactions can, however, produce signatures in density, velocity, and magnetic field that resemble the observations. In particular, we find north-south variations of the flow velocity with a per iod that is approximately half that of the period of the variation in flow speed: a result insensitive to the thickness of the velocity shear layers. A depletion in density (and magnetic field magnitude) relative to the expec ted Parker value is predicted by the simulations that generate the north-so uth flow via velocity shear. The Voyager spacecraft observed a similar depl etion in the outer heliosphere during the last two solar minima. When the e ffective tilt of the plasma sheet is increased, corotating interaction regi ons produce shock waves and other complex time-dependent evolution. We conc lude that at solar minimum the observed north-south oscillations are a robu st phenomenon that can form from either the interaction of fast and slow so lar wind streams or from velocity shear. Which mechanism dominates is a con sequence of the degree of tilt of the heliospheric current sheet, the magni tude of the velocity shear, and other physical parameters. However, the dep letions seen in density and magnetic flux in the Voyager data suggest that velocity shear in the outer heliosphere at solar minimum may be the dominan t cause of the observed north-south flow patterns.