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

Abstract

Abstract. 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 explanations have been proposed: compressive interactions between streams and velocity shear interactions that produce a Kfirmfin vortex street. The latter interpretation has been challenged on the basis that the velocity jumps are supersonic, thereby suppressing the Kelvin-Helmholtz (K-H) instability. Here we examine this issue using a time-dependent compressible magnetohydrodynamics code solved in spherical coordinates in the two-dimensional r -0 plane. We conclude that supersonic flow does suppress small-scale instabilities and that the classic Kfirmfin vortex street cannot be excited. Both velocity 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 period 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 expected Parker value is predicted by the simulations that generate the north-south flow via velocity shear. The Voyager spacecraft observed a similar depletion in the outer heliosphere during the last two solar minima. When the effective tilt of the plasma sheet is increased, corotating interaction regions produce shock waves and other complex time-dependent evolution. We conclude that at solar minimum the observed north-south oscillations are a robust phenomenon that can form from either the interaction of fast and slow solar wind streams or from velocity shear. Which mechanism dominates is a consequence of the degree of tilt of the heliospheric current sheet, the magnitude of the velocity shear, and other physical parameters. However, the depletions seen in density and magnetic flux in the Voyager data suggest that velocity shear in the outer heliosphere at solar minimum may be the dominant cause of the observed north-south flow patterns.

Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23

“… Burlaga [1990] and Veselovsky [1990] suggested that a vortex street was observed by V2 at ≈25 AU during 1987, and Burlaga and Richardson [2000] provided additional evidence that a vortex street was observed by V2 at 47 AU during 1995, approaching solar minimum. The model of Goldstein et al [2001] that includes both velocity shears and compressive MHD effects predicts a depletion of magnetic flux and density in the distant heliosphere near solar minimum. Thus, it is possible that the departures of the observations from Parker's model in 1987 and 1997 in Figure 2 are associated with the presence of a vortex street at those times.…”

First measurements of electron vorticity in the foreshock and solar wind

“…Indirect inference of plasma vorticity has been made in many studies, generally by coupling observations with specific models or simulations (e.g., see Goldstein et al, 2001). Deducing vorticity from a single-point measurement implies that only one scalar component can be estimated.…”

Particle Diffusion on Vortices in Nearly Incompressible Magnetohydrodynamics

“…One could expect such large-scale structures to be observed in the heliosphere. There is an indication of north-south flows in Voyager 2 observations at 47 AU (Burlaga & Richardson 2000;Goldstein et al 2001). The flow pattern showed features of a heliospheric vortex street.…”

“…These structures are characterized by large-amplitude modulation of the background plasma parameters, nonoscillatory shape, and usually finite size. There are indications of such coherent structures in a form of vortex streets in solar wind observations (Burlaga & Richardson 2000;Goldstein et al 2001). The Kubo number R ¼ ð B=B 0 Þðl k =l ?…”