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North‐south flows at 47 AU: A heliospheric vortex street?

A north-south flow pattern with a recurrence period approximately equal to the solar rotation period was observed by Voyager 2 during 1995 at 47 AU, nearly a solar cycle after the north-south flows were first observed near 21 AU during 1986. The north-south flow deflections observed during 1995 were less regular than those observed in 1986. One finds the same qualitative associations between the north-south flows and bulk speed and temperature that were observed in 1986, but the correlations are weaker at the larger distances and higher latitudes. Thus it appears that the flow observed during 1995 is basically the same kind of flow observed in 1986, but it is much less regular owing to evolutionary effects as the flow moves from 21 AU to 47 AU. The observations are consistent with the several predictions of the vortex street model. They fail to support some predictions of both the interaction region model and the vortex street models. It is possible that both a vortex street and interaction regions are significant near solar minimum between 20 and 50 AU. Further development of the models is needed. The interaction models must be extended to include time variations, and the effects of pickup protons should be included in future models of the north-south flows.