Viscously driven plasma flows in the deep geomagnetic tail

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Abstract
We present an analysis, based on the principles of stress balance in a 1‐dimensional current sheet, which considers the problem of closed magnetic flux transport into the deep tail by a “viscous”‐like interaction between the solar wind and the magnetosphere. We illustrate our analysis with an example of ISEE‐3 data showing strong tailward plasma sheet flows on apparently closed field lines in the deep tail. Apart from narrow regions adjacent to the magnetopause, these flows are not driven by the scattering of magnetosheath plasma into the magnetosphere. We estimate the fraction of the magnetosheath momentum flux needed to be anomalously transferred into the plasma sheet to drive the flows. In our example this is ∼6%. No previously suggested mechanism (e.g., the Kelvin‐Helmholtz Instability) has been shown capable of providing anomalous momentum transport of this magnitude. Our current understanding of the “viscous” interaction between the solar wind and magnetosphere is thus insufficient to explain these observations.