Origin of some anisotropic tailward flows in the plasma sheet

Abstract. We use a test particle model to explore anisotropy and fast flows in the central plasma sheet (CPS) that are a consequence of plasma sheet boundary layer (PSBL) ion beam dynamics. Ion distributions and flows (velocity mo-ments) in the CPS and equatorial current sheet (CS) are compared and we find that mirroring of initially earthward beams from the PSBL, and their subsequent convection to the CS region, results in strong anisotropy throughout the CPS. At higher latitudes, velocity moments are field-aligned and feature earthward flow. Deeper in the CPS, velocity mo-ments yield flows in the anti-earthward direction. There is no clear distinction between the PSBL and CPS, since ve-locity distributions with large streaming components occur throughout the model CPS, but in the CS region they are anisotropic and nongyrotropic. In the CS region velocity moments can feature anti-earthward cross field flows. These tailward flows (> 400 km/s) are observed in the CS region between X = −20 to −30 RE due to nonadiabatic effects. Model results suggest that fast tailward plasma flows can be obtained without necessarily appealing to magnetotail pro-cesses associated with dynamic geomagnetic activity. Key words. Magnetospheric physics (magnetospheric con-figuration and dynamics) – Space plasma physics (charge particle motion and acceleration; numerical simulation stud-ies)