Plasma flow channels at the dawn/dusk polar cap boundaries: momentum transfer on old open field lines and the roles of IMF B_y and conductivity gradients

Abstract. Using DMSP F13 data in conjunction with IMF data we investigate the newly discovered channels of enhanced (1.5–3 km/s) antisunward convection occurring at the dawn (06:00–09:00 MLT) or dusk (15:00–18:00 MLT) flanks of the polar cap for different combinations of IMF B_y polarity, hemisphere (NH/SH) and the dawn/dusk MLTs. Dawn-side cases where this flow channel appears occur for the following combinations: NH-dawn/B_y>0 and SH-dawn/B_y<0. The dusk-side cases are: NH-dusk/B_y<0 and SH-dusk/B_y>0. The flow channels are placed in the context of particle precipitation regimes/boundaries and ionospheric conductivity gradients. They are found to be threaded by "old open field lines" ("time since reconnection" >10 min) characterized by polar rain precipitation. In the dawn-side cases (NH-dawn/B_y>0 and SH-dawn/B_y<0) and in a Parker spiral field, the polar rain contains the "solar wind strahl" component. The convection enhancement is attributed to the Pedersen current closure of Birkeland current sheets (C1 and C2) in the polar cap (C1) and at the polar cap boundary (C2). The low ionospheric conductivity in the polar cap, particularly in the winter hemisphere, is compensated by an enhanced electric field driving the flow channel there. This is momentum transfer from the solar wind via dynamo action taking place in the combined current system of the high- and low-latitude boundary layers (HBL/LLBL). The conductivity gradient at the polar cap boundary contributes to establishing the convection channel and the associated enhancement of the dawn-dusk convection asymmetry extending beyond the dawn-dusk terminator during intervals of nonzero IMF B_y component. The HBL/LLBL-ionosphere coupling via Birkeland currents C1/C2 is a source of dawn-dusk convection asymmetry and Svalgaard-Mansurov effect which must be added to the effect of magnetic tension acting on "newly open field lines".