Abstract
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 By polarity, hemisphere (NH/SH) and the dawn/dusk MLTs. Dawn-side cases where this flow channel appears occur for the following combinations: NH-dawn/By>0 and SH-dawn/By<0. The dusk-side cases are: NH-dusk/By<0 and SH-dusk/By>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/By>0 and SH-dawn/By<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 By 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".
Highlights
In previous studies we aimed at a re-vitalization of a long tradition in the field of solar wind-magnetosphere-ionosphere coupling to which little attention has been directed in recent years
In case examples using data acquired by DMSP F13 in crosspolar cap passes, we have documented the presence of channels of enhanced antisunward convection at the polar cap boundary whose position with respect to noon in a given hemisphere depends on the IMF By polarity www.ann-geophys.net/27/1527/2009/
This flow channel, which we refer to as FC 2, is located immediately poleward of the zone of BPS/LLBL-type auroral precipitation, within the regimes of mantle and polar rain precipitation. From this observation we infer the convection channel to be associated with old open magnetic field lines
Summary
In previous studies we aimed at a re-vitalization of a long tradition in the field of solar wind-magnetosphere-ionosphere coupling to which little attention has been directed in recent years. The convection channel is present in the dawn (06:00– 09:00 MLT) or dusk (15:00–18:00 MLT) sides of the polar cap depending on the combination of By polarity and hemisphere (N/S) It is accompanied by polar rain electron precipitations (with or without the “solar wind strahl” component), i.e., it is threaded by “old open field lines” which means that they were reconnected some time (>10 min) in the past. We will check the different FC 2 variants presented in Fig. 1 by reporting new data examples in five different cases (1 October 2002, 20 August 1998; 12 December 1999; 21 January 2001; and 10 January 1997) This is followed by a summary of the observations, where we identify the characteristics of the different variants (categories) of the flow channel as sorted by IMF By polarity, hemisphere, and dawn-dusk side of the polar cap.
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