F-region winds over the central polar cap

Abstract

F-region neutral winds across the polar cap have been studied by utilizing a Fabry-Perot interferometer located at Eureka (89° CGM), Nunavut Territory, over a 2-winter period 1997/98 and 1998/99. The dependence of neutral winds on the interplanetary magnetic field (IMF) was studied with simultaneous solar wind data from the WIND satellite. A detailed comparison was made between the neutral winds and simultaneous ion drifts measured from the DMSP F13 polar orbiting satellite during January and December, 1998. Spatially resolved 4 vector winds for 4 campaign days were compared to vector ion flows, to study the contributions of various forcing terms including ion drag to the neutral motions, under a variety of IMF conditions. The major results are as summarized below. (1) The average neutral winds across the central polar cap were 165 m/s antisunward. For IMF Bz southward the average was 295 m/s and for Bz northward only 90 m/s. The By component influenced the neutral wind direction; for By positive the average wind direction was about 3.4° duskward relative to the direction for By negative. (2) The observed time for neutral winds to respond to major IMF Bz transient changes was on average about 57 minutes. The calculated neutral e-folding time constant is 3.5 hours for a typical winter F-region electron density. The observed response time ranged from 1.5 to 7 hours. (3) For steady IMF Bz conditions, the measured ratios of the neutral wind velocity in the polar cap to the ion velocity were 0.46 and 0.33 respectively for Bz southward and northward. The empirical relations between these velocities and Bz were Vn=-204+19.3 Bzm/s Bz innT Vi=-504+43.4 Bzm/s. (4) Sunward winds occur frequently in the dayside of the polar cap under prolonged IMF Bz northward conditions. For B z strongly northward, the neutral flow is sunward in the dayside due to sunward ion flows from reverse convection cells. The nightside flow is antisunward approximately along the 14-02 MLT meridian. (5) When B y is positive and a dominant component of the IMF, the dayside neutral flow is mainly dawnward due to the dawnward ion drifts from the expanded dusk convection cell. The nightside flow remains along the 14-02 MLT meridian. (6) For IMF Bz strongly northward and By strongly positive conditions, the contributions of the pressure gradient force and Coriolis force to the dayside neutral flow are apparent in addition to ion drag. The nightside flow is mainly controlled by the pressure gradient force.