Thermospheric winds in the cusp: Dependence of the latitude of the cusp |
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| Authors: | RW Smith K Henriksen CS Deehr D Rees FG McCormac GG Sivjee |
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| Institution: | 1. Ulster Polytechnic, Jordanstown, Co. Antrim, U.K.;2. University of Tromsø, Tromsø, Norway;3. University of Alaska, Fairbanks, Alaska, U.S.A.;4. University College London, Gower Street, London, U.K. |
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| Abstract: | The dayside thermospheric wind pattern as observed from Spitsbergen generally shows moderately strong westward winds with a small poleward component. The flow is almost zonal, frequently with sufficient westward velocity that parcels of air cross the noon meridian travelling towards the morning before turning antisunward towards the nightside across the polar cap. There have been some exceptions which are characterized by much weaker winds having been increased in the poleward direction but with a very much reduced westward component. Making use of the meridian scanning photometer data obtained simultaneously on the same site, it is clearly shown that the normal behaviour occurs when the cusp, as indicated by the region of high and photometric intensity ratios, is to the North of the station. Just below the latitude of the cusp, the strong thermospheric flow generated by neutral coupling to the strong westward convection in the dusk sector continues across the dayside. It is maintained in the zonal direction because of the balance between the poleward Coriolis force and the equatorward pressure force caused by cusp heating. Poleward of the high pressure region at the cusp the flow is diverted northward and initially makes much slower progress across the Polar Cap. When the auroral oval has expanded such that the cusp is well to the South of our Spitsbergen station, the thermosphere in the sampled region has been found to be within this slow flow zone. On such occasions, the nightside speeds are well in excess of those on the dayside, in contrast to the normal behavior of comparable dayside and nightside wind speeds. |
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