|
|
 |
|
|
Ann. Geophys., 20, 487-499, 2002
www.ann-geophys.net/20/487/2002/
© European Geosciences Union 2002
HF radar polar patch formation revisited: summer and winter variations in dayside plasma structuring
S. E. Milan, M. Lester, and T. K. Yeoman
Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
Correspondence to: S. E. Milan (Steve.Milan@ion.le.ac.uk)
Abstract. Three intervals of polar
patch formation, as observed by the CUTLASS Finland HF coherent radar, are
presented. Simultaneous observations from a vertical ionosonde located at
Longyearbyen on Svalbard, situated in the dayside convection throat region,
allow for F-region plasma structuring, leading to polar cap patch formation to
be determined. Solar wind and interplanetary magnetic field (IMF) precursors of
polar patch formation are investigated with MFI and SWE measurements from the
Wind spacecraft. We find that in the cases studied polar cap patches are formed
in response to changes in the orientation of the IMF, especially in the By
component. The resultant changes in the dayside convection pattern alter the
source of plasma drifting through the convection throat region into the polar
cap. When the convection flow is directed predominantly polewards, high density
sub-auroral or mid-latitude plasma enters the polar cap; when flow is directed
zonally, low density plasma entrained in the convection return flow replaces
it. This mechanism can act to significantly structure the plasma density at
sub-auroral or mid-latitudes as well as in the polar cap. In winter months,
polar patches appear to be produced by depletions in an otherwise high plasma
density tongue of ionisation. In summer months, patches are enhancements of an
otherwise low density tongue of ionisation.
Key words. Ionosphere (ionospheric
irregularities; plasma convection; polar ionosphere)
Full Article in PDF (844 KB) |
|
|
