Large scale structuring of polar cap plasma in the nightside ionosphere: the influence of the orientation of the interplanetary magnetic field

Wood, A. G.1, Pryse, E.1, Middleton, H. R.1 and Howells, V.2

1 University of Wales, Aberystwyth, UK
2 Rutherford Appleton Laboratory, Oxford, UK

The polar ionosphere is a highly structured medium comprising electron density enhancements on a large range of horizontal spatial scales. On the larger scales high density dayside plasma may be drawn antisunward within the high latitude convection pattern. This plasma can form regions of enhanced ionisation known as polar cap patches which may be drawn into the nightside ionosphere. It is well established that the occurrence and subsequent evolution of these patches can be influenced by the interplanetary magnetic field, season and solar cycle. To date most studies have focussed on either the dayside or the central region of the polar cap, but this presentation focuses on nightside observations.

Modelling predicts that the offset of the geographic and geomagnetic poles will introduce a UT dependence on the ionisation drawn from the dayside. Therefore it is predicted that patches in the night sector will be most clearly observed over Northern Europe. The EISCAT TOI/NIGHT UK special programme was designed to investigate nightside patch morphology under a range of geophysical conditions. Results are presented from case studies illustrating the influence of the orientation of the interplanetary magnetic field.