Polar cap sporadic E and electric field

Nygrén, T.1, Aikio, A.1 and Voiculescu, M.2

1 Department of Physical Sciences, University of Oulu, Oulu, Finland
2 Department of Physics, University Dunarea de Jos, Galati, Romania

It is now well known that electric fields of the order of tens of millivolts per metre are capable of producing sporadic-E layers, provided the field points in a proper direction. The polar cap electric field has a sufficient intensity and the field direction has a diurnal variation which predicts a diurnal variation in the Es occurrence as well. Since the polar cap convection pattern strongly depends on the IMF, an IMF dependence of Es occurrence is also expected.

In two earlier papers, a statistical study on the relation of the IMF, the polar cap convection pattern and the Es occurrence was made. The data consisted of two years of ionosonde observations from Longyearbyen and Thule and simultaneous satellite-based IMF observations. A convection model was used in calculating the polar cap electric field from the IMF observations. As expected, the diurnal variations of the Es occurrence at the two sites were different, but their mean features could be explained by the electric field theory. The study has revealed a mechanism for an IMF control of the polar cap ionosphere.

In the present study, an effort will be made to check the IMF connection to Es occurrence in individual cases. Examples of ESR observations of sporadic E have been selected and IMF observations will be used in calculating the time dependent electric field at the radar site. This electric field will then be used in calculating the vertical plasma convergence and the layer growth and the results will be compared with the observed Es layers.