ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-24-887-2006 IMF effect on sporadic-E layers at two northern polar cap sites: Part I – Statistical study Voiculescu M. 1 2 Aikio A. T. 2 Nygrén T. 2 Ruohoniemi J. M. 3 Department of Physics, Faculty of Sciences, University “Dunărea de Jos" Galati, 800008 Galati, Romania Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014, Finland Applied Physics Laboratory, The Johns Hopkins University Applied Physics Laboratory, Laurel MD, 20723, USA 19 05 2006 24 3 887 900 Copyright: © 2006 M. Voiculescu et al. 2006 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/24/887/2006/angeo-24-887-2006.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/24/887/2006/angeo-24-887-2006.pdf

In this paper we investigate the relationship between polar cap sporadic-E layers and the direction of the interplanetary magnetic field (IMF) using a 2-year database from Longyearbyen (75.2 CGM Lat, Svalbard) and Thule (85.4 CGM Lat, Greenland). It is found that the MLT distributions of sporadic-E occurrence are different at the two stations, but both are related to the IMF orientation. This relationship, however, changes from the centre of the polar cap to its border. Layers are more frequent during positive <i>B<sub>y</sub></i> at both stations. This effect is particularly strong in the central polar cap at Thule, where a weak effect associated with <i>B<sub>z</sub></i> is also observed, with positive <i>B<sub>z</sub></i> correlating with a higher occurrence of Es. Close to the polar cap boundary, at Longyearbyen, the <i>B<sub>y</sub></i> effect is weaker than at Thule. On the other hand, <i>B<sub>z</sub></i> plays there an equally important role as <i>B<sub>y</sub></i>, with negative <i>B<sub>z</sub></i> correlating with the Es occurrence. Since Es layers can be created by electric fields at high latitudes, a possible explanation for the observations is that the layers are produced by the polar cap electric field controlled by the IMF. Using electric field estimates calculated by means of the statistical APL convection model from IMF observations, we find that the diurnal distributions of sporadic-E occurrence can generally be explained in terms of the electric field mechanism. However, other factors must be considered to explain why more layers occur during positive than during negative <i>B<sub>y</sub></i> and why the <i>B<sub>z</sub></i> dependence of layer occurrence in the central polar cap is different from that at the polar cap boundary.