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Ann. Geophys., 24, 887-900, 2006
www.ann-geophys.net/24/887/2006/
© European Geosciences Union 2006
IMF effect on sporadic-E layers at two northern polar cap sites: Part I – Statistical study
M. Voiculescu1,2, A. T. Aikio2, T. Nygrén2, and J. M. Ruohoniemi3
1Department of Physics, Faculty of Sciences, University “Dunărea de Jos" Galati, 800008 Galati, Romania
2Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014, Finland
3Applied Physics Laboratory, The Johns Hopkins University Applied Physics Laboratory, Laurel MD, 20723, USA
Abstract. 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 By at both
stations. This effect is particularly strong in the central polar cap at
Thule, where a weak effect associated with Bz is also observed, with
positive Bz correlating with a higher occurrence of Es. Close to the
polar cap boundary, at Longyearbyen, the By effect is weaker than at
Thule. On the other hand, Bz plays there an equally important role as
By, with negative Bz 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 By and why the Bz dependence of layer
occurrence in the central polar cap is different from that at the polar cap
boundary.
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