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Ann. Geophys., 24, 901-913, 2006
www.ann-geophys.net/24/901/2006/
© European Geosciences Union 2006
IMF effect on sporadic-E layers at two northern polar cap sites: Part II – Electric field
T. Nygrén1, A. T. Aikio1, M. Voiculescu1,2, and J. M. Ruohoniemi3
1Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014, Finland
2Department of Physics, Faculty of Sciences, University “Dunărea de Jos" Galati, 800008 Galati, Romania
3Applied Physics Laboratory, The Johns Hopkins University Applied Physics Laboratory, Laurel MD, 20723, USA
Abstract. This paper is the second in a series on a study of the link between IMF and
sporadic-E layers within the polar cap. In Paper I (Voiculescu et al., 2006),
an analysis of the sporadic-E data from Thule and Longyearbyen was
presented. Here we concentrate on the electric field mechanism of sporadic-E
generation. By means of model calculations we show that the mechanism is
effective even at Thule, where the direction of the geomagnetic field departs
from vertical only by 4. The model calculations also lead to a
revision of the electric field theory. Previously, a thin layer was assumed
to grow at a convergent null in the vertical ion velocity, which is formed
when the electric field points in the NW sector. Our calculations indicate
that in the dynamic process of vertical plasma compression, a layer is
generated at altitudes of high vertical convergence rather than at a null.
Consequently, the layer generation is less sensitive than previously assumed
to fluctuations of the electric field direction within the NW sector. The
observed diurnal variations of sporadic-E occurrence at Longyearbyen and
Thule are compared with the diurnal variations of the electric field, calculated
using a representative range of IMF values by means of the statistical APL
model. The results indicate that the main features of Es occurrence can be
explained by the convection pattern controlled by the IMF. Electric fields
calculated from the IMF observations are also used for producing
distributions of sporadic-E occurrence as a function of electric field
direction at the two sites. A marked difference between the distributions at
Thule and Longyearbyen is found. A model estimate of the occurrence
probability as a function of electric field direction is developed and a
reasonable agreement between the model and the experimental occurrence is
found. The calculation explains the differences between the distributions at
the two sites in terms of the polar cap convection pattern. The conclusion is
that the electric field is the major cause for sporadic-E generation and,
consequently, IMF has a clear control on the occurrence of sporadic E within
the polar cap.
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