Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> A self-consistent method for daytime F-region modelling was applied to EISCAT observations during two periods comprising the very disturbed days 3 April 1992 and 10 April 1990. The observed strong N<i><sub>e</sub></i> decrease at F2-layer heights originated from different physical mechanisms in the two cases. The negative F2-layer storm effect with an <i>N<sub>m</sub></i>F2 decrease by a factor of 6.4 on 3 April 1992 was produced by enhanced electric fields (<strong>E</strong>≈85 mV/m) and strong downward plasma drifts, but without any noticeable changes in thermospheric parameters. The increase of the O<sup>+</sup> + N<sub>2</sub> reaction rate resulted in a strong enrichment of the ionosphere with molecular ions even at F2-layer heights. The enhanced electric field produced a wide mid-latitude daytime trough on 03 April 1992 not usually observed during similar polarization jet events. The other strong negative storm effect on 10 April 1990 with a complete disappearance of the F2-layer maximum at the usual heights was attributed mainly to changes in neutral composition and temperature. A small value for the shape parameter <i>S</i> in the neutral temperature profile and a low neutral temperature at 120 km indicate strong cooling of the lower thermosphere. We propose that this cooling is due to increased nitric oxide concentration usually observed at these heights during geomagnetic storms.<br><br><b>Key words.</b> Atmospheric composition and structure · Thermosphere · Ionosphere · Ion chemistry and composition · Atmosphere interactions
Highlights
Ionospheric F2-layer negative storms have been studied for years and general mechanisms of their origin are well known
A self-consistent method for daytime F-region modelling has been applied to EISCAT observations during two disturbed periods with a strong Ne decrease at F2layer heights
Additional contribution to the NmF2 reduction can result from a strong downward plasma drift, it is uncertain how much of this is due to geometrical eects.This is dierent from midlatitude F2-layer storms where the vertical plasma drift is usually more positive due to enhanced equatorward thermospheric wind
Summary
Ionospheric F2-layer negative storms have been studied for years and general mechanisms of their origin are well known. Mid- and low-latitude F2layer storm eects have been studied more extensively than high-latitude ones. This is partly due to the diculties with ground-based ionosonde observations during geomagnetically disturbed periods. The high-latitude F2 region is very variable, being strongly inuenced by magnetospheric processes; in particular, substantial electric ®elds are usually present during geomagnetic storms. These electric ®elds and the corresponding horizontal E B plasma drifts can strongly perturb the electron density distribution at F2-layer heights. Two periods of EISCAT observations comprising the very disturbed days 03 April 1992 and 10 April 1990 with a very strong electron concentration depletion are analyzed in the paper
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