Physical mechanism of strong negative storm eects in the daytime ionospheric F2 region observed with EISCAT

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 Ne decrease at F2-layer heights originated from di€erent physical mechanisms in the two cases. The negative F2layer storm e€ect with an NmF2 decrease by a factor of 6.4 on 3 April 1992 was produced by enhanced electric ®elds (E » 85 mV/m) and strong downward plasma drifts, but without any noticeable changes in thermospheric parameters. The increase of the O + N2 reaction rate resulted in a strong enrichment of the ionosphere with molecular ions even at F2-layer heights. The enhanced electric ®eld produced a wide mid-latitude daytime trough on 03 April 1992 not usually observed during similar polarization jet events. The other strong negative storm e€ect 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 S in the neutral temperature pro®le 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.