Observation of the F2-layer variability from the "Alma-Ata" observatory

Hourly values of the F-region critical frequency, 176172, obtained by ground-based ionosonde method at Alma-Ata station during 1988-1999 are used to study the F2-layer variability in terms of Delta foF2 obtained as a relative deviation of the current foF2 values from their background level. It has been found that in comparison with summer and equinox the winter ionosphere is more variable with an increase in the night time ionosphere day-to-day variability. Usually, the relative deviations Delta foF2 occurred in the +/- 20% range, with about 80% of them occurring in the +/- 10% range. The winter nighttime Delta foF2 values usually occurred in the range from -30% to 25%. Examples of the emission rate and rotational temperature for the O-2 (0-1) atmospheric band obtained at Almaty using the MORTI instrument over nights of January and May 1999 show that the range of the Delta E and Delta T deviations like those in the ionosphere are maximum in winter like those in the ionosphere suggesting that the winter mesosphere is also more variable than summer mesosphere. Ionospheric data indicate episodic events of pronounced planetary wave (PW)-like oscillations observed in winter ionosphere. Two of the events, which occurred in the period of December 1994-January 1995 and December 1998, are analysed and revealed planetary-wave (PW) like oscillations in foF2 with periods of 5-14 days. The midnight ionospheric data were compared with available optical data obtained at Almaty employing the MORTI instrument for the period of December 1998-January 1999. The comparison showed that the F2-layer electron density, 02 emission rate and rotational temperatures over the entire period of observations tend to behave similarly, indicating oscillations with a period of about 9 days. NCEP/NCAR-reanalysis temperature and zonal wind at 40N and at 10 hPa pressure level as well as geopotential heights at 1 hPa pressure level (UKMO assimilated data) were analysed to interpret the observed perturbations in the mesosphere and ionosphere. A good coincidence between PW-like oscillations in foF2 and those of geopotential heights for periods of about 5 and 9-14 days has been obtained. The number of geomagnetic storms registered at Alma-Ata has been analysed as a plausible source of the ionosphere variability. Potential mechanisms linking the PW-like activity in the ionosphere with the wave events in the stratosphere/mesosphere have also been discussed. (c) 2004 Elsevier Ltd. All rights reserved.