TIDAL VARIATIONS IN THE HIGH-LATITUDE E-REGION AND F-REGION OBSERVED BY EISCAT

During the MLTCS (Mesosphere-Lower Thermosphere Coupling Study) campai gn the EISCAT UHF radar was continuously operated over 7 days (30 July -5 August 1992) in the CP-1 mode. The long time series obtained of the fundamental ionospheric parameters field-aligned ion velocity (V-i), ion and electron temperature (T-i and T-e), and electron density (N-e) are useful in investigating tidal variations in the E- and F-region s ince the geomagnetic activity was particularly low during the time of measurement. Maximum entropy spectra of the parameters were calculated for the relatively quiet interval from 1 August to 4 August 1992 and indicated dominant variations with harmonics of 24 hours. In the elect ron density spectrum especially, harmonics up to the sixth order (4-h period) are clearly visible. The phase and amplitude height profiles ( 100-450 km) of the diurnal, semidiurnal, and terdiurnal variations wer e determined by Fourier transform for a 24-h data set beginning at 12: 00 UT on 3 August 1992 when the contaminating influences of electric f ields were negligible. The tidal variations of the ion temperatures ar e compared with the corresponding variations of the neutral temperatur e predicted by the MSISE-90 model. A remarkable result is the dominanc e of terdiurnal temperature oscillations at E-region heights on 3-4 Au gust 1992, while the measured diurnal and semidiurnal variations were negligible. The finding was confirmed by the analysis of further EIS-C AT data (2-3 August 1989, 2-3 July 1990, 31 March-1 April 1992) which also showed a dominant terdiurnal temperature tide in the E-region. Th is is different from numerous observations of tides in the E-region at mid-latitudes where the diurnal and especially the semidiurnal temper ature oscillations were dominant.