Observations of day-to-day variability in the meridional semi-diurnal tide at 70 deg N

Measurements of plasma velocity along the magnetic field line by EISCAT provide an estimate of the meridional neutral wind in the E-region. During March and April 1988 such observations were made during the pre-midnight period on fourteen separate evenings and the results were used to fit the amplitude and phase of the semidiurnal component of neutral velocity between 100 and 160 km altitude. Large variations in the dominating tidal modes were observed from day to day, during geomagnetically active periods. Smaller changes were observed during five successive days of quiet geomagnetic conditions. The most stable feature was found to be the phase at 110 km; on 10 out of 14 cases studied the maximum southward velocity was encountered at 1800 LT to within 30 minutes. On three successive days a phase-height variation corresponding to the (2,4) tidal mode dominated below 130 km with phase varying less than ten minutes from day-to-day, and the phase-height variation of the (2,2) mode was observed above 130 km. These observations indicate the presence of a stable tidal oscillation. However, even then the amplitude changed from day to day, which is explained as a modulation produced by a planetary wave having a two-day period.