Variability in the maximum height of the ionospheric F2-layer over Millstone Hill (September 1998-March 2000); influence from below and above

The basic aim of this 'case study' is to investigate the variability in the maximum height of the ionospheric F2-layer, hmF2, with periods of planetary waves (2-30days), and to make an attempt to determine their origin. The hourly data of hmF2 above Millstone Hill (42.6degrees N, 71.5degrees W) during 01 September 1998 - 31 March 2000 were used for analysis. Three types of disturbances are studied in detail: (i) the 27-day oscillations observed in the hmF2 above Millstone Hill are generated by the geomagnetic activity and by the global scale 27-day wave present in the zonal mesosphere/lower thermosphere (MLT) neutral wind. The time delay between the 27-day oscillation in the zonal wind and that in the hmF2 is found to be 5-6 days, while between the 27-day oscillation in the geomagnetic activity and that in the hmF2 is found to be 0.8-1 day; (ii) the 16-day oscillation in the hmF2 observed during summer 1999 is probably generated by the global scale 16-day modulation of the semidiurnal tide observed in the MLT region during PSMOS campaign in June-August. We found that if the modulated semidiurnal tide mediates the planetary wave signature in the ionosphere, this planetary wave oscillation has to be best expressed in the amplitude and in the phase of the 12-h periodicity of the ionosphere; and (iii) the third type of disturbances studied is the quasi-2-day activity in the hmF2 that increases during geomagnetic disturbances. The strong pseudo diurnal periodicities generated during the geomagnetic storms can interact between each other and produce the quasi-2-day oscillations in the ionosphere.