Abstract.A study has been carried out on the occurrence of bottom side equatorial spread F (ESF) and its dependence on the polarity and magnitude of the thermospheric meridional wind just prior to ESF occurrence during summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995), using ionosonde data of Trivandrum (8.5 • N, 76.5 • E, dip=0.5 • N) and SHAR (13.7 • N, 80.2 • E, dip ∼5.5 • N) in the Indian longitude sector. In this study, we have examined the changes in the threshold height of the base of the F layer for the triggering of ESF, irrespective of the magnitude and polarity of the meridional winds during the above periods. The study indicates that the threshold height above which ESF triggering is entirely controlled only by the collisional R-T instability is least for summer months, with higher values for winter and equinox, during the solar minimum period, whereas for the solar maximum period the threshold height is least for winter, with higher values for summer and equinox. But the range over which the threshold height varies is very narrow (<15 km) for solar minimum in relation to the large range of variation (>50 km) in the solar maximum epoch. Further to this, the study also reveals a clear-cut increase in threshold height with solar activity for all seasons. Clear-cut seasonal variability is also observed in the threshold height, especially for solar maximum. The study quantifies the level of the base of the F layer below which neutral dynamical effects play a decisive role in the triggering of ESF during different seasons and solar epochs.

The response of ionospheric E-region electric fields and currents to solar flare related X-ray flux enhancements are studied at the magnetic equatorial location of Trivandrum (8.5 • N, 77 • E; dip 0.5 • N) using VHF (54.95 MHz) coherent backscatter radar observations in the altitude region of 95-110 km conducted during daytime. The amplitude of the Solar Flare Effects (SFE) observed in the earth's magnetic field variations at Trivandrum have been examined in relation to that at Alibag and it is found that the ratio of the SFE amplitudes at the two stations lies in the range of 1.8-2.6. The backscattered power of the VHF radar showed a substantial reduction during the peak phase of all the strong X-rank flares studied. It has also been observed that there is a sharp fall in the ratio of the field line integrated Hall conductivity (σ 2 ) to the field line integrated Pedersen conductivity (σ 1 ) in the dynamo region ( σ 2 ds/ σ 1 ds) during strong flare times in relation to normal times. The time variations of mean Doppler frequency ( f D ) of the backscattered signals have been observed to indicate a fall close to the peak phase of the strong flare events. Another new result is the radar observed presence of westward electric field for a long duration of 2-3 hours during a partial counter electrojet (CEJ) event that occurred on one of the days (8 July 1992) studied. The implications of the observations are discussed in detail.

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Abstract. The electrodynamic effects on the low-mid latitude ionospheric region have been investigated using GPS (global positioning system) data, ionosonde data and H values, during the very large solar flare (X17.2/4B) of 28 October 2003. The results bring out the flare induced unusual behaviour of the equatorial ionosphere on this day just prior to sunset. The important observations are i) Large and prolonged N e enhancements observed from ionosonde data just after the flare-related peak enhancement in EUV flux. The observed enhancement in N e is due to the increase in ionization production due to the enhanced EUV flux and the persistence of the enhancement is probably due to the prompt penetration related upliftment of the F layer (just prior to the flare peak phase) to higher altitudes, where recombination rates are lower. ii) A significant enhancement in total electron content (TEC) (∼10 TEC units) at regions around the Equatorial Ionization Anomaly (EIA) crest region (Ahmedabad) during the flare in association with the flare related EUV flux enhancement. iii) Similar enhancements seen at stations of Jodhpur and Delhi in the mid latitude sector. iv)The flare related flux enhancements in different longitude sectors in the equatorial electrojet region have been shown to produce positive and negative variations in electrojet strength indicating the presence of current systems having positive and negative polarities in different longitude sectors. Thus the flare effect reveals the longitudinal variation of the counter electrojet events in the Equatorial Electrojet (EEJ) region.

The present work pertains to the eclipse-induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde-derived f o F 1 and f o F 2 parameters are used to analyze the characteristics of the eclipse-induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30-100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave-induced winds in generating blanketing E s over the equator is also examined.