Acknowledgements

Inter-annual variability of GPS-TEC depicts the peak, descending phase of solar cycle 23 and minimum, ascending phase of solar cycle 24. Maximum TEC values are observed during 2002– 2004 and minimum TEC values are observed during 2008–2010. GPS TEC indicates a distinct daily, monthly, semi-annual and annual cycle. Sensitivity of TEC to solar activity is prominent in EIA region compared to mid-latitudes in northern hemisphere. TEC values recorded are consistent with large-scale electrodynamics associated with the equatorial electrojet (EEJ), plasma fountain, equatorial ionization anomaly (EIA), equatorial wind and temperature anomaly, which affect the ionosphere variability at equatorial and low latitude regions. The high variability of equatorial and low latitude ionosphere are due to the perfect horizontal alignment of the geomagnetic field lines at the dip equator and the shifting between the

GPS-TEC values increase from geomagnetic equator to the crest of EIA region (17 N geomagnetic latitude) after which they gradually decrease toward mid-latitudes in the northern hemisphere. Latitude variability of ionosphere is more pronounced during the high solar activity years (2002–2004) when compared to low solar activity years (2008–2010). Diurnal peak TEC value has longer duration between 0 and 9 N geomagnetic latitude. Diurnal maxima have pronounced peaks and diurnal minima is observed for longer duration in the northern crest of EIA region and beyond. Ionosphere variability with longitude is observed for longitude difference of 19 E and above during the ascending phase of current solar cycle 24. Normally, solar radiation strikes the atmosphere more obliquely with increasing latitude decreasing its intensity and production of free electrons, whereas near the geomagnetic equator its strikes horizontally with eastward electric field during day and westward during night. This causes plasma diffusion along magnetic field lines at approximately 15 geomagnetic latitudes forming crests on both the hemispheres (EIA region). Hence, TEC increases gradually from geomagnetic equator to the EIA crest, beyond which it decreases toward the mid-latitude regions. Intensity of EIA and its latitude of crest development vary with the strength of EEJ, season and solar activity. Our study indicates that the northern crest of EIA region extends up

Diurnal variability of ionosphere depends on the intensity of solar activity, season and strength of geomagnetic field with high TEC values recorded in 2004 and 2011 and low values in 2009. Day-to-day variability is more pronounced for the high solar activity years when compared to low solar activity years. Maxima occurs during midday (7–13 h UT) with longer duration for geomagnetic latitudes between 0 and 9 N and pronounced peaks for greater than 9 N. Minima occurs after midnight (20–24 h UT) between 0 and 9 N geomagnetic latitude whereas it is for longer duration (15–24 h UT) for northern crest of EIA region and beyond. Day-to-day variability of maxima is more pronounced in the crest of EIA regions (9–18 N geomagnetic latitude). Day-to-day variability of diurnal TEC is high and random during December 2004 and September 2011 due to seismo-ionosphere disturbance caused by 2004 Sumatra and 2011 Sikkim earthquake. Also anomalous day-to-day TEC variability is observed for Gujarat stations (MABU, KHAV, BELP) in 2011 which needs further detailed study. Diurnal maxima and minima vary significantly during the equinox and solstice of summer and winter seasons with lower values during summer solstice in EIA region and higher values during equinox and winter solstice. Beyond EIA (>18 N), maxima with pronounced peak occurs in the equinox

geographic and geomagnetic equator.

202 Multifunctional Operation and Application of GPS

to about 17–18 N geomagnetic latitude in Indian region.

This is a CSIR-4PI ARiEES contribution. We thank the anonymous reviewers for their time and effort.
