Study of day to day variability in foF2 at low latitudes in the Indian region 
 
H Chandra and Som Sharma 
 
Physical Research Laboratory 
Ahmedabad 380 009 
 
Long series of ionospheric data obtained from regular radio soundings of 
ionosphere all over the globe provide valuable database for studying the 
climatology of ionosphere under different geophysical conditions. While the 
features like daily, seasonal, solar cycle and latitudinal variations are 
understood fairly well, the day-to-day variability both on geomagnetic quiet and 
disturbed days is yet to be completely understood. Space weather effects and 
pre-seismic anomalies are some of the topics of recent interest. The variability 
of ionosphere, both on geomagnetic quiet and disturbed conditions, is also 
important as the ionospheric effects like the time delays in radio propagation 
are essential in navigation. The ionization distribution in the F-region at low 
latitudes is largely controlled by the equatorial electrodynamics that gives 
rise to the Equatorial Ionization Anomaly (EIA). EIA is also one of the 
important parameters associated with the onset of equatorial spread-F an 
 d scintillations. With this aim, hourly values of the critical frequency of F2 
layer (foF2) over Ahmedabad, situated near the anomaly crest are analyzed to 
estimate the variability. Normalized deviations in foF2 and the standard 
deviations at different hours are computed for different seasons covering 
different solar activity years. For comparison the variability in foF2 over 
Thumba, situated close to the magnetic equator are also examined. The 
variability is more pronounced during the nighttime. Case studies on few 
geomagnetic storms are also presented. Findings of the study will be discussed 
in term of relevance to ionospheric effects at low latitudes.