foF2 variability over New Delhi during different solar cycle conditions and comparison with IRI-2001 model Original Research Article

In this paper, the variability of F2-layer critical frequency foF2 is examined, using modern digital ionosonde observations at a low-middle latitude station, New Delhi (28.6°N, 77.2°E, Dip 42.4°N). Data used for the present study are the hourly values of foF2 for the periods of high (2001–2002) and low (2004–2006) solar activity. To investigate the foF2 variability, the values of median, lower quartile and upper quartile are estimated at each hour for different seasons, during both the solar activity periods. The variability index (henceforth referred to as VI) of foF2 is then obtained by using inter-quartile difference and its percentage ratio to the median. Results show that VI is in general, lower during daytime hours than by night during high solar activity for all the seasons, while during low solar activity, VI behavior is irregular during all the seasons. A solar activity dependence of VI is found with maximum values during periods of low solar activity in all the seasons except for winter during pre-dawn and post-sunset hours. Comparisons of observations with IRI-2001 model [Bilitza, D. International Reference Ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001] show in general good agreement during both the solar activity periods, with model values lying within inter-quartile range.