Solar Flare and F Layer Critical Frequency

ABSTRACT

In an earlier study of the effect of some Mögel-Dellinger type of fade-outs at several stations, it was found that, after the fade-out, the trend of variation of the deviation Δf of F-2 critical frequency from the mean had a phase reversal at about 14°N geomagnetic latitude¹. Three of the selected fade-outs and flares associated with them had started before ground sunrise at the Indian Stations. This paper deals with further investigation of the possible effect of solar flares on F layer critical frequency, f₀F during the presunrise period. The analysis carried out for a large number of flares shows consistent results at stations in India and Japan. The flare associated changes in f₀F at lower latitude stations are found to be opposite in phase to those at higher latitude ones. At Ahmedabad (14°N representing a lower latitude station) Δf becomes negative after flares corresponding to the period 0100–0400 hrs. local time, whereas for flares corresponding to 0400–0500 hrs. local time it becomes positive. At Yamagawa (20°N representing a higher latitude station), an opposite type of variation is observed. Δf becomes positive after flares corresponding to the period 0100–0400 hrs. local time, while it becomes negative after flares corresponding to the period 0400–0500 hrs. local time.

A critical examination of the data has been carried out to test the validity of the observations made in this paper.