SOME STUDIES ON LOW-FREQUENCY SIGNAL IN RELATION TO X-RAY FLARES AND CLIMATIC CONDITIONS

The statistical behaviour of the sudden enhancement in signal strength (SES) in relation to solar X-ray flares has been studied for the near east-west propagation of 40 kHz radio waves from Sanwa. (36 degrees 1 1'N; 139 degrees 51'E) in Japan to Calcutta (22 degrees 34'N; 88 degre es 24'E) over a long distance path of 5100 km for a period of two year s. The period has been divided into four phases - P1, P2, P3 and P4, a ccording to the position of the overhead sun. The change in signal Str ength during X-ray flares is dependent on the solar zenith angle and c limatic conditions. The statistical modal values of the time lag of th e SES peak with respect to that solar X-ray flare is found to increase as solar zenith angle increases. The relative rates of increase and d ecrease of the signal strength (RRISS and RRDSS respectively) have bee n evaluated for a-number of SES which are related to large X-ray flare s. Their characteristics have also been investigated. The modal values of the relaxation time have been found to be highly correlated with c limatic conditions like temperature and humidity of the propagation pa th.