STUDY OF VLF WAVE-PROPAGATION AT 12.3 KHZ - PRELIMINARY-RESULTS

An experimental set-up consisting of a loop antenna, VLF tracking rece iver, rubidium frequency standard for phase reference and two-channel strip chart recorder for recording the phase and strength of the incom ing signals has been in operation at Rajkot since July 1991. The set-u p is being used to monitor continuously the signal strength and phase of the 12.3 kHz VLF OMEGA signal from La Re-Union (20-degrees-58'S; 55 -degrees-17'E). The data so far obtained show the typical diurnal vari ation of phase and amplitude. The diurnal phase change is typically 50 mus and the diurnal amplitude change is 14 dB which are found to vary from month to month. In addition, effect of solar flare is seen as a sudden decrease in phase by about 20 mus. Results of the analysis of a mplitude and phase changes due to several flares are presented. Many a bnormal features seen in the phase and amplitude records, such as peri odic oscillations in the phase and amplitude, sudden spiky changes in amplitude and phase, fluctuations in phase alone but not associated wi th amplitude changes etc. are also presented.