SOLAR X-RAY FLARES INFLUENCE ON VLF/LF SIGNALS IN DIFFERENT MIDDLE-LATITUDES PATHS

Abstract. Sudden Phase Anomalies (SPAs) of VLF/LF signals recorded in Graz (Austria), Sheffield (UK) and Moscow (Russia) stations during two solar flares of intensity class X1.0 and M3.8 in November 2013 are examined. Data from seven transmitters in frequency range from about 20 to 45 kHz are used for the analysis. SPAs were observed in middle-latitudes paths with length from 200 to 7000 km and different orientation. Solar X-ray burst data were taken from the satellite GOES observations in the band of 0.1-0.8 nm. SPA characteristics like types, onset times, growth times, duration of SPA events and their amplitudes are studied. Observed SPAs are positive in 24 cases and they are negative in 2 cases regardless of the length or orientation of paths or signal frequency. The amplitude of SPAs in different paths varies from 10 to 230 degrees and no clear correlation between amplitude of SPAs and intensity of the solar flares or length of the paths are found. The SPAs begin within 3 and 4 minutes after the start of the first and second flare events, respectively. The time of growth for the first event is from 4 to 23 minutes and for the second event it is only 2-5 minutes. The duration is longer for the first event (~1h20 min-2 h) and it is shorter for the weaker flare (~30 min-1h30min)