Abstract
Abstract. In this paper we analysed sudden phase anomalies (SPAs) of VLF–LF signals recorded at Graz (Austria), Birr (Ireland) and Moscow (Russia) stations during two strong solar flares in September 2017. The first X-class 9.3 flare occurred on 6 September at 12:02 UT, and the second X-class 8.2 flare was observed on 10 September 2017 at 16:06 UT. Data from seven transmitters in a frequency range between 20 and 45 kHz are used for the analysis. The SPAs were observed in all middle-latitude paths (differently orientated) with path lengths from 350 to 7000 km. Solar X-ray burst data were taken from GOES satellite observations in the wavelength band of 0.05–0.4 nm. It was found that (i) the amplitude of SPAs in different paths varies from 10 to 282∘, and (ii) the correlation between the amplitudes of SPAs, the lengths of paths and the signal frequency is weak. The change in effective height of reflection due to lowering of the reflecting layer during the flares was found to be about 12 km for the first event and about 9 km for the second event. Spectral analysis of the X-ray and LF data, filtered in the range between 5 s and 16 min, showed that the LF signal spectra are very similar to X-ray spectra. Maxima of both X-ray and LF spectra are in 2–16 min interval.
Highlights
Solar flares are one of the most influential space weather events
In this work we investigate sudden phase anomalies (SPAs) characteristics from the VLF–LF signals recorded during two strong solar flares in September 2017 in the three European middle-latitude VLF stations – Graz (Austria), Birr (Ireland) and Moscow (Russia)
The amplitude and phase anomalies due to the solar flares were observed in all the paths
Summary
Solar flares are one of the most influential space weather events. During the flares, electromagnetic and corpuscular radiation of significant power is emitted. Very low and low frequency (VLF–LF) radio wave measurements are used to study the D-region behaviour These waves are reflected by the D region, and changes in ionisation of the D region result in amplitude and phase variations of the received VLF signal. Thomson et al (2005) modelled the D-region electron densities as functions of X-ray flux up to the level of the largest, X45 flare During this largest flare, they found that the reflecting height was about 17 km below the normal midday level. Grubor et al (2008) investigated changes in the ionosphere during X-ray flares (ranging from class C to M), which have the corresponding VLF recordings from Belgrade in 2004–2007 They found the decrease in ionosphere reflection height of up to 7 km and β = 0.47 during the strongest events. The event began at 15:35 achieving the maximum at 16:06 as X8.2 and ended at 16:31 UT
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