Correlation between the delay and rise time of VLF/LF amplitudes during 20 solar X-ray flares observed in February 2014 at mid-latitude

During daylight hours, the concentration of electrons in the ionosphere can be amplified by solar flares, which may subsequently influence the propagation of radio waves. Previous research on Very Low Frequency (VLF) signals focused on X-class and M-class flares. This study expands the scope to include a broader frequency range and C-class flares. During 20–28 February 2014, signals from 15 transmitters (18.3–81.0 kHz) were recorded by a receiver in Bath, UK. 20 solar flares captured during this period are investigated. A new methodology was employed to determine the rise times of the received amplitudes for comparison with the solar X-ray flux recorded by the Geostationary Operational Environmental Satellite geostationary satellite. The time delays between the onset of the X-ray flux and the onset of received amplitude changes are calculated. The general trend shows that shorter delays are linearly correlated to longer rise times of the amplitudes. It is found that the absolute slopes of the linear correlation between the delay and the rise time of M-class flares are larger than those of C-class flares. Two flares showed onset times of received amplitudes preceding the X-ray flux onset. A possible explanation for this is that the received signals might also be influenced by hard X-rays rather than the analyzed soft X-rays. In summary, this study demonstrates the effects of small C-class and M-class flares on the propagation of VLF/LF signals and offers insights for further research on solar flare impacts on radio waves and the lower ionosphere.