Low-latitude sub-ionospheric VLF radio signal disturbances due to solar flares: Effects on the attenuation and phase velocities of the waveguide modes

Abstract

Solar flares are sudden bursts of X-rays and UV rays emitted from coronal magnetic loops in active regions near sunspots on the sun’s surface. Soft X-rays below 1 nm penetrate the D-region ionosphere, causing excess ionization and altering its conductivity profile. This study examines solar flares (C-class and M-class) from the 25th solar cycle using two Very Low Frequency (VLF, 3–30 kHz) radio receivers in the low-latitude Indian region, located in Cooch Behar (CHB) and Kolkata (CUB). The work focuses on the Indian VLF transmitter VTX at a frequency of 18.2 kHz to study the effects of solar flares on the D-region ionosphere and VLF signal propagation characteristics in the earth-ionosphere waveguide. The solar zenith angle at the CUB station has a significant impact on the magnitude of VLF amplitude disruption caused by solar flares, showing a positive correlation (r = +0.82, r = +0.61) with flare power during low and high solar activity, respectively. In contrast, CHB exhibits both positive and negative amplitude perturbations, with a negative correlation (r = −0.83, r = −0.78) between flare power and VLF amplitude under similar conditions. The Long Wave Propagation Capability (LWPC) code has been used to explain the differences in the observed amplitude perturbations due to solar flares in both receivers. Solar flares weaker than C2.0 at CHB reduce attenuation and phase velocity of the propagating waveguide modes in the earth-ionosphere waveguide, causing positive amplitude perturbations, while stronger flares increase these parameters, resulting in negative perturbations. In contrast, solar flares of all classes cause an increase in phase velocities and a decrease in attenuation coefficients of the propagating waveguide modes along the VTX-CUB propagation path, resulting in positive VLF amplitude responses. This study highlights distinct responses of VLF signals to solar flares in different propagation paths, emphasizing the complex interactions between solar activity and earth-ionosphere waveguide properties.