Modeling of lower ionospheric response during solar X-ray events using propagating radio wave signal

The excess solar X-ray radiation during solar flares causes an enhancement of ionization in the lower ionospheric D-region and hence affects sub-ionospherically propagating Very Low Frequency (VLF) radio wave signal amplitude and phase. VLF signal amplitude and dynamic phase perturbation $(\Delta A)$ and amplitude time delay $(\Delta t)$ (also the corresponding solar X-ray as measured by GOES-15) of several VLF transmitters such as NWC/19.8 kHz, VTX/18.2 kHz etc. signals have been computed for solar flares. In the first part of the work, using the well-known Long Wave Propagation Capability technique, we simulated the flare induced excess amount of lower ionospheric electron density profile by amplitude perturbation method [1]. Unperturbed D-region electron density is also obtained from simulation with the help of the 2-component D-region model and compared with International Reference Ionosphere-model results. Further, in the second part, we compute the corresponding ‘sluggishness’ through ionospheric time delay and effective electron recombination coefficient $(\alpha_{eff})$ analysis [1]. We find that while the time delay is anti-correlated with the flare peak energy flux $(\varphi_{max})$ which is independent of solar zenith angle values [2, 3].