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Excess solar X-ray radiation during solar flares causes an enhancement ofionization in the ionospheric D-region and hence affects sub-ionosphericallypropagating VLF signal amplitude and phase. In first part of the work, usingthe well known LWPC technique, we simulated the flare induced excess lowerionospheric electron density by amplitude perturbation method. UnperturbedD-region electron density is also obtained from simulation and compared withIRI-model results. Using these simulation results and time delay as keyparameters, we calculate the effective electron recombination coefficient($\alpha_{eff}$) at solar flare peak region. Our results match with the sameobtained by other established models. In the second part, we dealt with thesolar zenith angle effect on D-region during flares. We relate this VLF datawith the solar X-ray data. We find that the peak of the VLF amplitude occurslater than the time of the X-ray peak for each flare. We investigate thisso-called time delay ($\bigtriangleup t$). For the C-class flares we find thatthere is a direct correspondence between $\bigtriangleup t$ of a solar flareand the average solar zenith angle $Z$ over the signal propagation path atflare occurrence time. Now for deeper analysis, we compute the $\bigtriangleupt$ for different local diurnal time slots $DT$. We find that while the timedelay is anti-correlated with the flare peak energy flux $\phi_{max}$independent of these time slots, the goodness of fit, as measured by$reduced$-$\chi^2$, actually worsens as the day progresses. The variation ofthe $Z$ dependence of $reduced$-$\chi^2$ seems to follow the variation ofstandard deviation of $Z$ along the $T_x$-$R_x$ propagation path. In otherwords, for the flares having almost constant $Z$ over the path a tighteranti-correlation between $\bigtriangleup t$ and $\phi_{max}$ was observed.

astrophysics and space science

Effective recombination coefficient and solar zenith angle effects on  low-latitude D-region ionosphere evaluated from VLF signal amplitude and its  time delay during X-ray solar flares