  Excess solar X-ray radiation during solar flares causes an enhancement of
ionization in the ionospheric D-region and hence affects sub-ionospherically
propagating VLF signal amplitude and phase. In first part of the work, using
the well known LWPC technique, we simulated the flare induced excess lower
ionospheric electron density by amplitude perturbation method. Unperturbed
D-region electron density is also obtained from simulation and compared with
IRI-model results. Using these simulation results and time delay as key
parameters, we calculate the effective electron recombination coefficient
($\alpha_{eff}$) at solar flare peak region. Our results match with the same
obtained by other established models. In the second part, we dealt with the
solar zenith angle effect on D-region during flares. We relate this VLF data
with the solar X-ray data. We find that the peak of the VLF amplitude occurs
later than the time of the X-ray peak for each flare. We investigate this
so-called time delay ($\bigtriangleup t$). For the C-class flares we find that
there is a direct correspondence between $\bigtriangleup t$ of a solar flare
and the average solar zenith angle $Z$ over the signal propagation path at
flare occurrence time. Now for deeper analysis, we compute the $\bigtriangleup
t$ for different local diurnal time slots $DT$. We find that while the time
delay 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 of
the $Z$ dependence of $reduced$-$\chi^2$ seems to follow the variation of
standard deviation of $Z$ along the $T_x$-$R_x$ propagation path. In other
words, for the flares having almost constant $Z$ over the path a tighter
anti-correlation between $\bigtriangleup t$ and $\phi_{max}$ was observed.
