  Solar flares significantly impact the conditions of the Earth's ionosphere.
In particular, the sudden increase in X-ray flux during a flare penetrates down
to the lowest-lying D-region and dominates ionization at these altitudes
(60-100 km). Measurements of very low frequency (VLF: 3-30kHz) radio waves that
reflect at D-region altitudes provide a unique remote-sensing probe to
investigate the D-region response to solar flare emissions. Here, using a
combination of VLF amplitude measurements at 24kHz together with X-ray
observations from the Geostationary Operational Environment Satellite (GOES)
X-ray sensor, we present a large-scale statistical study of 334 solar flare
events and their impacts on the D-region over the past solar cycle. Focusing on
both GOES broadband X-ray channels, we investigate how the flare peak fluxes
and position on the solar disk dictate an ionospheric response and extend this
to investigate the characteristic time delay between incident X-ray flux and
the D-region response. We show that the VLF amplitude linearly correlates with
both the 1-8 A and 0.5-4 A channels, with correlation coefficients of 0.80 and
0.79, respectively. Unlike higher altitude ionospheric regions for which the
location of the flare on the solar disk affects the ionospheric response, we
find that the D-region response to solar flares does not depend on the flare
location. By comparing the time delays between the peak X-ray fluxes in both
GOES channels and VLF amplitudes, we find that there is an important difference
between the D-region response and the X-ray spectral band. We also demonstrate
for several flare events that show a negative time delay, the peak VLF
amplitude matches with the impulsive 25-50 keV hard X-ray fluxes measured by
the Ramaty High Energy Solar Spectroscopic Imager (RHESSI).
