  In this paper, we present a model for determination of a weakly time
dependent effective recombination coefficient for the perturbed terrestrial
ionospheric D-region plasma. We study consequences of a class M1.0 X-ray solar
flare, recorded by GOES-15 satellite on February 18, 2011 between 14:00 UT and
14:15 UT, by analyzing the amplitude and phase real time variations of very low
frequency (VLF) radio waves emitted by transmitter DHO (located in Germany) at
frequency 23.4 kHz and recorded by the AWESOME receiver in Belgrade (Serbia).
Our analysis is limited to ionospheric perturbations localized at altitudes
around 70 km where the dominant electron gain and electron loss processes are
the photo-ionization and recombination respectively.
