The whistlers are right-handed electromagnetic waves which propagate below the plasma frequency and electron cyclotron frequency. We study the whistlers induced by lightning discharges and their penetration to the ionosphere. They propagate approximately along the geomagnetic field lines and their amplitude can be attenuated mainly owing to the particle collisions in the lower ionosphere. We developed a method for automatic detection of the fractional-hop whistlers in the spectrograms computed from the VLF data recorded by the DEMETER satellite. By comparison of detected whistlers with lightning data from the EUCLID network, we are able to assign causative lightning to the observed whistlers. Processing large amount of data, we statistically investigate the relation of the detected whistler amplitudes to the lightning currents, distance between the discharge and penetration point to the ionosphere and the daytime. In our study, we use data from ~200 satellite passes over the European region. We find that mean whistler amplitude monotonically decreases with horizontal distance up to ~1000 km from the lightning source. The maximum of whistler intensities is shifted ~1° from the satellite magnetic footprint owing to the oblique propagation. We find that during the morning, the mean whistler amplitude is about three times lower than during the local evening for the same lightning current, which is probably caused by different ionization profiles of the ionosphere.