Geophysical control of amplitude and phase parameters of VLF propagation along mid-latitude paths

The effective height of the VLF waveguide was analyzed as a function of geomagnetic activity variations for different models deduced from VLF phase measurments. It is confirmed once again that the effective height is controlled at night by the current Ap-values. As Ap increases, the mean height decreases but shows stronger variations. The radio signal amplitude is not as strongly dependent on the geomagnetic activity, being controlled by the intensity of galactic cosmic rays. A relation was established between anomalous night-time variations in the VLF amplitude and cosmic ray variations during the Forbusch-effect.