Geomagnetic disturbances and associated phase anomalies of a VLF radio wave at midlatitudes

Phase anomalies of VLF radio waves were investigated in relation to geomagnetic disturbances, using data obtained from the reception of the NLK station during 37 months from August 1970 to December 1974. The horizontal component of geomagnetic field observed at Memambetsu (Japan) together with the Ap index is used as an index of intensity of a geomagnetic disturbance. Primary storm effects on the phase anomalies during nighttime with an average value of 8μsec have a tendency to occur 2 hours later than the decrease of horizontal component of geomagnetic field with an average value of 70 gammas (hourly mean value). Storm after-effects which are the main features at midlatitudes continue for 3 days with large phase anomalies and further continue for about 5 days with small ones. Phase anomalies were observed during the daytime when the solar zenith angle was about 70° and observed during severe storm periods even with a solar zenith angle of 30°. Sudden phase anomalies were observed just after magnetic Pi pulsations.