A Probe of Magnetosphere-Ionosphere Coupling using the Propagation Characteristics of Very Low Frequency Signal

The amplitude and phase of VLF/LF radio signal are sensitive to changes in the electrical conductivity of the lower ionosphere when propagated in the Earth-ionosphere waveguide. This unique characteristic makes it useful in studying sudden ionospheric disturbances and/or anomaly especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbances and/or storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region ionosphere. Other than X-ray flux enhancement of amplitude and phase, diurnal VLF signature may convey other important information especially those related to geomagnetic disturbance/storm induced ionospheric changes. In this paper, using the data of three propagation paths (at latitudes 40-54), we performed detail analysis of the trend of variations of aspects VLF diurnal signal under varying solar and/or geomagnetic space environmental conditions for identification of possible geomagnetic footprint on the ionosphere. We found that trend of variations significantly reflected the prevailing space weather conditions of various time scales. The `dipping' of the signal diurnal amplitude have shown noteworthy consistency with significantly geomagnetic perturbed and/or storm conditions in the time scale of 1-2 days. We also found that dipping of most MDP signal occurred irrespective of the time (of the day), which an event happened, while those of MBSR, MASS, SRT and SST appear to largely depend on event occurrence time and/or duration. Pre-sunset event had more influence on the SST and MASS (dusk signal), while pre-sunrise event had more influence on the SRT and MBSR (dawn signal), and depending on the duration of the event, impact could be extended to the neighbouring point/component in succession. The induced dipping varied with geomagnetic activity/event intensity and/or duration, as well as the signals' propagation paths. With lesser or significantly reduced geomagnetic activity on the day following a main event, the post-storm day signal exhibited a tendency of recovery to pre-storm level. We further investigated correlation between ionospheric anomalies attributed to seismic events and solar-induced magnetosphere variations.