Patrick ’ s day geomagnetic storm effect on mid-low-equatorial D-region ionosphere using very low-frequency radio waves

The amplitude of VLF signal is sensitive to change in the D-region electrical conductivity; hence the VLF diurnal signal suggested showing an imprint of space weather disturbances in the D-region. In this work, an attempt is made to study the dynamics of St Patrick ’s Day severe geomagnetic storm of March 2015 using VLF diurnal signal under varying solar and geomagnetic conditions. We have used two VLF transmitter signals GBZ (19.6 kHz; L=2.38) from the Great Britten and NWC (19.8 kHz, L=1.42) from Australia recorded at low latitude station Allahabad (L=1.15), India. The D-region preparation time (DLPT), D-region evolution time (DLET), D-region reduction time (DLRT) and D-region disappearance time (DLDT) are the proposed diurnal VLF signal anomalies as the potential candidate to detect geomagnetic disturbances in the D-region. We have found that the general behavior of VLF diurnal anomalies reflects the storm effect on various time scales. In particular, DLPT and DLET show significant effect during the recovery phase. The DLRT and DLDT are effectively following storm main phase and show pronounced increase. The DLPT, DLET and DLRT also showed prolonged recovery and recovered on 28 March (3 days after storm recovery) whereas DLDT recovered before storm recovery. Significant increase is seen on a particular day in all parameters, which could be associated with solar flares caused D-region changes. The storm was associated with many flare events. The DLPT and DLET for NWC signal exhibit prolonged peaks which could be propagation path dependent.