Abstract:
D region effects of the 17–19 March 2015, a St Patrick’s Day super geomagnetic storm
(Dst = 223 nT), using a navigational transmitter very low frequency (VLF) signal (NWC, 19.8 kHz) recorded
at a low-latitude Indian station, Allahabad (geomag. lat., 16.45°N), have been analyzed and compared with
similar strength of the 22–25 June 2015 storm (Dst = 204 nT). During the March storm, NWC signal
amplitude decreased on 17 March (main phase of the storm) and recovered on 27 March, which is 1 day after
the recovery of the storm, whereas for the June storm, VLF amplitude decreased for 2 days only during its
recovery phase. The decrease in the amplitude was pronounced during evening terminator for both the
storms. The modeling of VLF signal anomaly on 17 March and on 25 June using Long-Wave Propagation
Capability code shows an increase in the D region reference height (h0) by ~2.6 km and ~2.5 km, for March
and June storms, respectively. The D region electron density (Ne) determined using storm time h0 and
sharpness factor β shows a decrease in the Ne during the main phase followed by a slow recovery during the
recovery phase of the March storm, whereas June 2015 storm showed a decrease in the Ne only on 25 and 26
June. Morlet Wavelet analysis of the amplitude for both the storms shows a presence of strong wave-like
signatures, suggesting propagation of atmospheric gravity waves/traveling ionosphere disturbances to the
low latitude D region due to the Joule heating at high latitudes.
