Abstract:
We present perturbations in very low frequency (VLF) signals
received at Ionospheric & Earthquake Research Centre (IERC) (Lat.
22.50N, Long. 87.48E) during and prior to two earthquakes, one
on 11 March 2011 at 11:16:24 LT (M ¼ 9) in Honshu, Japan and
another on 12 May 2015 at 12:35:19 LT (M ¼ 7.3) in Kodari,
Nepal. The VLF signal transmitted from JJI (22.2 kHz) in Japan
(Lat. 32.05N, Long. 131.51E) showed strong shift in VLF-sunrise
terminator times towards night-time few days prior to both the
earthquakes. These two earthquakes took place near the transmitter
JJI and receiver IERC respectively. We simulated the VLF sunrise
terminator time shifts using the long wavelength propagation
capability (LWPC) code. To effectively represent the D-region
ionospheric variabilities, we assumed a mean dynamic perturbation
over the path and presented them with a set of effective
Wait’s parameters (beff, h0
eff ). We have successfully reproduced the
temporal trend of the normalized VLF signal amplitude at VLF
sunrise terminators for a few days around both the earthquakes.
We used Wait’s exponential model for estimating the altitude profile
of D-region electron density Ne(h) at VLF sunrise terminator
times around both the earthquakes and quantified the changes
of those Ne(h) profiles.
