Transcript RESULTS OBTAINED FROM VLF-MALDA

CALIBRATION OF THE VLF SIGNAL OF VTX
18.2 KHz STATION BY A RECEIVER IN MALDA
Asit Kumar Choudhury, ICSP (Malda Branch)
Prof. Sandip Kumar Chakrabarti, SNBNCBS & ICSP,
Mr. Suman Ray, ICSP,
and
Mr. Sudipta Sasmal, ICSP,
It also has been reported that the time taken to
form the D-region or to dissolve it through
recombination also becomes anomalous
immediately prior to the earthquake (Chakrabarti
& Sasmal 2007 and Chatterjee, Bari &
Choudhury 2009).

In order to establish a consensus in this
issue, especially in the context of Indian
scenario, Indian Centre for Space Physics
(ICSP) has been recording the VLF signal
for the last several years transmitted from
the Indian Navy station VTX near the
southern tip of India

At ICSP we are involved in preparing such
Standardized Calibration Curves (SCCs) for a
large number of baselines in order that the
ionospheric anomalies may be pinpointed very
accurately

SCC for VTX-ICSP (Kolkata) baseline has
already been published (Chakrabarti et. al. Nat.
Hazards…2009) by ICSP .

Here we present SCC for VTX-ICSP (Malda)
baseline only.
Experimental
Setup
We have used a Gyrator II loop
antenna made on a square frame of
one-meter side. Several turns of a
shielded single core wire are wound
over the frame.
• DAQ card connected to a
computer for continuous data
acquisition
* Receiver (made at ICSP)
Transmitting Station
Receiving Station
Vijayanarayanam
Malda
Latitude – 08°26´ North
Latitude – 25° North
Longitude - 77°44´ East
Longitude - 88°08´ East
Some Classical data from VTX-station received at Malda centre.
Time (IST=UT+5:30:00)
The signal clearly showed recovery after the sunrise terminator and before the sunset terminator.
The “cosine” pattern of the solar flux is clearly visible in the daytime.
Comparison between Malda-data and Kolkata-data
Standardized Calibration Curves (SCCs)

In order to obtain the “mean” Sunrise terminators (SRT)
and Sunset terminators (SST) over the year we now
present four years of data (2005, 2007–2009).

We have not removed the data for those days in which
the SRT or SST may be anomalous due to seismic
activities, because we did not have any reference frame to
quantify the disturbance.

However, once the mean SCCs are drawn for the sunrise
and sunset, significant deviations from the mean may be
used to judge whether there is any real correlation between
the ionospheric and seismic activities
Calibration curve for Malda-data of 2005,07
Here we observed that generally SRTs and SSTs fall in between
the curves of local sunrise - sunset times, and VTX sunrise sunset time, though in some days there are exceptions.
Calibration curve for Malda-data of 2008,09
Standardized Calibration Curve
We note that the
SCC for sunrise is
situated roughly
midway between
the SR Malda and
SR VTX curves.
This indicates that
the terminators
may dominated by
a single hop and
the signal is
reflected from
halfway between
the transmitter
and receiver.
Superpose all the four years’ data into a single plot to obtain the standardized
calibration curve (SCC) for the VTX-Malda baseline.
The coloured curves are our standardized calibration curve for the VTX-MALDA
baseline, which we obtained by taking running mean of all the observed points.
Comparison of the original day length of Malda and VTX with
VLF-day-length (SST-SRT) using the data of 2005,07,08.
TD-MAL
TD-VTX
In the winter period the VLF day length is closer to TD (Malda ) in summer, it
is somewhat intermediate between TD (Malda) and TD(VTX).
The correlation with seismic activities.

To see if correlation between the length of the
“VLF day” and the seismic activities, we use the
data for the period of January to December 2008.

First, we plot the day-length as a function of day

To obtain the mean variation during the period, we took
the running mean of the all observed data

Then we calculate initially the standard deviation (σ)
using the above mean plot and we remove the data which
are away from σwhole .

Using rest of the data we again compute the standard
deviation σ and plotted curves which are σ, 1.5σ, away
from the mean.
Variation of the VLF-day-length

In some days the anomalous deviations, as high as
2σ , some seismic activities were generally found in
those or nearby days.
Variation of the VLF-day-length

We associated such activities with those anomalous
days and marked the association by larger circles.
These circles are kept at a constant distance away
from the observed points (squares), and thus, their
exact co-ordinates are not important.
Co-relation between the deviation of VLF-day-length
and the effective magnitude of earthquake

We compile the latitudes and
longitudes of the epicentre,
magnitude and depth of the
earthquakes, its magnitude,
from the Indian Meteorological
Department

The actual magnitudes of the
earthquakes are not important
since we are interested in the
energy deposition by these
earthquakes near the mid-point
of the great circle path instead
of taking the original magnitude

we calculate the energy of these earthquakes from
log10E=4.4+1.5Ms
(for earthquake less than 5.0 magnitude) and
log10E=5.24+1.44Ms
(for earthquake greater than 5.0 magnitude)
where, E=Energy of the earthquake in Jules, and
Ms=surface wave magnitude (Lowrie, 2007)

In the days when more than one earthquake occurs,
we calculate the total energy from the individual
earthquakes.

From this, by reverse process, we find out the
effective magnitude on each day.

We thus have the deviation of the VLF-daylength
from the mean and the effective magnitude on any
given day (effective magnitude is zero on nonseismic days) which enable us to compute the
correlation coefficients.
Co-relation between the deviation of VLF-day-length and the effective magnitude earthquake
Figure shows the correlation of these two quantities. We observe that a
weak correlation peaks at Three days ahead of the earthquake.
CONCLUSIONS
1) We obtained the SCCs for VTX-ICSP baseline
2) It appears that there are deviations of the VLF day length closed to the
days on or nearby seismically active days
3) We found that the Correlation coefficient shows a weak peak three days
prior to the seismic activities.
In future we shall try to co-relate our results with the data received
from other places which will help us to improve the predictability of
earthquakes.
THANK YOU.