Theoretical study of the diurnal behavior of VLF signal and comparison with VLF campaign data

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Theoretical study of the diurnal behavior of VLF
signal and comparison with VLF campaign data
Sujay Pal 1, Sudipta Sasmal2, and Sandip K.
Chakrabarti1,2 1 S.N. Bose National
Centre for Basic Sciences, Block JD, Sector
III, Kolkata 98 2 Indian Centre for
Space Physics, 43 Chalantika, Garia Station Rd,
Kolkata 84
The Sharjah-Stanford AWESOME VLF Workshop Feb
22-24, 2010, Sharjah, UAE
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• Outline of my talk
• Introduction
• Diurnal behaviour of amplitudes and phases of
  VLF
• signals
• Motivation of the present work
• Theoretical background
• Our theoretical results of the variation of
  amplitudes and phases variation
• A comparison with observational results
• Conclusions

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Introduction Very Low Frequency (3-30 kHz)
electromagnetic signal propagation through EIWG
depends strongly upon the lower ionospheric
ionization conditions. The ionization varies
from time to time due to variation of incident
radiation on the ionosphere and due to different
photo-chemical reactions. It changes the
effective reflection height and reflection
coefficient of a VLF signal. The reflection
takes place from the D-layer (60-90 km) during
day-time and from the lower E-region during
night-time. Accordingly, the amplitude and phase
of a VLF signal shows a diurnal variation.
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Typical diurnal behaviour of amplitude of VTX
(18.2 kHz) signal
ICSP
AWESOME
ICSP
VTX-Kathmandu 2300 km VTX-Kolkata 1932
km VTX-Pune 1209 km
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Motivation of the present work Monitoring of
VLF transmitter signals from different places of
India is being continuously carried out by ICSP,
Kolkata using AWESOME and ICSP made set up.
Significant difference in the amplitude behaviour
is found over different propagation
paths. Understanding of these data requires a
detailed theoretical knowledge. In this work, we
have tried to simulate and understand the diurnal
and seasonal behaviour of amplitudes and phases
of VLF signals based on wave-hop theory, Wakai
et. al, 2004.
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Theoretical Background
The effective field strength received by a small
loop antenna for a 1-hop sky-wave is,
in
where, Pt Transmitter power in kW, ?
launching elevation angle relative to
horizontal, Ri Ionospheric reflection
coefficient, Fi Ionspheric focusing factor, Ft,
Fr are the transmitting and receiving antenna
factors, L1 is the total sky-wave path length of
1-hop wave. For 2-hop sky-wave
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Assume Ri1 Ri2, Fi1 Fi2 and Fi Dg
1 Similarly, field strengths for higher-order
hop can be obtained. The resultant field strength
and resultant phase are calculated accordingly.
Models of ionospheric reflection heights and
reflection coefficients as functions of solar
zenith angle help us to simulate the diurnal and
seasonal behaviours of VLF signals over different
propagation paths. We introduce two parameters h
and dh such that hNhdh h daytime effective
reflection height hNnighttime effective
reflection height
summer
(ITU-R Recommendations)
winter
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Theoretical Results
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Diurnal behavior of amplitude of VTX (18.2 kHz)
at Kolkata in Summer
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Diurnal behavior of amplitude of VTX (18.2 kHz)
at Kolkata in Winter
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Typical phase behaviour of VTX (18.2 kHz) signal
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A comparison
VTX 18.2 kHZ
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Conclusions

• We made an effort to understand theoretically the
  VLF signal propagation characteristics over the
  Indian subcontinent.
• It is seen that the variation of VLF reflection
  height is more sensitive than the variation of
  reflection coefficient.
• By choosing appropriate parameters for both
  variations, observational results can be
  reproduced.
• We are also modelling the variation of received
  signal during total or partial solar eclipses,
  Gamma-ray bursts and fluctuation of ionization
  due to Earthquakes.
• More works have to be done in near future.

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THANK YOU