Transcript Loop antenna preamplifier transfer function

Features of Tweeks observed in Indian
Low Latitudes
Rajesh Singh, B. Veenadhari, A.K. Maurya, P. Pant1, A.K. Singh2
Indian Institute of Geomagnetism
New Panvel, Navi Mumbai - 410218 India
Manora Peak, Nainital – 263129, India
2Physics Department, B.H.U. , Varanasi – 221005 India
1ARIES,
Location of AWESOME sites in India
Nainital
Lat.20.48N Long.153.34E
May, 2007
Allahabad
Lat.16.49N Long.155.34E
March, 2007
Stanford University
Varanasi
Lat. 15.41N Long. 156.37E
October, 2007
IHY 2007/UNBSSI program
Sources of ELF / VLF waves
ELF/VLF waves has various Natural and Artificial origin :
 Natural sources of ELF/VLF waves:
Includes Lightning discharge from thunder storms , volcanic eruptions , dust
storm and tornadoes, etc
 Man Made Sources of ELF and VLF Radio Waves:



HF heating
Fixed frequency VLF transmitters
Nuclear explosions
However, on a global basis, by far the most significant
source of wave at ELF/VLF is that generated by lightning
discharges from thunderstorms.
 Global Lightning Flash rate ~ 50-100 sec-1 km-2
Propagates in a guided fashion by multiple reflections through
wave guide formed by lower boundary of ionosphere and earth
surface.
This guided propagation occurs with low attenuation rates (a few
decibel/1000 km) allowing VLF waves to be observed literally around
the world from their source lightning discharge
E-I waveguide boundary conductivity and distance traveled results
in appreciable dispersion at lower frequency end
Tweeks
Dispersion is caused because off wave group velocity becomes very
small near the cut-off frequencies due to multiple reflection between Dregion ionosphere and Earth’s surface
Frequency dispersion analysis of tweeks provide information about
ionospheric reflection height, equivalent electron densities and
propagation distance in Earth-Ionosphere wave guide
Mode Reflection Height Electron Density
Mode
Reflection Height Electron Density
(n)
(Km)
(el/cc)
(n)
(Km)
(el/cc)
1
95
21.5
1 2
9588
94
21.5
21.7
23.3
2 3
9085
89
22.8
23.1
24.2
3 4
8383
24.8
24.7
Propagation Distance
Propagation Distance
(Km)
(Km)
7439
7690
6751
6824
7618
6131
6297
5853
5956
 Looking at the application of Tweek atmospherics in the
studies of the D-region ionosphere, we decided to start by
studying First –
Characteristic Features of the tweeks recorded in Indian
low latitude region
 There has been couple of studies from very few selected tweeks
observations in Indian region by S. Kumar et al., [1994] and Singh et
al., [1992, 1996].
From the analysis of tweeks observed at Bhopal and Varanasi
stations in India, the estimated ionospheric reflection height and
distance traveled was found to vary in the range of 83-89 km and
1500-2500 km for Bhopal, and 84-94 km and 900-1400 km for
Varanasi, respectively.
 But a complete study using longer data base of tweeks was
lacking in Indian low latitude region.
 To fill this gap, in this work using large data base we discuss
in detail for first time characteristic features of the tweek radio
atmospherics observed in Indian low latitude region at
Allahabad (Geomag. lat. 16.490 N).
 Comprehensive analysis of about ~ 3300 tweeks observed
over the three month period during March – May, 2007 is done.
 The quiet location of site and the frequency of tweeks with
high harmonics provide good opportunity to study D and E
region of the ionosphere in Indian low latitude region.
TWEEK EXAMPLES
Total of ~3300 Tweeks
where analyzed
During March-May, 2007
Wave-Guide Mode Propagation
 A VLF wave propagation relies on the theory of waveguide mode
propagation [Budden, 1961].
 In electromagnetic waveguide, there are two types of mode,
transverse electric (TE) and transverse magnetic (TM) modes. Each
mode is associated with its cutoff frequency and there is a one single
mode with no cutoff frequency which is called the transverse
electromagnetic (TEM) mode.
Tweeks are supposed to be excited by vertical current, which occurs
in cloud-to-cloud lightning flashes. In an ideal parallel plate waveguide,
vertical sources excite only TM and TEM modes [Budden, 1961].
But in real condition the electrical properties of the Earth, the
ionosphere and the Earth’s magnetic field causes significant deviation
from ideal condition. Because of this pure TM and TE modes can not
exist in the earth-ionosphere waveguide.
 Instead, the VLF energy is constituted by a superposition of quasi-TM or
QTM and quasi-TE or QTE modes.
QTM modes are similar to TM modes except that it has a small longitudinal
component in the direction of wave propagation. Different modes have
different cutoff frequency, for example QTM mode has cutoff frequency ~1.8
kHz and similarly higher order modes have higher cutoff frequency in EIWG.
The QTEM mode has no cutoff frequency so this mode travels at
approximately with the speed of light with simultaneous arrival of all
frequencies.
The attenuation of the QTEM mode increase exponentially with frequency
and most of the QTEM frequency component are strongly attenuated above
~1.0 kHz, so below first mode cutoff (~1.8kHz) energy is propagated as
QTEM mode waves.
Occurrence pattern of tweeks
 March – May 2007
Tweek Harmonics
Time
1st
2nd
3rd
4th
5th
6th
Dusk
(18:00-21:00LT)
245
526
41
18
4
0
25.24
Night
(21:00-3:00LT)
87
1450
345
59
5
1
58.94
Dawn
(3:00-6:00LT)
171
286
63
0
0
0
15.80
Total Tweek
Percentage
(%)
15.22
68.48
13.59
2.33
0.27
0.031
Occurrence
Percentage%
 A total of ~3,300 tweeks analyzed, which showed that tweeks
occur only in nighttime (18:00-06:00 LT).
Occurrence pattern of tweeks
 Generally tweek occurs during whole night but there
occurrence percentage is higher in night-time (21:00-3:00 LT).
 Higher harmonic tweeks occur mostly in this period. Few
tweeks especially with 5th and 6th harmonics were found to
occur in this period.
 This is because D-region ionosphere is better reflector at night
than during day, and gyrotropy of the ionosphere is strong in
night time when D-region is elevated to 80-90 km height
 Tweeks are easily observed up to 5:00 LT after that their
occurrence becomes less, and after 6:00 LT it is difficult to
observe tweek.
 Also the time duration of the tweeks is found in the range of
10-50 ms.
Ionospheric reflection
height and propagation
distance from source
lightning discharge in
EIWG for all the tweeks ~
3300 analyzed
Tweek
Mode
(n)
Cut-off
Frequency
(kHz)
Mean
Reflection
Frequency (fcn/n) Height
(kHz)
(Km)
a
1
2
3
4
5
6
1.5893
3.2321
4.8750
6.5893
8.3036
10.0893
1.5893
1.6160
1.6250
1.6473
1.6607
1.6815
94.38
92.81
92.30
91.05
90.32
89.20
b
1
2
3
4
5
1
2
3
4
1.6260
3.3036
5.0893
6.8393
8.6607
1.6607
3.4107
5.2679
7.2231
1.6260
1.6518
1.6964
1.7098
1.7321
1.6607
1.7053
1.7557
1.8053
92.25
90.81
88.42
87.72
86.59
90.32
87.95
85.42
82.96
1
2
3
1
2
1.6250
3.4107
5.3750
1.6250
3.4464
1.6250
1.7053
1.7783
1.6250
1.7232
92.30
87.95
83.72
92.30
87.04
c
d
e
 We see that mean frequency for each mode is not same as it should,
rather it increases as mode number increases from n = 1-6 and this
increase is in step manner.
 Calculated reflection height for the different tweek shown in the
example, varies from ~ 94 to 83 km.
 This indicates that as mode number increases reflection height
decreases, this is because as mode number increase cut-off frequency
increases so reflection height decreases. Similar trend is being
observed for other examples also.
 This trend in reflection height indicates that the upper boundary of
the EIWG is not sharp medium rather it is diffuses boundary.
This explains the well known trend of ionosphere that ionization
density slowly increases with height.
 Next we also calculated propagation distance for 710
selected tweeks with very clear dispersion.
 Major part of the tweek analyzed (~46.90 %) has propagation
distance in the range of ~ 6000 km, and most tweek (~90%) have
propagation distance in the range of 4000-8000 km.
 This distance range from the observing station Allahabad in India,
lies in South-East Asia which is one of the three major regions of
lightning activity on globe
To explain why tweeks are mainly observed in the night time? We
have calculated attenuation factor for fundamental mode n=1
 The curve indicates that the attenuation increases as the frequency
approaches cut-off frequency of the EIWG, so waves below this cutoff frequency can not travel, and attenuation also increases when
ionospheric reflection height falls.
 During the daytime attenuation is high and in the nighttime
attenuation is less
 For the closer look of nighttime ionospheric conditions - Dusk,
Night, and Dawn periods attenuations is also calculated
 Attenuation values is nearly similar
for Dusk and Dawn periods
 And values are much lower for
night-time period - this causes higher
occurrence of tweek with higher
harmonics
Direction Finding
for identifying
source position of
lightning discharge
From the simultaneous observation of Tweeks at Indian station
Allahabad
Nainital
Geomag. Lat.16.49N
Geomag.Long.155.34E
Geomag. Lat. 20.48N
Geomag. Long.153.34E
Date: June 13, 2007; Time:15:00:00 – 19:00:00 UT
Local Night time: 20:30:00 – 00:30:00 LT
Analysis of 526 pairs (1052 total) of tweeks observed
simultaneously at both sites during the period
Examples of Tweek atmospherics Analysed
Distance traveled by tweeks from source lightning discharge w.r.t.
ALLAHABAD site
(Geomag. Lat.16.49N; Geomag. Long.155.34E)
Distance traveled by tweeks from source lightning discharge w.r.t.
NAINITAL site
(Geomag. Lat.20.48N Geomag. Long.153.34E)
The calculated distance traveled were then used to locate the
source position of causative lightning discharge of Tweeks
This is fixed by the intersection of two circles drawn by the
distance traveled (propagation distance) from Allahabad and
Nainital stations.
Ohya et al., 55, 627, 2003, Earth Planets Space
The location of the tweek atmospherics were confirmed by
looking into Lightning data
SUMMARY
 Lightning generated tweek atmospherics has the unique
advantage of being capable of estimating lower ionosphere
heights, electron densities at these heights in a wide area
surrounding observation site (~ 1000 – 15000 km)
 The method is useful for detecting changes in reflection
heights, electron density in the D-region ionosphere, which
could correspond to abnormal geophysical conditions.
Thank you for kind attention !
Theoretical Background
The refractive index of wave propagation in magnetoactive plasma
is expressed by famous Appleton-Hartree formula (Budden, 1961):
n  1
2
r
2 X (1  X  iZ )
2(1  iZ )(1  X  iZ )  Y sin   Y sin   4(1  X  iZ ) Y cos 
2
2
4
2
2
(1)
Where
nr = Refractive index of the medium
ω=Angular frequency of the wave
2
2

=Collision frequency of electron with neutrals
θ=Angle between the propagation direction of the
wave and the external magnetic field vector
 p =Angular plasma frequency
H =Angular electron gyro frequency H  eo H
m
2
p 

H

X  
Z
Y





Plasma frequency
fp
1

2
Ne 2
 9 .0 N
m o
Where N is the electron density pre cm3
The upper sing “+”in the denominator of equation (1) corresponds to
the Ordinary wave and lower sing “-” corresponds to the
Extraordinary wave in the magneto active plasma.
 The ordinary mode corresponds to the right hand circular
polarization and Extra ordinary wave corresponds to left hand
circular polarization ( Hayakawa, 1994).
 the possibility of full reflection for the Extraordinary waves in
the lower ionosphere is demonstrated by using the quasi
longitudinal approximation [Yedemsky et al.(1992) and
Hayakawa et al. (1994)]
Applying the quasi longitudinal approximation and excluding the
terms proportional to sin2θ ( under the condition sin2θ << 1) in equ.
(1).
We get the well known expression for refractive index:
X
n  1
(1  iZ )  Y cos
2
r
Taking into accounts the plasma parameters we can neglect the
losses (Z<<Y) (Shvets et al,1997)
The expressions for the refractive index of the Extraordinary and Ordinary
waves are given respectively as follows:
X
n  1
(3)
Y cos
2
e
X
n  1
Y cos
2
o
(4)
For extraordinary wave
For ordinary wave
From equation (4) it clear that refractive index for ordinary waves is always
positive so this wave can propagate into the ionosphere and will be received as
whistler wave .
For Extra ordinary wave refractive index decreases as the electron density ( with
altitude) increase and it will be zero at some height. the wave undergoes full
reflection at this height.
With the further increase of height refractive index for the E-waves becomes
imaginary so the wave can not propagate into the ionosphere .
The X value where n2r becomes zero is given by following equation
X=1
X  1 Y
(5)
(6)
Equation (5) and (6) represents O-mode and E-mode wave respectively
The E-mode wave corresponds to
X=1+Y
when Y > 1 ( ωH > ω)
The E-mode wave corresponds to
X=1- Y
when Y < 1( ωH < ω )
As in the case of ELF/VLF wave frequency of wave is below the
gyro frequency of electron so we use condition first i.e.
X=1+Y
The electron density N (el/cm3) at the tweek atmospherics
reflection height is derived from :
X  1 Y  Y
Under the assumptions of collision less plasma and Y >>1 is as
follows:
N
c  H
31.81 10
8
[el/cm-3 ]
Using the value of ωH=8.4 x 106 s-1 from IGRF model for the low
latitude, we have:
N=1.66 x 10-2fc
[el/cm-3 ]
(7)
Where fc is the waveguide first order cut-off frequency (in Hz)
obtained using tweek measurement
 Estimation of electron density
 The wave source is assumed to be vertical electric dipole located
on the ground and excite the QTM and QTEM waveguide modes.
which propagates with different phase velocities (cummer, 1997)
 Each of the mode is characterized by its cut-off frequency fcn except
for QTEM mode which has no cut-off frequency .
 For a waveguide having perfectly conducting boundaries the cutoff frequency of nth mode is given by (Budden, 1961):
nc
f cn 
(8)
2h
Where n is mode number, h is height of waveguide and c is speed of light in free
space.
 By accurately measuring cut-off frequency for first order mode fc
we can calculate the reflection height (h):
c
h
(9)
2 fc
 The group velocity υgn for the different modes of propagation
is given by (ohtsu, 1960) :
 gn

f
 c1 
f

2
cn
2



1
2
(10)
 The distance d propagated by tweek in the Earth-Ionosphere
waveguide is given as :

f 

d  dTc1 
f 

2
cn
2
1
2
(11)
Where dT is the dispersion time of tweeks measured from tweek
spectrogram , f is frequency of wave.
 By accurately measuring the dispersion time dT and cut-off
frequency fcn from tweek spectrogram we can estimate distance d
of tweek in EIWG from equation (11).
How to measure dispersion time dT ?
dT
Tweek
dT= td - to
to
td
dT = Dispersion time
to = Arrival time of tweek before dispersion
td = Arrival time of tweek after dispersion
Set of Equations used in present
study of D-region ionosphere
1. Electron density :
N=1.66 x 10-2fc
[el/cm-3 ]
2. Reflection height
c
h
2 fc
[km]
3. Distance traveled from source L-D

f 

d  dTc1 
f 

2
cn
2
1
2
[km]