Transcript AWESOME Tunisia - Stanford University

Magnetic Field
Frequency (kHz)
Characteristics of VLF Tweeks
Tunis 21-Aug-2007 UT
N/S Antenna
14
12
10
8
6
4
2
100
50
26.8
27
27.2
Time (seconds) after 00:05:00 UT
Tunis 21-Aug-2007 UT N/S Antenna
0.5
0
-0.5
-1
26.84
26.85
26.86
26.87
26.88
Time (seconds) after 00:05:00 UT
In this spectrogram recorded by our receiver during August 2007 we can recognize the
tweeks characteristics which present tails with the sferics.
 Magnetic Field variation
Nedra Tounsi & Hassen Ghalila
Laboratoire de Spectroscopie Atomique Moléculaire et Applications
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References
 Lightning and ionospheric remote sensing using VLF/ELF radio atmospherics
S.A. Cummer Thesis 1997
 Propagational Features of Higher harmonic Tweeks at low latitudes
R.P. Singh 1996
 Tracing Particles from the Sun to the Earth’s Ionosphere
S. Murray 2007
 Equivalent electron densities at reflection heights of tweek atmospherics in the low
middle latitude D-region ionosphere
H. Ohya 2003
 Observations of tweeks in the south Pacific region (2003) and lightning stroke
distance estimation from single station observation and validation with WWLLN data.
V. Ramachandran 2007
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Earth-Ionosphere Waveguide
Ionosphere
Earth
Ligthning - Sprites : Electrical source of electromagnetic waves
The waveguide acts as a filter -> remains only the waveguide modes
3
Detection of tweeks
Cutoff frequency
Zoom
Zoom
Velocity dispersion
The two first tails are clear enough to extract the cutoff
frequencies and the dispersion of the light velocity
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Cutoff frequency
The tweeks signals have caracteristic Cutoff
frequencies due to long distance propagation by the
f2 =2f1 ; f3=3f1 ; f4=4f1
Earth-ionosphere wave guide mode.
We measure the cutoff frequencies by the help of the
improfile tool of matlab which gives us a one dimensional plot
of the spectrogram.
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Ionospheric height , Group
velocity
On the basis of values of cutoff frequency we
can compute the Height and Group velocity
values.
 We estimate the heigh of ionosphere and the
groupe velocity through these expressions.
Date
Mode(n)
Height =>
h=
nc
2 fcn
Group velocity => Vg =c[1-(fc /f)2]1 /2
fc (Hz)
h (m)
Vg (m s-1)
21-08-2007
1
1703
91400.1396
2,9373 108
21-08-2007
2
3560
89223.9458
2,6772 108
21-08-2007
3
4980
86879.5763
2.2456 108
21-08-2007
4
6370
88959.1864
1,4648 108
The nighttime ionospheric reflection height estimated from tweeks varies in the range of
88-92km
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The groupe velocity is nearly equal to celerity of light.
Electronic density, mobility
conductivity
Date
Height
(km)
Electronic density
Ne (m -3)
∆Ne (m -3)
Mobility
µe(m2/V/s)
Conductivity
σe (S/m)
06-08-2007
 50
43.50 106
6.453 106
2.017 103
1.403 10-8
24-09-2007
70
31.13 106
6.453 106
2.093 104
1.042 10-7
24-09-2007
 100
20.60 106
6.453 106
3.565 106
1.175 10-5
Ne
(h)=1.241.10-8
FC Fh
µe =1.36N0/ Nn
(Ohya 2003)
Fh =B e/2πm (Cyclotron frequencies)
• Using the International Geomagnetic Reference field model and taking
tunis’s latitude (10°1024 E)
Fh =1.2294 MHz
•Using the Handbook atmospheric in summer at middel latitude.
• Atmospheric density decrease exponentially with Heigh.
σe =qe Ne µe
Estimated lightning distance
∆t = t1 - t2
Time interval between two close frequencies gives
the source distance of atmospherics which is written
as expression (1).
∆t = d/vg1 - d/vg2

d=
v g1 v g2
v g1 – v g2
t
(1)
For perfectly conducting boundary layers of Earthionosphere waveguide,the equation reduice to
expression (2).

c 2 n 2 1 / 2
c 2 n 2 1 / 2 
d  NC (t1  t 2 ) (1  2 2 )  (1  2 2 ) 
4 f1 h
4 f2 h


1
(2)
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Mode
n
Cutoff
frequency
(2%)
h (m)
t
f1 (2%)
f2 (2%)
d distance
(km)
1
1703
91400.1396
-
-
-
-
2
3560
89223.9458
0.005
3711
4688
1626.49528
3
4980
86879.5763
0.005
5469
5957
1392.43049
4
6370
88959.1864
0.005
7040
7715
1065.04197
The error of cutoff fréquency is about 2%
The delay time is measured from spectrogram of tweeks.
The tweeks shown have propagated a distance of arund 1300 Km
The average percentage deviation in estimating the distance is 30 % The
tail of the Tweeks which are not sufficently long to have a precise value of the
cutoff frequency.
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Validation with CAL data
Coupling of Atmospheric Layers:project concerns
thanderstorms,electrical And space radiation effects in
the stratosphere, mesosphere and lower thermosphere.
www.blitzortung.org
00:05:21,1279426
Long:16.290300
Lat : 44.363000
Storms in centrale Europe
∆t = t
00 :05 :21.131
record -
t CAL
∆t = 3,05 ms
D=C. ∆t
D = 916 km
The Sferic corresponding to the lightning should appear after
the stroke time due to the travel time delay.
(Long,Lat) D = 1084 km from Tunis
(www.movable-type.co.uk/scripts/latlong.html)
The storms which took place in
Bosnia seem to be responsible
for these tweeks
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Identification of the lightning
60 
Magnetic Field
Magnetic Field
Tunis 21-Aug-2007 UT N/S Antenna
N
1
0
50 
-1
21.12
21.125
21.13
21.135
Time (seconds) after 00:05:00 UT
N
21.14
Tunis 21-Aug-2007 UT E/W Antenna
40 
0.05
N
N
0
30 
-0.05
21.12
21.125
21.13
21.135
21.14
Time (seconds) after 00:05:00 UT
1.3
° E
O
N
10 
W
=arctan (BEW /BNS )
0


S

10 E

20 E
40

E
30 E
Two channel orthogonal Magnetic
loop antenna configuration
.
=1.3°
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Frequency (kHz)
Frequency (kHz)
Comparison over station
Tunis 21-Aug-2007 UT N/S Antenna
14
12
10
8
6
4
2
100
50
27.2
27
26.8
Time (seconds) after 00:05:00 UT
Algeria 21-Aug-2007 UT
•The dispersion of tweek
atmospherics is dependent on the
conductivity of the ground and
seawater.
N/S Antenna
14
12
10
8
6
4
2
100
50
9.5
9.6
9.7
9.8
9.9
Time (seconds) after 00:01:00 UT
The lower attenuation offered
by the waveguide between the
sea and the ionosphere than
that between the earth and the
ionosphere.
Tweeks have been observed only during the nighttime in tunisia and
algeria in the night of 21 August.
No indication for tweeks occurrence in Sebha south libya in the night of 21
August.
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Thank you
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