Characteristics of VLF Tweeks

Nedra Tounsi & Hassen GhalilaLaboratoire de Spectroscopie Atomique Moléculaire et Applications

1

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

Freq

uenc

y (k

Hz) Tunis 21-Aug-2007 UT N/S Antenna

Time (seconds) after 00:05:00 UT26.8 27 27.2

2468

101214

50

100

26.84 26.85 26.86 26.87 26.88-1

-0.50

0.5

Mag

netic

Fie

ld

Tunis 21-Aug-2007 UT N/S Antenna

Time (seconds) after 00:05:00 UT

References

Equivalent electron densities at reflection heights of tweek atmospherics in the low middle latitude D-region ionosphere H. Ohya 2003

Tracing Particles from the Sun to the Earth’s Ionosphere S. Murray 2007

Lightning and ionospheric remote sensing using VLF/ELF radio atmospherics S.A. Cummer Thesis 1997

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

Propagational Features of Higher harmonic Tweeks at low latitudes R.P. Singh 1996

2

Earth-Ionosphere Waveguide

Earth

Ionosphere

Ligthning - Sprites : Electrical source of electromagnetic wavesThe waveguide acts as a filter -> remains only the waveguide modes

3

Detection of tweeks

Cutoff frequency

Velocity dispersion

Zoom

Zoom

4

The two first tails are clear enough to extract the cutoff frequencies and the dispersion of the light velocity

Cutoff frequency

5

We measure the cutoff frequencies by the help of the improfile tool of matlab which gives us a one dimensional plot of the spectrogram.

The tweeks signals have caracteristic Cutoff frequencies due to long distance propagation by the

Earth-ionosphere wave guide mode. f2 =2f1 ; f3=3f1 ; f4=4f1

Date Mode(n) 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

6 The nighttime ionospheric reflection height estimated from tweeks varies in the range of 88-92kmThe groupe velocity is nearly equal to celerity of light.

Height => h = n c2 fcn

Vg =c[1-(fc /f)2]1 /2Group velocity =>

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.

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

Fh =B e/2πm (Cyclotron frequencies)

µe =1.36N0/ Nn

σσee =qe Ne µe

• Using the International Geomagnetic Reference field model and taking tunis’s latitude (10°1024 E)

•Using the Handbook atmospheric in summer at middel latitude.• Atmospheric density decrease exponentially with Heigh.

Fh =1.2294 MHz

(Ohya 2003)

Estimated lightning distance

∆t = t1 - t2

∆t = d/vg1 - d/vg2

d =

v g1 v g2

v g1 – v g2t (1)

(2)

8

1

2/1

222

222/1

221

22

21 )4

1()4

1()(

hfnc

hfncttNCd

Time interval between two close frequencies gives the source distance of atmospherics which is written as expression (1).

For perfectly conducting boundary layers of Earth- ionosphere waveguide,the equation reduice to expression (2).

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

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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.

The delay time is measured from spectrogram of tweeks.The tweeks shown have propagated a distance of arund 1300 Km

The error of cutoff fréquency is about 2%

00:05:21,1279426 Long:16.290300 Lat : 44.363000

Validation with CAL data

∆t = 3,05 ms D=C. ∆t

The storms which took place in Bosnia seem to be responsible for these tweeks

00 :05 :21.131

∆t = t record - t CAL

D = 916 km

www.blitzortung.org

(Long,Lat) D = 1084 km from Tunis (www.movable-type.co.uk/scripts/latlong.html)

Coupling of Atmospheric Layers:project concerns thanderstorms,electrical And space radiation effects in the stratosphere, mesosphere and lower thermosphere.

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Storms in centrale Europe

The Sferic corresponding to the lightning should appear after the stroke time due to the travel time delay.

11

1.3°O E

S

N

21.12 21.125 21.13 21.135 21.14-1

0

1

Mag

netic

Fie

ld

Tunis 21-Aug-2007 UT N/S Antenna

Time (seconds) after 00:05:00 UT

21.12 21.125 21.13 21.135 21.14-0.05

0

0.05

Mag

netic

Fie

ld

Tunis 21-Aug-2007 UT E/W Antenna

Time (seconds) after 00:05:00 UT

Identification of the lightning

Two channel orthogonal Magnetic

loop antenna configuration. =arctan (BEW /BNS )

=1.3°

12

Freq

uenc

y (k

Hz)

Algeria 21-Aug-2007 UT N/S Antenna

Time (seconds) after 00:01:00 UT9.5 9.6 9.7 9.8 9.9

2468

101214

50

100

Comparison over station

•The dispersion of tweek atmospherics is dependent on the conductivity of the ground and seawater.

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.

The lower attenuation offered by the waveguide between the sea and the ionosphere than that between the earth and the ionosphere.

Freq

uenc

y (k

Hz)

Tunis 21-Aug-2007 UT N/S Antenna

Time (seconds) after 00:05:00 UT26.8 27 27.2

2468

101214

50

100

Thank you

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