1. Characteristics of VLF Tweeks Nedra Tounsi & Hassen Ghalila Laboratoire 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

2. 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

3. Earth-Ionosphere Waveguide Earth Ionosphere Ligthning - Sprites : Electrical source of electromagnetic waves The waveguide acts as a filter -> remains only the waveguide modes 3

4. Detection of tweeks Cutoff frequency Velocity dispersion Zoom 4 The two first tails are clear enough to extract the cutoff frequencies and the dispersion of the light velocity

5. 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. f 2 =2f 1 ; f 3 =3f 1 ; f 4 =4f 1

6. Date Mode(n) f c (Hz) h (m) V g (m s -1 ) 21-08-2007 1 170391400.1396 2,9373 10 8 21-08-2007 2 356089223.9458 2,6772 10 8 21-08-2007 3 498086879.5763 2.2456 10 8 21-08-2007 4 637088959.1864 1,4648 10 8 6  The nighttime ionospheric reflection height estimated from tweeks varies in the range of 88-92km  The groupe velocity is nearly equal to celerity of light. Height=> V g =c[1-(f c /f) 2 ] 1 / 2 Group 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.

7. Electronic density, mobility conductivity Date Height (km) Electronic density N e (m -3 ) ∆N e (m -3 ) Mobility µ e (m 2 /V/s) Conductivity σ e (S/m) 06-08-2007  50 43.50 10 6 6.453 10 6 2.017 10 3 1.403 10 -8 24-09-2007  70 31.13 10 6 6.453 10 6 2.093 10 4 1.042 10 -7 24-09-2007  100 20.60 10 6 6.453 10 6 3.565 10 6 1.175 10 -5 N e (h)=1.241.10 -8 F C F h F h =B e/2πm ( Cyclotron frequencies) µ e =1.36N 0 / N n σ e σ e =q e N e µ e 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. F h =1.2294 MHz (Ohya 2003)

8. Estimated lightning distance ∆t = t 1 - t 2 ∆t = d/v g1 - d/v g2 (1) (2) 8 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).

9. Mode n Cutoff frequency (2%) h (m)  t f 1 (2%)f 2 (2%)d distance (km) 1170391400.1396---- 2356089223.94580.005371146881626.49528 3498086879.57630.005546959571392.43049 4637088959.18640.005704077151065.04197 9  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%

10. 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. 10 Storms in centrale Europe  The Sferic corresponding to the lightning should appear after the stroke time due to the travel time delay.

11. 11 1.3 ° OE S N Identification of the lightning Two channel orthogonal Magnetic loop antenna configuration.  =arctan (B EW /B NS )  =1.3°

12. 12 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.

13. Thank you 13