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

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

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

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

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

Date Mode(n) f c (Hz) h (m) V g (m s -1 ) , , ,  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.

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)    N e (h)= 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 = MHz (Ohya 2003)

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).

Mode n Cutoff frequency (2%) h (m)  t f 1 (2%)f 2 (2%)d distance (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.  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, Long: Lat : 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 : ∆t = t record - t CAL D = 916 km (Long,Lat) D = 1084 km from Tunis ( 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.

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

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.

Thank you 13