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Characteristics of VLF Tweeks . 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
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Characteristics of VLF Tweeks • 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
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
Ionosphere Earth-Ionosphere Waveguide Earth Ligthning - Sprites : Electrical source of electromagnetic waves The waveguide acts as a filter -> remains only the waveguide modes
Zoom Zoom Detection of tweeks Cutoff frequency Velocity dispersion The two first tails are clear enough to extract the cutoff frequencies and the dispersion of the light velocity
Cutoff frequency • 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 • We measure the cutoff frequencies by the help of the improfile tool of matlab which gives us a one dimensional plot of the spectrogram.
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. Height => Group velocity => Vg =c[1-(fc /f)2]1 /2 • The nighttimeionosphericreflectionheightestimatedfromtweeks varies in the range of 88-92km • The groupe velocityisnearlyequalto celerity of light.
Electronic density, mobility conductivity σe=qe Ne µe µe =1.36N0/ Nn Ne (h)=1.241.10-8 FC Fh (Ohya 2003) • Using the International GeomagneticReferencefield model and takingtunis’s latitude (10°1024 E) • Using the Handbookatmospheric in summeratmiddel latitude. • AtmosphericdensitydecreaseexponentiallywithHeigh. Fh =B e/2πm (Cyclotron frequencies) Fh =1.2294 MHz
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 (1) For perfectly conducting boundary layers of Earth- ionosphere waveguide,the equation reduice to expression (2). (2)
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.
00:05:21,1279426 Long:16.290300 Lat : 44.363000 Validation with CAL data Coupling of AtmosphericLayers:projectconcernsthanderstorms,electrical And space radiation effects in the stratosphere, mesosphere and lowerthermosphere. www.blitzortung.org Storms in centrale Europe ∆t = t record - t CAL ∆t = 3,05 ms D=C. ∆t 00 :05 :21.131 D = 916 km The storms which took place in Bosnia seem to be responsible for these tweeks • 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)
N 1.3° O E S Identification of the lightning =arctan (BEW/BNS ) Twochannel orthogonal Magneticloopantenna configuration. =1.3°
Comparison over station • The dispersion of tweek atmospherics is dependent on the conductivity of the ground and seawater. • 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.
