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Y. Bouderba, S. Naitamor, O. Boumia

Research Center on Astronomy, Astrophysics and Geophysics. CRAAG (Algeria).

International School of Space Science, Frontiers of Space Science: from Solar activity to NEOs.L’Aquila , April 17-22, 2011

E-mail : y.bouderba@craag.dz

19 Avril 2011

VLF signals perturbations due to solar flares

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Our vision

I will present in this talk: solar flares, the ionosphere and VLF

signals perturbations recorded till 2010.

Study the disturbances of the D layer of the ionosphere due to the

solar flares.

Understand the behavior of the VLF EM waves in response with

the solar eruptions.

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Solar flares

Northern lights (Norway).

Solar flar and earth size comparison

• Generate enormous flow of waves

emissions at high energies (UV, X-ray,…)

• Radiations from the solar flares affect the terrestrial radioelectric

transmissions, and causes appearance of the polar lights while

entering in interaction with the terrestrial magnetic field.http://img.fotocommunity.com/Landscape/Skies/Aurora-Borealis-over-Trondheim-Norway-a20791924.jpg

• Paramount events of the sun activity .

• Equivalent to gigantic nuclear explosions

which occur on the sun surface.

• Eject an ionized matter (plasma) in the space.

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A : I < 10-7

B  :  I < 10-6

C  : 10-6 < = I < 10-5

M : 10-5 < = I < 10-4

X : I > = 10-4

Solar flare (NASA-2007)

Solar eruptions classes

• They are classified in five categories(A,

B, C, M, X) according to the maximum

intensity of their flow (in Watt/m2) in the

band of X-ray (1 to 8 A°).

• Each category corresponds to a solar eruption of an intensity 10 times more important than the preceding one.

• Each category is divided into 10 subcategories (for example: C1.0 to C1.9).

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The ionosphere

Various layers of the terrestrial atmosphere

It is well known that the solar activity causes disturbances on the earth atmosphere. As the ionosphere is one of the atmosphere layers, it was widely studied using different techniques and instrumentations

The part of the Earth lit by the Sun is then subjected to a bombardment of X-rays and UV radiations. These radiations penetrate to the D layer causing supplementary ionization.

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The ionosphere Layers

Simplified sight of ionosphere layers on 24 h

E layer: (90 to120 km).

We distinguish four principal layers:

D layer: (60 to 90 km)

F layer: (120 to 800 km).

• It disappears quickly after sun lying.

F layerBy night

Ionosphere

E Layer disappearsAlmost at night.

D layer disappears by night

• The lowest layer of the ionosphere.

During the day, F layer is decomposed into to sub-layers F1 and F2

• It is influenced by the solar flares.

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Study of VLF waves

Transmitter

ReceiverEarth

Ionosphere

D layer

Atmosphere

X-rays from solar flares

Wave propagation

The Very low frequencies (VLF) band are used by the naval transmission, and range

between 3 and 30 KHz (wavelength 100 to 10 km), these waves are very sensitive to

small changes in the D layer conductivity, and constitute a powerful tools to study

this low layer density.

VLF waves propagating

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Worldwide VLF receivers

In our case, we used the Very Low Frequencies (VLF) signals coming from naval

transmitters and received by the North African AWESOME network to study the

VLF signal perturbations due to the solar flares.

The VLF signals analysis was carried out during the quite period of the solar activity

i.e. from 2007 to 2010

Data from VLF AWESOME Network

Worldwide VLF Transmitters

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VLF signal without perturbation

During a quite day only the sunrise and sunset effects are observed .

Amp(dB)

Transmitter: GQD (Great Britain)

A typical graph of a quiet day resembles like this

sunrise

sunset

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2007/06/04 at : 05 :05 :00 (Class : M8.9)

Date tonst (min) trecovery (min) Amplitude(dB)

NS NSCNS NRK

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60100

77.5

VLF signal with perturbation

Amp(dB)

A typical graph of an active day with one event (M8.9) resembles like this

The rise of the signal power corresponds to a solar flare.

Transmitters: NSC(Italy), NAA(USA), NRK( Iceland)

M8.9

M8.9

Amp(dB)

Amp(dB)

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2007/07/10 at : 17:50 :00 (Class : C5.2)

Date tonst (min) trec(min) Amp(dB)

NS NAANS NRK

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4563

4.54.1

C1.2C1.4

C2.8

C7.4

VLF signals disturbances due to several solar flares which were happened in the same day

VLF signal with perturbations

Transmitters:NSC(Italy), NAA(USA), NRK( Iceland)

C5.2

C5.2Amp(dB)

Amp(dB)

Amp(dB)

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Example of three AWESOME receivers recorded for the same flare: M4.0

Some transmitters signals showed a decreasing in the amplitude followed by an increasing signal(NSC).

Other transmitters signals showed an increasing one( GQD).

The arrival time of the minimum signal amplitude varies from a receiver to another, so it appears first in Tunis receiver’s, then in Crete and finally in Algiers one.

Transmitters: NSC(Italy), GQD(Great Britain )

VLF signal with perturbation

Amp(dB)

Map of Some transmitters and receivers

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Seasonal dependence of VLF perturbations proprieties for the

same flare C2.3

In May (spring), the perturbation proprieties (amplitudes and duration times) are more important than in January (Winter).

Transmitters: DHO(Germany, GQD(Great Britain ), NSC(Italy), ICV(Italy).

Seasonal VLF signal effects

Amp(dB)

Amp(dB)

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Summary

 And finally,following  the  variation of  the D  region parameters during  the  solar  cycle by 

using the LWPC code. 

When a solar flare occurs, the VLF signal is disturbed, and this

perturbation is a function of the flare intensity and the season during

which the flare occurs.  The  seasonal  dependence  can  be  interpreted  by  the  changes  in  the  ionosphere 

conductivity during the year.

 The shape of the perturbation (increasing or decreasing in the signal amplitude) depends 

on the wave modal structure near the receiver or near the transmitter.

 It is interesting to continue to analyze more data to determine the lower limit of the flare 

class that causes a measurable perturbation.