1. Y. Bouderba, S. Naitamor, O. Boumia Research Center on Astronomy, Astrophysics and Geophysics. CRAAG (Algeria). 1 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.dzy.bouderba@craag.dz 19 Avril 2011

2. 2 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.

3. 3 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.

4. 4 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).

5. 5 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.

6. 6 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 layer By night Ionosphere E Layer disappears Almost 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.

7. 7 Study of VLF waves TransmitterReceiver Earth 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

8. 8 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

9. 9 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 sunrisesunset

10. 10 2007/06/04 at : 05 :05 :00 (Class : M8.9) Date t onst (min) t recovery (min) Amplitude (dB) NS NSC NS NRK 3535 60 100 7 7.5 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 Amp (dB) Amp (dB)

11. 11 2007/07/10 at : 17:50 :00 (Class : C5.2) Date t onst (min) t rec (min) Amp (dB) NS NAA NS NRK 4343 45 63 4.5 4.1 C1.2 C1.4C2.8 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 Amp (dB) Amp (dB) Amp (dB)

12. 12 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

13. 13  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)

14. 14 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.