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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 26, issue 7
Ann. Geophys., 26, 1731–1740, 2008
https://doi.org/10.5194/angeo-26-1731-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 26, 1731–1740, 2008
https://doi.org/10.5194/angeo-26-1731-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  18 Jun 2008

18 Jun 2008

Classification of X-ray solar flares regarding their effects on the lower ionosphere electron density profile

D. P. Grubor1, D. M. Šulić2, and V. Žigman3 D. P. Grubor et al.
  • 1University of Belgrade, Belgrade, Faculty of Mining and Geology, Physics Cathedra, Djusina 7, 11000, Belgrade, Serbia
  • 2Institute of Physics, Belgrade, Pregrevica 118, Zemun, 11001, Belgrade, Serbia
  • 3University of Nova Gorica, Vipavska cesta 13, 5001 Nova Gorica, Slovenia

Abstract. The classification of X-ray solar flares is performed regarding their effects on the Very Low Frequency (VLF) wave propagation along the Earth-ionosphere waveguide. The changes in propagation are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-ray solar flares. All flare effects chosen for the analysis are recorded by the Absolute Phase and Amplitude Logger (AbsPal), during the summer months of 2004–2007, on the single trace, Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path (GCP) D≈2000 km in length.

The observed VLF amplitude and phase perturbations are simulated by the computer program Long-Wavelength Propagation Capability (LWPC), using Wait's model of the lower ionosphere, as determined by two parameters: the sharpness (β in 1/km) and reflection height (H' in km). By varying the values of β and H' so as to match the observed amplitude and phase perturbations, the variation of the D-region electron density height profile Ne(z) was reconstructed, throughout flare duration. The procedure is illustrated as applied to a series of flares, from class C to M5 (5×10−5 W/m2 at 0.1–0.8 nm), each giving rise to a different time development of signal perturbation.

The corresponding change in electron density from the unperturbed value at the unperturbed reflection height, i.e. Ne(74 km)=2.16×108 m−3 to the value induced by an M5 class flare, up to Ne(74 km)=4×1010 m−3 is obtained. The β parameter is found to range from 0.30–0.49 1/km and the reflection height H' to vary from 74–63 km. The changes in Ne(z) during the flares, within height range z=60 to 90 km are determined, as well.

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