1. The D-region Ionospheric Perturbations Caused by Solar Flares:
Inferred from Very Low Frequency (VLF) Waves observations
in Mid-latitude region
Famil Mustafa1 , Elchin S. Babayev1, Namig Dzhalilov 1,
Stefaan Poedts 2, Rajesh Singh3, Ajeet K. Maurya4, Ilgar Alakbarov1
1Shamakhy Astrophysical Observatory named after N.Tusi, Azerbaijan National Academy of Sciences, AZERBAIJAN
2 Department of Mathematics, Centre for Mathematical Plasma-Astrophysics, KU Leuven, Leuven, Belgium
3K.S. Krishnan Geomagnetic Research Laboratory (KSK GRL), Indian Institute of Geomagnetism, Allahabad, India
4School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
RESULTS
Results show that solar flare effects on the sub-ionospheric VLF signals are identified by the sudden increase in their amplitudes. These sudden changes are manifestations of rapid enhancements in the D-region ionospheric plasma density associated with flares. It was revealed that solar flares of C, M and X classes affect VLF signal amplitude and lead to its increase consequently from ~1dB to ~10 dB. All selected examples (JJİ, DHO and GBZ) showed that the amplitude of VLF signals were perturbed by solar X-ray flares occurrence. D region ionospheric parameters (H/, β) show significant dependence on the local time of flare’s occurrence and their classes.
ACKNOWLEDGEMENTS
This work was partially supported by the International project of Solar and Space Weather Network of Excellence - SOLSPANET, project number FP7-PEOPLE-2010-IRSES
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INTRODUCTION
The measurement of Very Low Frequency (VLF) signals, generated by navigational transmitters is one of the reliable and cost-effective tools for remote sensing of the D-region electron density perturbations associated with solar flares as well as for understanding the dynamical processes in the D-region ionosphere.
MATERIAL AND METHODS
The Automatic Weather Electromagnetic System for Observation Modelling and Education (AWESOME, Stanford University, USA) VLF receiver located in middle latitude geographical station at the Shamakhy Astrophysical Observatory, Pirgulu, Azerbaijan (latitude 40°46’204 N, longitude 48°35’04E) was used to record the amplitude and phase perturbations of three VLF signals transmitted by GBZ transmitter (frequency f = 19,6 kHz) at Anthorn, Great Britain, JJI transmitter (f=22.2 kHz) at Ebino, Miyazaki, Japan and DHO transmitter (f = 23,4 kHz) at Rhauderfehn, Germany and propagating along the Earth-ionosphere waveguide (EIWG).
Data on VLF waves, transmitted by navigational transmitters for years were used to study changes in the VLF signal amplitude and phase in D-region with respect to solar flares. The solar flares of C, M and X classes which occurred during (in total 133 flares – 110 C class, 21 M class, 2 X-class; low solar activity period of the solar cycle 24) are selected for comparative analysis of VLF data. Geostationary Operational Environmental Satellite (GOES) data were involved in this study. The local time dependence of solar flare effects on the change in the VLF amplitude, time delay between VLF peak amplitude and X-ray flux peak have been studied during morning, noon and evening periods of local daytime. Using the Long Wave Propagation Capability code V2.1 the D-region reflection height (H/) in km and and D-region electron density gradient or sharpness factor (β) in km-1 for each class of solar flare (C, M and X) have been estimated.
Worldwide AWESOME Sites
FLV sample data
Worldwide Transmitters Sites
Normal Day
Comparison X-ray flux with VLF amplitude
during Solar Flare
Day with Solar Flares