Study of solar flare induced D-region ionosphere changes using VLF amplitude observations at a low-latitude site
We obtained and analyzed 26 solar flare events from C2.56 to X3.2 classes at Tay Nguyen University, Vietnam (12.56° N, 108.02° E) during May – December, 2013 using Very Low Frequency remote sensing to understand the responses of low-latitude D-region ionosphere during solar flares. The observed VLF amplitude perturbations are used as the input parameters for the simulated LWPC program, using Wait’s model of lower ionosphere, to calculate two Wait’s parameters: the reflection height, H’ and the sharpness factor, b. Results reveal that when the X-ray irradiance increased, the b increased from 0.3 km-1 to 0.506 km-1, while the H' decreased from 74 km to 60 km. The electron density increases at the height of 74 km with 1-3 orders of magnitude during solar flares. These phenomena can be explained that the ionization due to X-ray irradiance becomes greater than that due to cosmic rays and Lyman-a radiation, which increases the electron density profile. The changes rules of the Wait’s parameters and electron density of D-region ionosphere of our results are in agreement with the studied results shown by other authors. The 3D representation of the electron density changes with altitude and time supports to fully understand the shape of the electron density changes due to X-ray flares. The shape variation of electron density is roughly followed to the variation of the amplitude perturbation and keeps this rule for the different altitude. We also found that the electron density versus the height in lower latitude D-region ionosphere increases more rapidly during solar flares.
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