Solar flare induced the parameter changes of lower ionosphere from VLF amplitude observations at a low-latitude site

Abstract:

We recorded and analyzed 43 solar flare events from C2.56 to X3.2 classes at Tay Nguyen University, Vietnam (12.56^o N, 108.02^o E) during May, 2013 - February, 2014 using the well known VLF technique to understand the behaviour of low-latitude D-region ionosphere during solar flares. The VLF amplitude perturbations of NWC/19.8 kHz signal and time delay between the VLF amplitude peaks and the X-ray flux peaks recorded by GOES satellites were computed. We found that almost peaks of VLF amplitude occurred about 1 - 4 minutes after the peak of X-ray flux. However, some solar flare events, the peaks of VLF amplitudes appearred about 1 - 5 minutes before the peaks of flare flux. The observed VLF amplitude perturbations are used as the input parameters for the simulated LWPC code, using Wait's model, to calculate two Wait's parameters of the Earth - ionosphere waveguide: the reflection height, and the exponential sharpness factor. Results reveal that when the X-ray irradiance increased, the exponential sharpness factor increased from 0.301 km^-1 to 0.506 km^-1, while the reflection height decreased from 73.6 km to 60 km. The variation of electron density is a logarithmic function of amplitude perturbation. The 3D representation of the electron density changes with altitude and time supports us to sufficiently understand the rules of the electron density changes during solar 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. During solar flares, the ionization due to X-ray irradiance becomes greater than that due to cosmic rays and Lyman-alpha radiation, which increases the electron density profile. Using amplitude perturbations and time delay, we also calculate the effective electron recombination coefficient at solar flare peak region. The effective electron recombination coefficient versus X-ray flux varies from ~ 10^-14m³/s to ~ 10^-12 m³/s for C1.0 to M9.0 classes. The changes rules of the Wait's parameters and electron density of lower ionosphere of our results are in accordance with the studied results shown by other authors. We also found that the electron density versus the height in lower latitude D-region ionosphere increases more rapidly during solar flares.