Anomalous effect of solar flares on the D-region: observed using very low frequency waves

The modern civilization increasingly depends on the space born and ground based technological system which are prone to energetic charge particles and radiation from the Sun. This interdependency given birth to a new kind of weather known as ^Space Weather or SW in short. SW refers to the conditions on the Sun, including solar wind, sunspots, solar flares, Coronal Mass Ejections (CMEs), which creates an adverse condition and are the sources of the disruption of satellites operations, communication, navigation, and failure of electrical power grids. Solar flare which occurs when the magnetic energy, that has built up in the solar atmosphere, is suddenly released appears as intense variation in brightness on the solar disk, become the major source of high energetic radiation (mainly X-rays) in the Earth's atmosphere. X-ray radiation from solar flares create extra ionization in the entire ionosphere but maximum effects are seen in the lower part of the ionosphere know as D-region. The additional ionization in the D-region caused by X-ray bursts of the wavelength (08-02 nm) which ionizes N₂ and O₂ molecules of atmosphere. This further results into changes in upper boundary of the Earth-Ionosphere waveguide, in which the VLF (Very Low Frequency) radio waves propagate. These are the electromagnetic waves lying in the frequency range of 3-30 kHz, and propagates with very low attenuation inside the EIWG. Thus, the amplitude and phase of the received VLF is altered when a solar flare occurs. The amplitude change (increase) in VLF signal follows X-ray flux, hence higher for X-ray with high flux. These changes are further analyzed to study solar flares and its effects on the Earth's ionosphere. In the present report, we have analyzed one year (2014) solar flare data and observed interesting diurnal and seasonal dependency of solar flare effect on the VLF signal. We have observed that the general rule of high X-ray flux causes high change in amplitude does not always followed and depends on many factors. Our analysis result provides a new insight in to the solar flare effect on the D-region, hence are very useful for ionosphere and space weather forecasting models. (author)