Numerical study of multiple solar flare induced modulation of Very Low Frequency (VLF) diurnal profile

Abstract

Earth's ionosphere is a perpetual detector of ionizing radiation received from celestial objects, particularly the Sun. Solar ionizing radiation in the form of extreme ultraviolet (EUV) and X-rays during both quiet and active phase of the Sun, and charged particles associated with a solar wind imprint their ionization signatures on the ionosphere. Although due to the bipolar nature of the geomagnetic field, the events, such as the solar coronal mass ejections (CMEs) and associated solar wind enhancement, usually disturb the polar ionosphere only, the UV and X-rays from the solar flares produce sudden ionospheric disturbances (SIDs) in low-mid-latitude part of the earth's ionosphere. Such ionospheric disturbances are studied with the help of the influence they exert on radio waves propagating through earth-ionosphere waveguide. For the lower part of the ionosphere, called the D region, prominent modification in electron-ion density during solar flares can be observed via deviation in earth bound Very Low Frequency (VLF) wave signal from its ambient diurnal profile. In earlier work, successful model of the deviation in VLF amplitude due to different classes of solar flares was formulated. There, calculation of rate of ionization with Monte Carlo simulation and ion-chemistry evaluation of plasma density enhancement followed by a radio propagation simulation was used. Presently, we attempt to numerically reconstruct the modulation in VLF signal from its diurnal pattern produced by multiple solar flares occurring over a single day. Successful reconstruction of the VLF signal modulation for such a complex flaring scenario points to the accuracy of our understanding of the ionization effect due to solar activity on the lower ionosphere, and strengthen our claim to use earth's ionosphere as a high energy space transient event detector.