Modeling of the Variability of D‐Region Ionospheric Electron Density During Solar Cycle‐24.

Solar cycle variation of earth's atmosphere, particularly the ionosphere is of particular interest in the field of space science and space weather studies. In this article, we present the outcome of our detailed quantitative study on solar cycle variation of lower ionospheric properties using numerical investigation. First, we seek to model and compare the collective D‐region ionization rates (q $q$'s) due to the individual contributions from the ionizing sources, namely, (a) solar extreme ultraviolet (EUV) radiation including the Lyman‐α $\alpha $ irradiation and (b) solar X‐ray irradiation throughout the 24th $2{4}^{th}$ solar cycle (C24). Then, we compute the electron density Ne $\left({N}_{e}\right)$ profiles using the ionization rates. We report significant solar cycle variation in ionization rate and electron density profiles for the entire span of C24. We use the sunspot number (SPN) profile to substantiate the finer details of Ne ${N}_{e}$ profile during C24. For the segments of the D‐region above two different geographic coordinates, we report that Ne ${N}_{e}$ profiles show a consistent "dual peak" nature. It is very similar to the SPN profile during C24. As a next‐order validation, we compare our modeled Ne ${N}_{e}$ profiles with their IRI‐2020 counterpart (Ne,iri ${N}_{e,iri}$s). Their overall trends are found to be in agreement. Finally, we discuss the response of the D‐region in terms of Ne ${N}_{e}$ due to C24. The work lays the foundation for our upcoming studies on D‐region response to solar cycle variation with Very Low Frequency (VLF) observation. Plain Language Summary: The solar cycle is a periodic change in solar activity with a periodicity of about 11 years. An observable indicator of the solar cycle is the temporal variation in Sunspot Number (SPN). During the supposed maxima period of most of the observed cycles, two peaks have been detected in the sunspot data. The gap between these two peaks is named as Gnevyshev gaps. The D‐region is the lowermost part of the ionosphere with the altitude distribution from, roughly, 60–90 km. We develop a numerical model to study the long‐term variability of the D‐region ionosphere during solar cycle 24 (C24). The D‐region is maximally ionized by solar extreme ultraviolet (EUV) and X‐ray radiation. We estimate the net ionization rate (q) $(q)$ due to both of these two types of solar radiation in this region. We model the electron density Ne $\left({N}_{e}\right)$ profile and check the long‐term effects due to C24. We found, just like the "dual peak" in SPN, Ne ${N}_{e}$ profiles also show similar "dual peak" consistently across different parts of the D‐region above different geographic coordinates. We also investigate the altitude (h) $(h)$ dependency of ionization rate (q) $(q)$ and Ne ${N}_{e}$. We infer that the D‐region ionosphere varies appositely with solar activity phases during C24. Key Points: Variability of solar Lyman‐α and X‐ray due to solar cycle 24D‐region ionization in presence of solar Lyman‐α and X‐raySolar cycle 24 induced D‐region electron density variation [ABSTRACT FROM AUTHOR]