Modeling Solar Eclipses at Extreme Ultra Violet Wavelengths and the Effects of Nonuniform Eclipse Shadow on the Ionosphere‐Thermosphere System

Abstract

Solar eclipses have drawn a lot of interest in ionospheric research because they significantly alter the photochemical and transport processes due to the abatement of solar X-ray and extreme ultraviolet (EUV) flux within the eclipse's shadow (penumbra). Observations provide exceptional opportunities for testing global models of the ionosphere-thermosphere (I-T) because numerical simulations can be done in advance by virtue of knowing eclipses' timing, duration, location, and magnitude centuries ahead, making eclipses natural experiments. However, the laboratory experiment notion proved very challenging as nicely summarized by Rishbeth (1968): "The ionospheric physicist might wish that the Sun could be regarded as a constant, uniform source of ionizing radiation; but investigations of the Sun show that it is not." The solar corona, the source of the ionizing X-ray and EUV flux is considerably larger than the photosphere, therefore there exist no total solar eclipses for the

2‐D Total Electron Content and 3‐D Ionospheric Electron Density Variations During the 14 October 2023 Annular Solar Eclipse

“…The eclipse-induced direct ionospheric effect includes a depletion of a few tenths in ionospheric electron density and TEC due to a local decrease of the photo-ionization (e.g., Afraimovich et al, 1998;Rishbeth, 1968;Tsai & Liu, 1999), as well as a cooling of several hundred K in plasma temperature resulting from the reduction of solar extreme ultraviolet (EUV) heating (e.g., Goncharenko et al, 2018;MacPherson et al, 2000). Moreover, the additional effects of solar eclipse include the enhanced downward plasma diffusion from the plasmasphere due to the reduction in the plasma equilibrium scale height and F region density (Huba & Drob, 2017;Wang et al, 2019), the modified neutral dynamics and associated electrodynamics in response to the eclipse-induced atmospheric cooling and composition changes (C.-H. Chen et al, 2019;Harding et al, 2018;St.-Maurice et al, 2011), as well as atmospheric bow waves and traveling ionospheric disturbances (TIDs) (e.g., Chimonas, 1970;Liu et al, 2011;Mrak et al, 2018Mrak et al, , 2022Nayak & Yiǧit, 2018;Sun et al, 2018;Zhang et al, 2017). Overall, the ionospheric response during a solar eclipse can be notably intricate due to the interplay of these dynamic photo-chemical, ambipolar diffusion, neutral wind, and electric field processes.…”

“…Moreover, the additional effects of solar eclipse include the enhanced downward plasma diffusion from the plasmasphere due to the reduction in the plasma equilibrium scale height and F region density (Huba & Drob, 2017; Wang et al., 2019), the modified neutral dynamics and associated electrodynamics in response to the eclipse‐induced atmospheric cooling and composition changes (C.‐H. Chen et al., 2019; Harding et al., 2018; St.‐Maurice et al., 2011), as well as atmospheric bow waves and traveling ionospheric disturbances (TIDs) (e.g., Chimonas, 1970; Liu et al., 2011; Mrak et al., 2018, 2022; Nayak & Yiǧit, 2018; Sun et al., 2018; Zhang et al., 2017). Overall, the ionospheric response during a solar eclipse can be notably intricate due to the interplay of these dynamic photo‐chemical, ambipolar diffusion, neutral wind, and electric field processes.…”

The Changing Shape of the Ionosphere During a Solar Eclipse

“…It has been known for some time that observations of the ionosphere during a solar eclipse can be used as a method for studying various aspects of the ionosphere [1][2][3]. In recent decades, modern ionosondes have been used for such observations at relatively small time resolution during various eclipses; see, for example, [4][5][6][7][8].…”

“…In different studies, particular aspects of the ionosphere have been investigated: [4] and [6] focused on wave-like disturbances, [5] presented observations of plasma drift together with the effects on different International Union of Radio Science (URSI) parameters (f o E, f o F 2 , and hmF 2 ), and [7] used NmF 2 data derived from ionograms to validate physics-based modeling work.…”

“…As already described in [1,9], the quick movement of the lunar shadow results in a nonequilibrium condition in the ionosphere. In addition, the altitude dependency of the obscuration during an eclipse [10] and the effects of the non-uniformity of the solar disk in the EUV spectrum [7,11] have to be taken into account. Thus, it can be expected that not only the characteristics of the peaks of the ionospheric layers will be affected by a solar eclipse but also the entire shape of the electron density profile.…”