Arecibo measurements of D-region electron densities during sunset and sunrise: implications for atmospheric composition

Abstract

Abstract. Earth's lower ionosphere is the region where terrestrial weather and space weather come together. Here, between 60 and 100 km altitude, solar radiation governs the diurnal cycle of the ionized species. This altitude range is also the place where nanometre-sized dust particles, recondensed from ablated meteoric material, exist and interact with free electrons and ions of the ionosphere. This study reports electron density measurements from the Arecibo incoherent-scatter radar being performed during sunset and sunrise conditions. An asymmetry of the electron density is observed, with higher electron density during sunset than during sunrise. This asymmetry extends from solar zenith angles (SZAs) of 80 to 100∘. This D-region asymmetry can be observed between 95 and 75 km altitude. The electron density observations are compared to the one-dimensional Sodankylä Ion and Neutral Chemistry (SIC) model and a variant of the Whole Atmosphere Community Climate Model incorporating a subset SIC's ion chemistry (WACCM-D). Both models also show a D-region sunrise–sunset asymmetry. However, WACCM-D compares slightly better to the observations than SIC, especially during sunset, when the electron density gradually fades away. An investigation of the electron density continuity equation reveals a higher electron–ion recombination rate than the fading ionization rate during sunset. The recombination reactions are not fast enough to closely match the fading ionization rate during sunset, resulting in excess electron density. At lower altitudes electron attachment to neutrals and their detachment from negative ions play a significant role in the asymmetry as well. A comparison of a specific SIC version incorporating meteoric smoke particles (MSPs) to the observations revealed no sudden changes in electron density as predicted by the model. However, the expected electron density jump (drop) during sunrise (sunset) occurs at 100∘ SZA when the radar signal is close to the noise floor, making a clear falsification of MSPs' influence on the D region impossible.

A D‐Region Ionospheric Imaging Method Using Sferic‐Based Tomography

“…Daytime, nighttime, and day-night terminator conditions are presented. Compared to other studies, Baumann et al (2022) observe lower electron densities during sunrise compared to sunset, while the model presented here shows the opposite phenomenon. However, they do note that the difference is not strongly present at 80 km, and they make no mention of any altitudes below 70 km.…”

Ground-based noontime D-region electron density climatology over northern Norway

“…In addition to that, it also shows an asymmetry behavior that the solar variations could not explain. For example, recently Baumann et al (2022) reported the electron density is higher during the sunset than the sunrise. The postulate that the change in the recombination rates is a plausible reason for this observed asymmetry.…”

Ground-based noontime D-region electron density climatology over northern Norway

“…Continuous observation of the D region is very challenging because of the low electron abundance. There are very few direct observational techniques such as sounding rockets (Mechtly, 1974), incoherent scatter radar (Chau and Woodman, 2005;Baumann et al, 2022), very low frequency (VLF) methods (Worthington and Cohen, 2021), and partialreflection radars or medium-frequency (MF) radars (Belrose, 1970;Igarashi et al, 2000;Singer et al, 2008) that are effectively used to measure the D-region electron density (see. e.g., Friedrich and Rapp, 2009, for a review).…”

“…In addition, D-region electron density also shows an asymmetrical behavior that can not be explained by the solar variation. For example, recently Baumann et al (2022) reported the electron density is higher during sunset than sunrise. They postulate that the change in the recombination rates is a plausible reason for this observed asymmetry.…”