A Statistical Study of Nighttime Ionospheric NmF2 Enhancement at Middle‐to‐High Latitudes in the Northern Hemisphere

Abstract

The electron density in the ionosphere usually continuously decreases during nighttime due to the vanishing of sunlit ionization. However, it sometimes increases unexpectedly, which has been called nighttime ionospheric enhancement. Previous researches studied this type of phenomenon mainly in the low‐to‐middle latitude regions. Here, we investigate the nighttime ionospheric NmF2 enhancement at middle‐to‐high latitudes in the northern hemisphere (NH) during solar cycles 23–24 by analyzing the ionosonde observations at Chilton (51.1°N, 359.4°E, geomagnetic 48.5°N), Juliusruh (54.6°N, 13.4°E, 51.7°N), Tromsø (69.9°N, 19.6°E, 66.9°N), and Qaanaaq (77.5°N, 290.8°E, 84.3°N), as well as comparing with the Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM). The observations show that the nighttime NmF2 enhancement occurs mainly in the winter solstice months (November–February), and the occurrence rate and relative amplitude of the enhancement are inversely related to solar activity. The observed results are in good agreement with the results obtained from E‐CHAIM. Furthermore, the model shows the spatial distribution of the enhancement at the middle‐to‐high latitudes in NH in winter, most obvious in geomagnetic latitude between 50°N and 65°N, and have longitudinal minima centered at 30°W and maxima centered at 120°W and 75°E. From these results, this unique phenomenon at middle latitude is explained probably by the plasma flux from the higher altitudes (e.g., topside ionosphere and plasmasphere) diffusing downward into the ionospheric F2 layer. However, at polar regions, under the influence of the abundant high‐density structures such as polar cap patches, auroral blob, particle precipitation, etc., the F region plasma density enhancement becomes more complicated.

Statistical Properties of Ion Upflow Associated With Subauroral Ion Drift (SAID) in the Northern and Southern Hemispheres

“…Zhao et al. (2022) find that the injection of plasma from the plasmasphere to the nightside ionosphere may leads to the enhancement of plasma density using statistical analysis and simulation (E‐CHAIM model). In their study, nightside electron density enhancement has a clear polar boundary which coincides with the polar boundary of the plasmasphere, and related to the position of mid‐latitude ionospheric trough.…”

Latitude Variation of the Post‐Sunset Plasma Density Enhancement During the Minor Geomagnetic Storm on 27 May 2021

“…When combined with the Ionospheric Observational Network for Irregularity and Scintillation in East and Southeast Asia (IONISE) (Li et al, 2019;Sun et al, 2020), various ionospheric phenomena as well as their possible mechanisms can be investigated in detail in the East Asia region. Among these phenomena, nighttime enhancement is a topic worthy of attention and has been extensively studied recently as regard to its temporal, spatial, seasonal, solar and geomagnetic activity dependences (e.g., Chen C. et al, 2012;Chen et al, 2015aChen et al, , 2015bLi et al, 2020;Liu et al, 2013Liu et al, , 2020Liu et al, , 2023Sahu et al, 2020;Wang et al, 2022;Zhao et al, 2022;Zhong et al, 2019). The mechanism of the nighttime enhancement is predominantly believed to be latitude dependent, indicating that equatorward neutral wind and field-aligned plasma transport dominate at middle latitude while westward electric field should be responsible for the low latitude feature observed like in Sanya (Le et al, 2014;Li et al, 2020;Lin et al, 2010;Liu et al, 2013Liu et al, , 2020.…”

The Altitudinal Dependences of the Inter‐Hemispheric Asymmetry in the Mid‐Latitude Ionospheric Post‐Midnight Enhancement During Equinox

“…Jakowski et al (2015) pointed out that the NWA is usually found in longitude sectors where the difference between the geomagnetic equator and the geographic equator is maximized. Utilizing the empirical model E-CHAIM, Zhao et al (2022) studied the ionosphere nighttime enhancement of the peak electron density of the ionospheric F2-layer (NmF 2 ) at mid-and high-latitude in the Northern Hemisphere and found that the longitudinal differences of the enhancement as the local minimum at 20°W and the local maximum at 75°E as well as 120°W were found. (The definition of the abbreviations used in this paper at parts to ease readability, is summarized in Table 1.)…”