A Statistical Study on the Winter Ionospheric Nighttime Enhancement at Middle Latitudes in the Northern Hemisphere

Abstract

The ionospheric electron density generally decreases after sunset under the effect of recombination, but sometimes, it increases, which forms an ionospheric nighttime enhancement (INE). In this research, the observations from two ionosondes in Mohe and Beijing and NASA's Jet Propulsion Laboratory‐Global Ionosphere Maps during the 24th solar cycle were used to explore the feature and mechanism of INE at midlatitudes. It shows that hmF2 always increases when INE occurs in summer, while it either rises or falls in winter. The increase of hmF2 has a significant contribution to INE in summer. For winter INE, however, the hmF2‐rising and the hmF2‐declining type INEs present no significant differences in durations and amplitudes, except that the former occurs slightly earlier than the latter. This suggests that the uplift of the ionosphere induced by equatorward thermospheric wind may not be a dominant factor for controlling the formation of winter INE at midlatitudes. We found that the electron density difference between the summer hemisphere and the winter hemisphere trends increases when the INE occurs in the winter hemisphere. This increased electron density difference between the two hemispheres is beneficial to the summer‐to‐winter interhemispheric plasma transport and the formation of INE in winter. Meanwhile, it is likely that the ionospheric equivalent slab thickness decreases during the winter INE due to downward transport from the topside. These indicate that downward plasma influxes from the topside ionosphere and the conjugate summer hemisphere play dominant roles for the formation of winter INE at middle latitudes.

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

“…The statistical results in Figure 2 show the nighttime NmF2 enhancement occurring predominantly in winter solstice months. These results are consistent with the earlier papers (e.g., Y. Chen et al., 2015b; Li et al., 2020; Zhang, Liu, et al., 2015). According to previous studies, there are two different mechanisms that cause different phenomena for the nighttime ionospheric electron density enhancement at middle latitude.…”

“…The phenomena caused by this mechanism was called WSA, later so called MSNA, were found mainly occuring in local summer, with electron density peaks appearing mainly at dusk or before midnight, photoionization plays a crucial role in it. The other mechanism is the downward plasma flux transport from the high altitudes (e.g., topside ionosphere and plasmasphere) to the ionospheric F2 region (e.g., Y. Chen et al., 2015b; Evans, 1965; Li et al., 2020; Luan et al., 2008). For the nighttime NmF2 enhancement, since it occurs mainly during the winter solstice months, and lacks solar photoionization when it occurs, the second mechanism should be the dominant one for its formation and development at middle latitude.…”

“…The other mechanism is the downward plasma flux transport from the high altitudes (e.g., topside ionosphere and plasmasphere) to the ionospheric F2 region (e.g., Y. Chen et al, 2015b;Evans, 1965;Li et al, 2020;Luan et al, 2008). For the nighttime NmF2 enhancement, since it occurs mainly during the winter solstice months, and lacks solar photoionization when it occurs, the second mechanism should be the dominant one for its formation and development at middle latitude.…”

Structure of Post‐Midnight Enhancements in Electron Density at the Low Latitude F‐Layer Ionosphere

“…For example, the nighttime enhancements in Ne are reported to be associated with complicated changes in the F2 peak height (hmF2), with an upward or downward evolution of hmF2 (W. Li et al, 2020). As a result, it is generally believed that the nighttime enhancements in the F2-layer peak electron density (NmF2) at middle and low latitudes are mainly caused separately or collectively by equatorward neutral winds (e.g., He et al, 2009;W. Li et al, 2020;Yakovets et al, 2009), electric fields (Balan et al, 1994;L.…”

“…Li et al, 2020;Yakovets et al, 2009), electric fields (Balan et al, 1994;L. Liu et al, 2013;Wang et al, 2022), plasma diffusion downward from the plasmasphere (e.g., Chen et al, 2016;Dabas & Kersley, 2003;Davies et al, 1979;W. Li et al, 2020;Q.-H. Li et al, 2023;Luan et al, 2008;Titheridge, 1968;Tsagouri & Belehaki, 2002), and traveling atmospheric waves (Jiang et al, 2016;Pezzopane et al, 2011).…”