Abstract
AbstractIn this study, we provide for the first time observation of the latitudinal four‐peak structure of F region electron density in the nightside ionosphere. The special configuration of Swarm satellites, Swarm B having the chance to resample the regions of Swarm A/C with successively increasing time differences, provides an unprecedented opportunity to check the evolution of these nightside electron density peaks. Overall, the latitudinal four‐peak structures have very low occurrence rates, only 4% of the Swarm orbits. The two mid‐latitude peaks prefer to appear close to ±40° magnetic latitude, while the two low‐latitude peaks appear within ±20° magnetic latitude. Such latitudinal four‐peak structures can persist throughout the night until sunrise hours. No clear seasonal dependence is found for the two mid‐latitude peaks, while the two low‐latitude peaks are almost symmetric about the magnetic equator during equinoxes but are located at slightly higher latitudes in the summer hemisphere around solstices. The two low‐latitude peaks at late‐night hours are believed not to be remnants of the dusk‐side equatorial ionization anomaly (EIA) crests, as (a) example shows that Swarm A/C observe the development of shoulders at the flanks of the two EIA crests after sunset hours, and the shoulders become peaks 3 h later when Swarm B resamples the same region; (b) statistic results reveal that the two low‐latitude peaks during post‐midnight hours do not propagate towards the magnetic equator, as expected for EIA crests, but move slowly poleward. We suggest that the enhanced meridional wind at postmidnight hours is one possible driver for causing such latitudinal four‐peak structure of F region electron density. In addition, the simultaneous magnetic measurements from Swarm satellites are also analyzed, but they show no obvious diamagnetic effect that could help to maintain pressure balance within these electron density peaks.
Highlights
A well‐known phenomenon of the low‐latitude ionospheric F region during daytime is the equatorial ionization anomaly (EIA), which is characterized by an electron density trough above the magnetic equator and double crests of enhanced electron density at about 15° north and south (e.g., Appleton, 1946; Liang, 1947; Namba & Maeda, 1939)
No clear seasonal dependence is found for the two mid‐latitude peaks, while the two low‐latitude peaks are almost symmetric about the magnetic equator during equinoxes but are located at slightly higher latitudes in the summer hemisphere around solstices
The two low‐latitude peaks at late‐night hours are believed not to be remnants of the dusk‐side equatorial ionization anomaly (EIA) crests, as (a) example shows that Swarm A/C observe the development of shoulders at the flanks of the two EIA crests after sunset hours, and the shoulders become peaks 3 h later when Swarm B resamples the same region; (b) statistic results reveal that the two low‐latitude peaks during post‐midnight hours do not propagate towards the magnetic equator, as expected for EIA crests, but move slowly poleward
Summary
A well‐known phenomenon of the low‐latitude ionospheric F region during daytime is the equatorial ionization anomaly (EIA), which is characterized by an electron density trough above the magnetic equator and double crests of enhanced electron density at about 15° north and south (e.g., Appleton, 1946; Liang, 1947; Namba & Maeda, 1939). By using occultation measurements from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites, Zhong et al (2019) showed that the nighttime electron densities at ±40° magnetic latitude (MLAT) are generally larger during equinoxes, forming two bands at all longitudes; while during solstices the band in the winter hemisphere is much more visible This nighttime ionospheric band structure at middle latitudes should be the same phenomena as earlier reported nighttime enhancements of the F2 electron density peak, NmF2 (e.g., Arendt & Soicher, 1964; Balan & Rao, 1987; Jakowski & Förster, 1995), which are more often observed in the winter hemisphere and are suggested to be caused by the enhanced equatorward meridional wind and downward diffusion from the plasmasphere at nighttime (Mikhailov et al, 2000; Zhong et al, 2019). The Swarm's in situ measurements show that two electron density peaks at low latitudes are still discernable during postmidnight hours, and the other two peaks are sometimes observed at northern and southern middle latitudes, forming a latitudinal four‐peak structure in the nightside ionosphere.
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