In 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.
The daytime plasma density disturbances in the low-latitude ionosphere, referred to as plasma irregularities, mainly occur during the nighttime and are an unusual phenomenon. Based on the observations from multiple low Earth orbiting (LEO) satellites, e.g., the Defense Meteorological Satellite Program (DMSP) F13 and F15, the first Satellite of the Republic of China (ROCSAT-1), the Gravity Recovery and the Climate Experiment (GRACE), and Challenging Mini-satellite Payload (CHAMP) satellites, as well as the ground-based Global Positioning System (GPS) receivers, we report a special event of low-latitude plasma irregularities that were observed after sunrise in the Pacific longitudes on 18 August, 2003, following a moderate geomagnetic storm. Observations from three ground-based GPS stations in both hemispheres showed remarkable total electron content (TEC) disturbances during 20:00 to 21:00 UT (around local sunrise), agreeing well with the in situ plasma density irregularities recorded by the nearby flying LEO satellites. The plasma irregularities observed by these LEO satellites showed quite different depletion intensities at different altitudes. We suggest that the plasma irregularities were freshly generated near sunrise hours due to the disturbance of the dynamo electric field (DDEF), evolving into the post-sunrise and morning sector, but were not the remnant of the plasma irregularities generated during the previous nighttime.
The effects of north‐south hemispheric asymmetry of background ionosphere and the neutral wind and declination varying with longitudes to the development and evolution of generalized Rayleigh‐Taylor instability are studied, using the method of flux‐tube integration in geomagnetic northern‐southern hemispheres independently. The results indicate that the flux‐tube integrated linear growth rate of generalized Rayleigh‐Taylor instability shows a significant characteristic of south‐north asymmetry. The hemispherical‐asymmetric neutral wind may be a key factor for the south‐north asymmetric distribution of ionospheric irregularities, and the neutral wind and the declination varying with longitudes have important impacts on the longitudinal variation of Rayleigh‐Taylor instability. They could be the major control factors to cause the longitudinal effects of ionospheric irregularities.
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