Diurnal evolution of the F region electron density local time gradient at low and middle latitudes resolved by the Swarm constellation

Abstract

In this study we have provided new insights into the local time gradient of F region electron density (ΔNe) derived from the lower pair of Swarm satellites flying side by side. Our result shows that the electron density (Ne) increase starts just at sunrise, around 06:00 LT, simultaneously at low and middle latitudes due to the increased photoionization. At equatorial latitudes the increase in electron density gets even steeper after 07:00 LT, and the steepest increase of electron density (about 3 · 1010 m−3 within 6 min) occurs around 09:00 LT. We suggest that the upward vertical plasma drift in connection with the buildup of the equatorial fountain effect plays a major role. We also found that the local time variations of the equatorial ionization anomaly (EIA) crest electron density during daytime are similar to the respective evolutions at the equator, but about 1–2 h delayed. We relate this delay to the response time between the equatorial electric field and the buildup of the plasma fountain. At equinox months a fast decrease of the F region electron density is seen at the EIA trough region during the prereversal enhancement, while an increase is found meanwhile at crest regions. Afterward, a fast decrease of the EIA crest electron density occurs between 19:00 and 23:00 LT, with seasonal dependence. The local time gradient between Swarm A and C shows also prominent longitudinal wave‐4 pattern around August months, and the phase of DE3 in ΔNe is found to be delayed by 6 h compared to that in Ne.

Climatology of the Occurrence Rate and Amplitudes of Local Time Distinguished Equatorial Plasma Depletions Observed by Swarm Satellite

“…Comparing to the peaks of the occurrence rate, the peaks of the depletion amplitudes appear at higher latitudes and at earlier MLT, which implies that the highest occurrence of EPDs does not always coincide with the largest depletion amplitude. The EPD amplitudes show similar MLT variations to those of the background plasma density at nightside to that as reported by (Xiong, Zhou, et al, , their Figure 5). Around sunset at low latitudes, the enhanced eastward electric field in F region enlarges the equatorial fountain effect, causing the two crests of the EIA to get stronger and the trough above the dip equator to become deeper.…”

A solar activity correction term for the IRI topside electron density model

“…The Swarm A and C are flying side-by-side at the same altitude with a longitudinal separation of about 1.4°, therefore, nearly identical in situ electron density are observed by the two satellites. This was confirmed by Xiong et al (2016), who have statistically compared the electron density between Swarm A and C during a two-year period, and found that the ratios between their measured electron densities are about 1.013, 1.010, 1.019 for three different seasons, indicating really consistent measurements between the two satellites. Therefore, we only used the Swarm A and B data within this study.…”

Magnetic Methods, Principles

“…Examination of data from magnetic observatories, it was evident that changes in amplitudes of the geomagnetic field, which recovered to original values could span over a few minutes to 11 and 22 year cycles (Cliver 2015). With the availability of the vast dataset from dedicated satellite missions, it became possible to analyze the global patterns of external field changes (Friis-Christensen et al 2006;Xiong et al 2016;Fathy et al 2019;Kim et al 2019).…”