Detecting Post-Midnight Plasma Depletions Through Plasma Density and Electric Field Measurements in the Low-Latitude Ionosphere

Plasma depletions in the low-latitude ionosphere are irregularities of special interest in space weather research, as they are highly detrimental to the operation of satellite-based communication and navigation systems. In this frame, we present the results of a systematic study of the low-latitude topside ionosphere, based on in situ measurements of both electron density (Ne) and electric field provided by the Langmuir Probe (LP) and the Electric Field Detector (EFD) onboard the first China Seismo-Electromagnetic Satellite (CSES-01). Specifically, by exploiting in situ measurements from 1 January 2019 to 31 May 2024, we devised two different techniques for the automatic detection of post-midnight plasma depletions at about 500 km of altitude: one using only Ne observations, the other using only electric field measurements. We validated these new techniques against each other and performed a statistical investigation of the main characteristics of the observed plasma irregularities, such as their latitudinal extension, longitudinal distribution, and monthly and seasonal occurrence. To test the robustness and reliability of our algorithms, we also applied them to well-established Swarm B satellite observations. In particular, we first investigated both the monthly and the seasonal occurrences of post-sunset plasma depletions detected between 18:00 and 04:00 local time (LT), by LP onboard the Swarm B satellite at about 500 km of altitude. In addition, we compared ionospheric irregularities detected by Swarm B with those detected by CSES-01. For the comparison, we considered Swarm B LP data collected for the same period as the CSES-01 dataset and under the same conditions by selecting Swarm B observations in the range 01:00 ≤LT< 03:00. Our results prove the robustness and reliability of both LP and EFD algorithms in detecting plasma depletions, and their good agreement suggests their complementarity in detecting such kinds of plasma irregularities. Results also confirm consistency between CSES-01 and Swarm B observations (once the same LT orbits have been considered) and with the relevant literature on the topic.