###### Abstract  
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 (\(N_{e}\)) 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 \(N_{e}\) 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 \(\leq LT<\) 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. low-latitude ionosphere; post-midnight plasma depletions; ionospheric irregularities; China seismo-electromagnetic satellite (CSES-01) 202517152917  
the absence of sunlight leads to a much faster ionospheric recombination at lower altitudes with respect to higher ones, thus causing the establishment of steep upward plasma density gradients, which in turn result in the growth of the collision-dominated Rayleigh-Taylor (R-T) instability (see, e.g., [1; 2; 3]). This process triggers the formation of plasma-depleted regions at the bottom side of the F-layer, which then rise buoyantly upwards to the top side of the F-layer [4; 5]. Generally, these irregularities occur within a narrow band of \(\pm 20^{\circ}\) of magnetic latitude, where they align with the geomagnetic field, stretching hundreds of kilometers in the magnetic north-south direction [6; 7]. Most of such depletions have a favored scale size near 100 km [8], but they may have spatial extents between 150 km and 450 km [9; 10], evolving as single or multiple structures. The importance of equatorial plasma depletions (hereafter, we use the nomenclature Equatorial Plasma Bubble (EPB) for such irregularities to align with the literature on the topic) relies on their capability in affecting many modern-day technologies, especially those dependent on space-based navigation systems, such as the Global Navigation Satellite System (GNSS), which transmit on the L-band (see, e.g., [11; 12]). Since the ionosphere acts as a propagation medium, such irregularities may cause disturbances in both the phase and the amplitude of radio signals. In severe cases, they can also lead to the loss of signal lock when the ground-based receiver tries to record the signal from a satellite as it enters the field of view [13; 14; 15; 16; 17].