## 2 Data and Methods  
### Data  
CSES-01 is a scientific mission dedicated to monitoring electromagnetic fields and plasma variations in the topside ionosphere, as induced by natural sources and anthropogenic emissions, and to study their possible correlations with the occurrence of seismic events [39]. CSES-01, launched on 2 February 2018 and still in operation, orbits sun-synchronously at an altitude of \(\sim\)500 km, with an orbital period of \(\sim\)94 min and an inclination of 97.4\({}^{\circ}\). It has an average speed of \(\sim\)7.2 km/s, descending and ascending nodes at \(\sim\)14:00 LT and \(\sim\)02:00 LT, respectively, and a revisit period of 5 days. CSES-01 carries a full suite of instruments for measuring the ionospheric electromagnetic fields and plasma properties, among which, as used in the present study, the LP and the EFD.  
The LP, designed for in situ measurements of bulk parameters of the ionospheric plasma, provides continuous measurements of the electron density (\(N_{e}\)) and temperature (\(T_{e}\)) between 5 \(\times\) 10\({}^{2}\) cm\({}^{-3}\) and 1 \(\times\) 10\({}^{7}\) cm\({}^{-3}\) and between 500 K and 10,000 K, respectively, both with an accuracy of 10% [40; 41] and a sampling frequency of 3 s in survey mode.  
The EFD is designed to take measurements of the ionospheric vector electric field in several frequency bands. More specifically, in the Extremely Low Frequency (ELF) band [42; 43] used in this work, EFD provides continuous measurements with a sampling frequency of 5 kHz. Measurements of all instruments onboard CSES-01 are taken continuously only between \(-65^{\circ}\) and \(+65^{\circ}\) of geographic latitude due to telemetry constraints.  
The Swarm constellation, launched at the end of 2013 and still in operation, consists of three low-Earth-orbit satellites that provide high-precision and high-resolution measurements of the amplitude, direction, and variation in magnetic field and plasma properties (such as temperature and ion density [44; 45; 46]).  
The Swarm satellites are located at two different quasi-circular polar orbits, two flying side-by-side at an initial altitude of 460 km (Swarm A and C), and the third one flying at an initial altitude of 530 km (Swarm B). Unlike CSES-01, Swarm satellites drift in LT, covering a full 24 h LT cycle in about 130-140 days. Swarm LPs provide continuous measurements of ion density (\(N_{i}\)) and electron temperature (\(T_{e}\)) at a sampling frequency of 2 Hz in harmonic mode [47]. At the Swarm satellite's altitude, the plasma quasi-neutrality is so well verified that it is safe to assume \(N_{i}\simeq N_{e}\) and compare the CSES-01 \(N_{e}\) with the Swarm B \(N_{i}\). For the present work, we analyzed the \(N_{i}\) data acquired by Swarm B LP, orbiting roughly at the same altitude as CSES-01 during the analyzed period (2019-2024), downsampled to 1/3 Hz to allow a better comparison with the CSES-01 LP data. Despite the plethora of calibrations and corrections recently delivered about Swarm LP observations [48; 49; 50; 51; 52; 53], we decided to use the original LP observations because the algorithm for the EPBs identification does not depend on the absolute magnitude of LP observations, but on the ratios with respect to a baseline calculated from actual observations themselves.