## 2 Materials and Methods  
This study used data from ground-based Magnetic Data Acquisition System (MAGDAS) magnetometer stations in Davao (DAV, GM Lat.: \(-\)2.22\({}^{\circ}\), GM Lon.:197.90\({}^{\circ}\)) and Langkawi (LKW, GM Lat.: \(-\)3.30\({}^{\circ}\), GM Lon.:172.44\({}^{\circ}\)) in the west and east of the Southeast Asian, respectively (refer Figure 1). In data processing, a series of EUEL index derivations on one-minute resolution data from 2008 to 2018 was performed as a representative analysis of the entire solar cycle 24. This index also referred to as the EEJ index was derived from a series of calculations, starting with the subtraction of the raw data with the median value of the northward geomagnetic component, H. The result gave the ERs value for each equatorial station, which was used to obtain another index, namely EDst. EDst, which served as the global magnetic variation indicator, was also the mean of ERs during the nighttime (LT = 18:00-06:00) from all available stations along the magnetic equator region.  
EUEL was the local index which is the final outcome after ER was subtracted with the EDst index [20; 21].  
To avoid solar-terrestrial disturbances such as geomagnetic storms, only geomagnetically quiet days were considered in the analysis by referring to the global geomagnetic activity index, which is Kp-index at lower values (i.e., Kp \(\leq\) 3).  
Over the course of 11 years, two levels of solar activity were introduced: low (F10.7 ave = 75 s.f.u.)--2008, 2009, 2010, 2016, 2017, 2018; high (F10.7 ave = 125 s.f.u.)--2011, 2012, 2013, 2014, 2015. However, intermittent data from the LKW station from 2008 to 2010 were deemed insufficient to cover the whole year. Hence, the three consecutive years were excluded from the analysis for this station, leaving only the data from 2016, 2017, and 2018 in the low solar activity level category.  
Following that, a set of criteria was established to ensure the accuracy of CEJ identifications. Based on daily preliminary inspections, it was determined that a CEJ could be identified when the EUEL value fell below--5 nT for at least 2 h, adapted from past studies [22]. The duration criterion was chosen to minimize false detections, since transient geomagnetic field depletion could occur from time to time due to other factors, such as instrument failures. In addition, depletion periods with any data gaps were excluded from the analysis to provide continuous CEJ.  
Subsequently, CEJ that had been successfully identified were sorted based on several local time intervals. Since no occurrence was detected at noon (11:00-13:00 LT), the intervals proposed in this study were morning CEJ (MCEJ, 06:00-11:00 LT), and afternoon CEJ (ACEJ, 13:00-18:00 LT). PSDs were identified between 18:00 LT and 21:00 LT to consider possible night-time occurrences. Figure 2a depicts an observation of MCEJ and ACEJ, while Figure 2b shows an example of PSD. The number of observation days collected from Davao and Langkawi stations year by year was illustrated in Figure 3 and the percentage of all occurrences was calculated according to their types using the following formula [8]:  
\[\text{Occurrence }\%=\frac{\text{No. of occurrence}}{\text{No. of observation days}}\times 100\%\]  
Figure 1: Location of the ground magnetometer stations used in the study: DAV (Davao) and LKW (Langkawi).