# A ROTI-Aided Equatorial Plasma Bubbles Detection Method
## 3 Results  
Figure 3 presents an example of the EPBs detection method in this study using station HKKT and satellite R04 on 28 February 2014. The blue line in panel (a) is the observed TEC series. The black line in panel (b) is the corresponding ROTI series calculated using Equations (1) and (2). The blue dotted line and red dotted line mark the thresholds of the up ROTI (0.2 TECU/min) and down ROTI (0.04 TECU/min), respectively. As shown in Figure 3b, the ROTI series during the period of \((T_{1},~{}T_{2})\) meet the condition of the up and down thresholds (the maximum is 0.49 TECU/min). So, ionospheric irregularity during this period can be marked as an EPB candidate. The partially fitted TEC series, using a third-order polynomial, are also presented in panel (a) for comparison (green line). The red line in panel (c) is the detrended TEC series obtained by subtracting the fitted from the observed TEC series. As shown in Figure 3c, the depths (5.5 TECU) and duration (24 min) of the EPB candidates meet the defined thresholds. So, it can be confirmed that there is an EPB event in the ionosphere during the period of \((T_{1},~{}T_{2})\). Figure 4 presents the temporal distribution of the EPBs (top panel) and the percent distribution of EPBs occurrence (bottom panel) detected by the station HKKT, during 2014. As shown in Figure 4a, EPBs generally occur after 20:00 local time and are observed throughout the nighttime. As shown in Figure 4b, the occurrence of EPBs during the spring and autumn equinoxes is significantly greater than their occurrence during the summer and winter solstices. The observed temporal distribution and occurrence of EPBs over the Hong Kong area are also reported in previous studies [12; 13], suggesting that the detection method is feasible. As shown in Figure 4a, part of the values of the depletion series is positive. Theoretically, the values of the depletion series should be negative. So, the maximum positive value (MPV) of each depletion series can serve as a reference index to assess the accuracy of the detection result. Figure 5 presents the percent distribution of  
Figure 3: Example of the EPBs detection method in this study using station HKKT and satellite R04 on 28 February 2014. (**a**) Observed total electron content (TEC, blue line) and partially fitted TEC using third-order polynomial (green line); (**b**) the rate of TEC variation index (ROTI, black line) and the up threshold (red dot line) and down threshold (blue dot line) for irregularity; (**c**) detrended TEC (red line). The annotations “T1” and “T2” indicate the initial and end time of EPB. the MPV by the station HKKT during 2014. As shown in Figure 5, most of the MPVs are smaller than 1.5 TECU. The mean value of the MPV is 0.63 TECU. As indicated in the introduction, an important difference between the EPBs detection method in this study and that of the IBS method is the strategy to derive the detrended TEC series. Compared to the IBS method, the third-order polynomial fitting but not linear fitting (namely first-order polynomial) is employed in this study. Figure 6 presents the detrended TEC series using first-order and third-order partial polynomial fittings for the same example in Figure 3. The green lines and red lines represent the first-order and third-order fitting results, respectively. The duration of this EPB event is about 24 min, which can serve as a short-duration case. As shown in Figure 6, the TEC depth difference is insignificant between the different fitting orders. Figure 7 presents a case of a long-duration EPB as observed by station HKKT and satellite G09. As shown in Figure 7, the duration of this EPB event is about 2 h. The fitted TEC series by the third-order polynomial are  
Figure 4: Temporal distribution of EPBs (top panel) and percent distribution of EPBs occurrence (bottom panel) detected by station HKKT during 2014. “LT” represents local time in Hong Kong.