*3.2. Second Set of Days (Lightning Days)* 3.2.1. 17th July

This was the first of the lightning days. Lightning happened for the whole day, with a peak count around 15–17 LT. After 22 LT, the lightning activity reduced to a minimum. Lightning activity is shown in Figure 11. TEC and DTEC of PRN 13 and 21 are shown in Figures 12 and 13 to check the changes in DTEC for the first peak expected at 18LT and second peak expected at 01LT, respectively.

**Figure 11.** Lightning activity on 1*7*th July. (**Upper panel**) shows the counts; (**lower panel**) is the current.

Since it takes about three hours for lightning to effect changes in TEC, and the change lasts for about three hours, amplitude changes for the first peak (15–17LT) can be expected to be around 20–23LT, while for the second peak (22LT) they can be expected to show changes of around 00–02LT. In panels e–g in Figure 12, P3, P5 and P6 show the same amplitude values throughout. No increase is seen in the DTEC amplitude values. For P10 in panel f, DTEC amplitude increases to ±0.5TECu at the expected time. P10 was thus able to detect the lightning event on PRN 13. In Figure 13, the DTEC amplitude drops sharply at the expected hour of 01LT from 1 to −2TECu for P5, P6 and P10, signifying a detection. P3 also records a similar observation, though this is not as obvious as P5, P6 and P10.

For the sgf\_3 parameters, only sgf\_3\_30 and 60 could detect the lightning event on PRN 13 in Figure 12. Their amplitudes increased to ±0.04 within the expected time range. Sgf\_3\_90 and 120, on the other hand, had constant DTEC amplitude. For the second lightning peak at 22LT, all of the sgf\_3 parameters could detect it within the expected time range. Their amplitudes increased to between ±0.2–0.4TECu, as seen in panels m to p in Figure 13.

Sgf\_6\_30 could not detect the first lightning peak but did detect the second. Sgf\_6\_60, 90 and 120 detected both lightning peaks as increase in amplitude, as seen in panels v to x in Figures 12 and 13.

**Figure 12.** TEC and DTEC for PRN 13 on 17th July 2015. Information about figure panels is same as that of Figures 5 and 6.

**Figure 13.** TEC and DTEC for PRN 21 on 17th July 2015. Information about figure panels is same as that of Figures 5 and 6.

P5, P6 and P10 could detect lightning events; although the change in amplitude for lightning and non-lightning days were similar, the DTEC amplitude values of sgf parameters indicating the presence of lightning were higher compared to the quiet nonlightning days.

#### 3.2.2. 18th July

This was the second of the lightning days. Lightning happened for the whole day, with peaks around 15LT. At 21LT, a small peak was also seen. Figure 14 shows the lightning event. DTEC-TEC are shown for PRN 5 in Figure 15, as its time of availability covered the expected time of the amplitude changes of the two peaks, 18LT for the peak at 15LT and 23LT for that at 21LT.

**Figure 14.** Lightning activity on 18th July. (**Upper panel**) shows the counts; (**lower panel**) is the current.

**Figure 15.** TEC and DTEC for PRN *5* on 1*8*th July 2015. Information about figure panels is the same as that of Figures 5 and 6.

With peak counts around 15 and 21LT, changes in DTEC amplitude were expected around 18 and 23LT. P3, P5 and P6 had constant amplitudes of ±3TECu, and hence were not able to detect the lightning event. P10 first saw an increase in amplitude of about ± 0.5TECu, then a reduced amplitude of ±0.2TECu, and finally another increase to ±0.5TECu. These increments all happened at the expected time of amplitude change, indicating that P10 could detect the lightning event. From panels m to p in Figure 15, sgf\_3 parameters all saw amplitude changes at the expected time. Sgf\_3\_30 recorded ±0.06TECu, while sgf\_3\_60, 90 and 120 all recorded ±0.2TECu at the expected time of DTEC changes. Sgf 6 parameters also showed an increase in DTEC amplitude at the expected time. Sgf\_6\_30 recorded ±0.03TECu and sgfs 6\_60, 90 and 120 all recorded ±0.07TECu.

P10 and all of the sgf parameters therefore detected the lightning activity on this day.

#### 3.2.3. 19th July

This was the last of the lightning days. Lightning happened from 08–09LT and reoccurred from 16–07LT. The peak periods were 16–17, 19–20 and 23–03LT, with DTEC amplitude changes expected at 11, 23 and 02LT, respectively. PRN 15 was available at 20–02LT, covering the expected time changes of the second peak period and hence able to be investigated. Figures 16 and 17 show the lightning activity and DTEC for PRN 15, respectively.

**Figure 16.** Lightning activity on 1*9*th July. (**Upper panel**) shows the counts; (**Lower panel**) is the current.

Less useful information could be derived from the DTEC of P3 (panel e of Figures 17 and 18). P5, P6 and P10 made similar observations to PRN 15. An increase in amplitude of ±2TECu was seen around 22–23LT, and the amplitude changes occur at the expected time.

**Figure 17.** TEC and DTEC for PRN 15 on 19th July 2015. Information about figure panels is the same as that of Figures 5 and 6.

**Figure 18.** Lightning activity on 1st August. (**Upper panel**) shows the counts; (**lower panel**) is the current.

Sgf\_3\_30, 60, 90 and 120 all saw an increase in DTEC amplitude at the expected time. Similar observations were also made by the sgf\_6 parameters. Sgf\_3\_30 recorded ±0.06TECu, while sgf\_3\_60, 90 and 120 all recorded ±0.2TECu at the expected time of DTEC changes. Sgf 6 parameters also showed an increase in DTEC amplitude at the expected time. Sgf\_6\_30 recorded ±0.03TECu, sgf 6\_60 and 90 recorded ±0.1TECu, and 6\_120 recorded ±0.2TECu.

P5, P6,P5, P10 and all the sgf parameters were able to detect the lightning activity on this day.

#### *3.3. Third Set of Days (Quiet Days after Lightning Events)*

### 3.3.1. 1st August

This day was the first of the non-lightning days after lightning days. Few lightning counts were seen at 11–12, 16–18 and 21–22 LT. Figures 18 and 19 show the lightning activity, TEC and DTEC of PRN 11.

**Figure 19.** TEC and DTEC for PRN 11 on 1st August 2015. Information about figure panels is the same as that of Figures 5 and 6.

P3, P5, P6 and P10 all had constant DTEC amplitude of ±0.2 on PRN 7, as seen in panels e to h of Figures 20 and 21. The Sgf\_3\_and 6 parameters all had constant DTEC amplitudes. Sgfs 3\_30, 3\_60, 6\_30, 6\_90 and 6\_120 had amplitudes of about ±0.02TECu, while 3\_90 and 3\_120 all had ±0.1TECu. All of the DTEC techniques used were able to show that this day was a quiet day, having constant DTEC amplitudes.

#### 3.3.2. 2nd August

This day was the second non-lightning day after a lightning day. Figure 20 shows the lightning activity in terms of current and count. Few lightning counts are seen at 14–19 or 03–04 LT. Figure 21 shows TEC and DTEC for PRN 5.

Less useful information could be derived from the DTEC of P3 in panel e of Figure 21. P5, P6 and P10 had a constant DTEC amplitude of about ± 0.1TECu. Sgf parameters also

had constant amplitude throughout. Sgf\_6\_30 recorded ±0.02TECu. Sgfs 3\_30, 6\_60, 6\_90 and 6\_120 recorded ±0.05TECu, while 3\_90 and 3\_120 had ±0.1TECu.

The constant amplitude shown by the detrending techniques indicate that this day was a quiet day.

**Figure 20.** Lightning activity on 2nd August. (**Upper panel**) shows the counts; (**lower panel**) is the current.

**Figure 21.** TEC and DTEC for PRN 5 on 2nd August 2015. Information about figure panels is the same as that of Figures 5 and 6.

#### 3.3.3. 3rd August

This was the last non-lightning day in the third set of days. Figure 22 shows the lightning activity for this day. Few lightning counts were seen, mostly at 14–19LT. Figure 23 show the time of passage of PRN 4 and its respective TEC and DTECs.

**Figure 22.** Lightning activity on 3rd August. (**Upper panel**) shows the counts; (**lower panel**) is the current.

**Figure 23.** TEC and DTEC for PRN 7 on 3rd August 2015. Information about figure panels is the same as that of Figures 5 and 6.

All detrending parameters for PRN 4 had constant DTEC amplitudes at the time of its passage. The polynomials had higher values compared to the those of sgf. The constant amplitude once again indicates that this day was a quiet a day. The amplitudes recorded by the detrending parameters were similar to those of other non-lightning days.

Table 2 provides a summary of which DTEC method and parameters were able to detect lightning events on lightning days and to represent the quiet nature of non-lightning days according to the 2DC approach for the results enumerated above.

**Table 2.** Summary of which DTEC parameters could indicate non-lightning and lightning events. R means represent, D means detected, ND means not detected.


#### **4. Discussion**

From the results in Section 3 and Table 2, all of the detrending methods had individual DTEC amplitudes that were mostly the same or constant during both sets of non-lightning days at the time of passage of the satellites (panels e–h, m–p, and w–x of Figures 6, 8, 10, 19, 21 and 23). Days without changes in amplitude show that the ionosphere was quiet, which truly reflects the weather events, as there were no geomagnetic storms, lightning events, or sunspots.

During the lightning days, P5, P6 and P10 could detect lightning events using the 2DC approach. DTEC amplitude increased, as seen in panels e to h of Figures 13 and 17. This observation agrees with Rahmani, et al. [11] and Ogunsua, et al. [12], who used P10 and P6 respectively to detect the occurrence of lightning. The amplitude values, however, were no different from those of non-lightning days. The amplitude for non-lightning days was on average between ±0.5–5TECu. Lightning days at the time of expected DTEC changes also recorded increases in amplitude values between ±0.5–5TECu. This does not show a clear distinction between the lightning days and non-lightning days. Thus, the distinguishing condition of 2DC was not met. This non-distinction could be the reason Kumar, et al. [29] reported no difference between lightning and non-lightning days, and makes polynomials less suitable for distinguishing lightning days.

Coster, et al. [30], suggests the accuracy of DTEC is about ±0.05TECu. Any fluctuations above this could be a disturbance. The Savitzky–Golay parameters mostly had ±0.05TECu on non-lightning days and saw an increase to about ±0.06–2TECu on lightning days (panels m–p and w–x of Figures 12, 13, 15 and 17), in agreement with this suggestion. This further suggests that the Savitzky–Golay filters were better at detecting lightning activity and representing the quiet activity of non-lightning days. Sgf\_3\_30, 6\_30, 6\_60 and 6\_120 mostly had amplitudes of ±0.05TECu on non-lightning days and saw an increase to ±0.06–0.2TECu at the time of expected DTEC amplitude changes on lightning days. These parameters were therefore able to detect lightning events and distinguish lightning days from non-lightning days using 2DC. Sgf\_3\_90 and 3\_120 had amplitudes of ±0.1TECu on non-lightning days (panels o and p of Figures 6, 8, 10, 19 and 21) and an increase to the same value at the time of expected DTEC changes on lightning days (panel o and p of Figures 12, 15 and 17). These two parameters, like the polynomials, could only detect lightning activity, not distinguish between lightning and non-lightning days. The time window of 90 to 120 min is the typical period of TIDs. Sgfs 3\_90, 3\_120, 6\_90 and 6\_120 being able to detect DTEC changes affirms that lightning can induce TIDs as, suggested by Mahmud M [13]. Another interesting observation can be seen in Figure 5: DTEC amplitude changes are observed on PRN 13, although 9th July was a non-lightning day. PRN 13 passed at a time about 1–2 h after the few lightning counts on 9th July, as seen in Figure 8. It could be that PRN 13 passed directly over the location of the lightning strokes, and was

therefore able to detect them. This also confirms the observation by Qin, et al. [31] that even a small lightning stroke can effect changes in the ionosphere.

As the Savitzky–Golay parameters sgf\_3\_30, 6\_30, 6\_60, 6\_90 and 6\_120 met all the conditions in 2DC, further evaluation through statistical means was deployed to choose the most suitable parameter. Linear correlations between lightning count and DTEC on lightning days for the PRNs presented in Section 3 above were conducted. The significant level (α) for accuracy assessment was 0.05, and the correlation coefficients and p-values for each PRN are presented in Table 3. Figure 24 shows the respective scatter diagrams of the correlations for these parameters.

**Table 3.** Correlation coefficients between lightning counts and DTEC values from the various parameters during lightning days. PRNs are placed in brackets. E is the scientific notation for base ten.


**Figure 24.** Scatter diagram for correlation between lightning count and DTEC amplitude in TECu on lightning days. Columns 1 to 4 are for the parameters sgf\_3\_30, 6\_30, 6\_60, 6\_90 and 6\_120, respectively. Row 1 (panels **a**–**e**) is for PRN 13 on 17th July. Row 2 (panels **f**–**j**) is for PRN 21 on 17th July. Row 3 (panels **k**–**o**) is for PRN 5 on 18th July. Row 4 (panels **p**–**t**) is for PRN 15 on 19th July.

From Table 3 and Figure 24, the DTEC amplitudes were mostly moderately positively correlated to lightning. The sgf\_6\_120 parameter was the most consistent with lightning count for all the days and PRNs, with an average moderate positive correlation of about 0.5. With such consistency and moderate positive correlation, sgf\_6\_120 was selected as the most suitable after meeting the conditions of 2DC. The time frame of 120 min further suggests that the disturbances could be TIDs, with lightning as the potential source. The coefficient of 0.4–0.5, though moderate, could be deemed as significant. Gravity waves in equatorial regions correlated with equatorial plasma bubbles (EPB) was 0.2. Though a weak correlation, this was consistent over a long period of time, and given the multiple sources of EPBs it should not be neglected. [32] Lightning is also a source of gravity waves [14], and a relative higher correlation of 0.4 could equally be deemed significant. The findings from the sgf confirms studies in other disciplines, such as medicine, that a Savitzky–Golay filter can represent physical parameters and events and provide detail which could otherwise be missed.
