*3.1. First Set of Days (Quiet Days before Lightning Events)* 3.1.1. 9th July

This day was the first day of non-lightning days before lightning days. The upper and lower panels of Figure 4 show the lightning count and current, respectively. Few counts are seen from 00–01 local time (LT: UT + 8). Figures 5 and 6 show the TEC-DTEC for satellites of pseudorandom noise code (PRN) 1 and 13. PRN 1 was chosen as it covered the time with no lightning (09–20LT), while PRN 13 was specifically chosen as it covers the time of the few lightning counts.

**Figure 4.** Lightning activity on 9th July. The (**upper panel**) shows the counts; the (**lower panel**) is the current.

From panel e of Figure 5, P3 recorded DTEC values between ±3TECu for PRN 1. The amplitudes remained same during the time the satellite was available. For P5, PRN 1 had an amplitude of DTEC of about ±0.5TECu, which remained same throughout the periods. On PRN 13, P5 and P6 had similar characteristics to P3. For P10 the amplitude of DTEC was about ±0.3TECu, which is 0.2 less than that of P5 and P6 on PRN 1. Similar observations to P5 on PRN 13 were made by P10.

Observations on PRN 1 reveal the quiet day phenomena, as no change in amplitude was seen. Increased amplitude on PRN 13 also reveals that P3, P5, P6 and P10 could detect low lightning counts, around 00–01 LT.

sgf\_3\_30 had an amplitude of DTEC of about ±0.03TECu on PRN 1, shown in panel m of Figure 5, which remained same throughout the day. For sgf\_3\_60, the amplitude of DTEC was about ±0.1TECu compared to sgf\_3\_30. sgf\_3\_90 had an amplitude of DTEC of ±0.5TECu, which was constant for the whole day for PRN 1. sgf\_3\_120 also had a DTEC amplitude of ±0.2TECu on PRN 1. With PRN 13, sgf\_3 parameters saw an increase in amplitude about the same time as lightning. Sgf\_3\_30 saw an increase to ±0.1TECu, and that of sgf\_3\_60 to ±0.6TECu. The increases in amplitude for sgf\_3\_90 and 120 are no different than the amplitude recorded on PRN 1.

**Figure 5.** TEC and DTEC for PRN 1 on 9th July, 2015. Original TEC are in black solid lines and modelled TEC are in dashed colored lines. In panels (**a**–**d**) are the original TEC and modelled TEC of P3, P5, P6 and P10, respectively. Their respective DTECs are in panels (**e**–**h**) with colors corresponding to their modelled TEC. Panels (**i**–**l**) show the original TEC and modelled TEC of sgf\_3\_30, sgf\_3\_60, sgf\_3\_90, and sgf\_3\_120. Their respective DTECs are in panels (**m**–**p**), with colors corresponding to their modelled TEC. Original and modelled TEC of sgf\_6\_30, sgf\_6\_60, sgf\_6\_90, and sgf\_6\_120 are in panels (**q**–**t**), with their respective DTEC in corresponding colors in panels (**u**–**x**). The horizontal axis is hours in local time (LT: UT + 8). The vertical axis is in TEC units.

The DTEC amplitude of sgf\_6\_30 was mostly between ±0.01TECu, and remained constant for the whole day on PRN 1. That of sgf\_6\_60 was between ±0.03TECu compared to sgf\_6\_30. For sgf\_6\_90 and 120, the amplitude was about ±0.05TECu compared to sgf\_6\_30 and sgf\_6\_60. On PRN 13, sgf\_6\_30 saw an increase in amplitude of about ±0.05TECu, while sgf\_6 30, 90 and 120 saw an increase of about ±0.2TECu during the time of the lightning count.

In all, most parameters had their DTEC amplitude constant throughout the day, and were thus able to represent a quiet day. Sgf parameters had lower amplitudes compared to polynomials.

#### 3.1.2. 10th July

This day was the second of the non-lightning days before lightning days. Figure 7 shows the lightning count and current. Few lightning counts were seen at 08–09 and 18–20 LT. Figure 8 shows the TEC-DTECs for PRN 13.

**Figure 6.** TEC and DTEC for PRN 13 on 9 July 2015. Original TEC are in black solid lines and modelled TEC are in dashed colored lines. In panels (**a**–**d**) are the original TEC and modelled TEC of P3, P5, P6 and P10, respectively. Their respective DTECs are in panels (**e**–**h**), with colors corresponding to their modelled TEC. Panels (**i**–**l**) show the original TEC and modelled TEC of sgf\_3\_30, sgf\_3\_60, sgf\_3\_90, and sgf\_3\_120. Their respective DTECs are in panels (**m**–**p**), with colors corresponding to their modelled TEC. Original and modelled TEC of sgf\_6\_30, sgf\_6\_60, sgf\_6\_90, and sgf\_6\_120 are in panels (**q**–**t**), with their respective DTECs in corresponding colors in panels (**u**–**x**). The horizontal axis is hours in local time (LT: UT + 8). The vertical axis is in TEC units.

**Figure 7.** Lightning activity on 10th July. The (**upper panel**) shows the counts; the (**lower panel**) is the current.

**Figure 8.** TEC and DTEC for PRN 13 on 10th July 2015. Information about figure panels is same as that of Figures 5 and 6.

From panels f–h of Figure 8, P5, P6 and P10 all had a constant amplitude of ±0.5TECu. P3 had the highest DTEC value at ±4TECu. The amplitudes remained the same for the whole period.

Sgf\_3\_30 had an amplitude of DTEC of about ±0.05TECu on PRN 13 in Figure 8, and remained constant. Panels n–p in Figure 8 show that sgf\_3\_60, 90 and 120 had a constant amplitude of ±0.2TECu on PRN 13.

For sgf\_6\_30, the DTEC amplitude was ±0.02TECu and remained constant for the whole period on PRN 13 (panel u of Figure 8). Sgf\_6\_60, 90 and 120 recorded DTEC amplitudes of ±0.05TECu throughout the period of PRN 13

Similar to 9th July, most parameters had their DTEC amplitude constant throughout the day, and were thus ably to represent a quiet day. Sgf parameters had lower amplitudes compared to polynomials.

#### 3.1.3. 11th July

This day was the last of the non-lightning days before lightning days. Compared to the previous days in this set of days, 11th July virtually had no lightning count, making it an ideal example of a truly quiet day. Figure 9 shows the lightning count and current. Figure 10 shows the TEC-DTECs for PRN 2. Most satellites had similar observations to PRN 2.

**Figure 9.** Lightning activity on 11th July. The (**upper panel**) shows the counts; the (**lower panel**) is the current.

**Figure 10.** TEC and DTEC for PRN *2* on 11th July 2015. Information about figure panels is same as that of Figures 5 and 6.

From Figure 10, the polynomial and sgf parameters had constant amplitudes throughout the period of PRN 2. DTECs for all the parameters had similar characteristics to those of 9th and 10th July; this example thus represents a typical quiet day.
