## 4 Results
### Missing FRP inside MODIS Swath Gaps  
Active fires inside the MODIS swath gaps contribute substantial FRP. Figure 9a and b show yearly summed MODIS and VIIIRS FRP at 1\({}^{\circ}\) grids in the MODIS observed area (outside swath gaps) in 2017. The FRP spatial pattern shows that fires mainly burn in areas between 4\({}^{\circ}\)N and 16\({}^{\circ}\)N in the Northern Hemisphere, Africa, and between 2\({}^{\circ}\)S and 20\({}^{\circ}\)S in the Southern Hemisphere, Africa.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Grid Size (Degree)**} & \multicolumn{3}{c}{**Model Parameters**} & \multirow{2}{*}{**RMSE**} \\ \cline{2-2} \cline{5-6}  & \(\beta_{0}\) & \(\beta_{1}\) & \(\beta_{2}\) & \\ \hline
0.05 & \(1.054\pm 0.02\)** & \(-0.045\pm 0.08\) & \(-0.223\pm 0.07\)** & 0.93 & 0.026 \\
0.10 & \(1.133\pm 0.02\)** & \(0.030\pm 0.08\) & \(-0.265\pm 0.07\)** & 0.91 & 0.028 \\
0.25 & \(1.313\pm 0.02\)** & \(-0.006\pm 0.09\) & \(0.141\pm 0.07\)** & 0.74 & 0.030 \\
0.50 & \(1.401\pm 0.04\)** & \(0.004\pm 0.16\) & \(0.074\pm 0.14\) & 0.23 & 0.054 \\
1.0 & \(1.456\pm 0.05\)** & \(-0.085\pm 0.22\) & \(0.369\pm 0.18\)** & 0.72 & 0.070 \\
2.5 & \(1.564\pm 0.08\)** & \(-0.295\pm 0.35\)** & \(0.672\pm 0.29\)** & 0.68 & 0.113 \\
5.0 & \(1.690\pm 0.12\)** & \(-0.851\pm 0.48\)** & \(1.219\pm 0.40\)** & 0.67 & 0.146 \\ \hline \hline \end{tabular} Note that ** and * represent \(p\)-value \(<0.001\) and \(p\)-value \(<0.1\), respectively, and the term after “\(\pm\)” is the 95\% level confidence interval.  
\end{table}
Table 1: Best-fitted models for VIIIRS to MODIS FRP ratio as a function of MODIS VZA at seven grid sizes.  
Figure 9: Yearly summed MODIS and VIIIRS daytime FRP at a 1\({}^{\circ}\) grid across Africa in 2017. **(a)** The observed MODIS FRP, **(b)** the observed VIIIRS FRP in the MODIS-sensed areas (outside MODIS swath gaps), **(c)** the total number of days without Aqua MODIS observations (inside MODIS swath gaps), and **(d)** the VIIIRS FRP observed inside MODIS swath gaps.  
Figure 9c shows that MODIS swath gaps occur in ~80% of the areas between 30\({}^{\circ}\)S and 30\({}^{\circ}\)N across Africa, of which 41% was not observed by MODIS for more than 30 days in 2017. Figure 9d shows the yearly summed VIIRS FRP observed inside MODIS swath gaps. It indicates that fires observed by VIIRS inside MODIS swath gaps account for 10.7% of yearly summed FRP (\(9.0\times 10^{7}\) MW) from all VIIRS fire detections across Africa. Indeed, MODIS could miss fire observations in most of the areas where a swath gap occurs, especially fire-prone regions with a highly frequent occurrence of gaps. Therefore, to a large extent, the amount of FRP missed by MODIS is related to fire frequency and the number of days when swath gaps appear.  
VIIRS FRP observed inside MODIS swath gaps varies seasonally (Figure 10). The monthly sum VIIRS FRP inside the gaps peaks in August and December when peak fire seasons generally occur in Southern and Northern Hemisphere Africa [52], which reflects that MODIS misses 0.4-1.4 \(\times\) 10\({}^{6}\) MW FRP monthly (an average of ~0.8\(\times\)10\({}^{6}\) MW). This monthly missing FRP in gaps explains 9.4-18.3% (an average of 12.5%) of monthly VIIRS FRP observed outside MODIS swath gaps. In, MODIS missing FRP accounts for 10.7% and 12.8% of VIIRS FRP observed outside swath gaps, respectively, in Southern Hemisphere Africa during fire season from July to October [52] and Northern Hemisphere Africa during fire season from November to March (next year). The relatively larger percent of missing FRP in Northern Hemisphere Africa is due to the fact that MODIS swath gap occurs in many more days over fire-prone regions in Northern Hemisphere Africa than that in Southern Hemisphere Africa (Figure 9c).