## 4 Discussion  
We compared the RASTer and AVA detections in both nighttime and daytime conditions over three different active volcanic areas. Table 2 summarizes the results of this comparison, performed over the period May 2000-August 2017. The first column shows that at Mt. Etna and Shinmoedake, RASTer and AVA flagged the presence of thermal anomalies on the same number of ASTER scenes, although the number of hot pixels was significantly different (see second column). Common thermal anomaly detections characterized up to 86 ASTER scenes over Mt. Etna (see third column) where, however, higher was the number of hot pixels flagged by RASTer. On the other hand, the number of unique detections (fourth column) shows a possible complementarity of the two systems, which could increase the continuity of information. In general, the higher number of hotspots flagged by RASTer reveals a higher sensitivity to mid-low intensity thermal anomalies than AVA. This is also confirmed by results achieved at Shishaldin volcano, where only two thermal anomalies were flagged by AVA (see also results section). Thermal anomalies flagged by RASTer, including those undetected by AVA, were in good agreement with information from volcanological reports, as shown for instance in Figure 9 in reference to the Mt. Etna activity. The figure displays the thermal anomalies detected by RASTer on ASTER data of 2000-2020 and three different levels of thermal activity derived from field reports, independent observations and scientific papers [33; 34; 35; 36; 37; 38; 39; 49]. It should be pointed out that RASTer detections were mostly associated with documented \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Number of ASTER Images**} & \multicolumn{2}{c}{**Number of Pixels**} & \multicolumn{2}{c}{**Common Detections**} & \multicolumn{2}{c}{**Unique Detections**} \\ \cline{3-8}
**Volcano** & \multicolumn{2}{c}{**with Detected Thermal**} & \multicolumn{2}{c}{**Flagged as Thermal**} & \multicolumn{2}{c}{**Number of Hot**} & \multicolumn{2}{c}{**Number of Scenes**} & \multicolumn{2}{c}{**Number of Scenes**} \\ \cline{2-8}  & **Anomalies** & **(Until August 2017)** & **Anomalies** & **R** & **A** & & **R** & **A** & **R** & **A** \\ \cline{2-8}  & **RASTer (R)** & **AVA (A)** & **R** & **A** & & & & & & \\ \hline Etna & 100 & 100 & 4070 & 3036 & 86 & 3947 & 3017 & 14 & 14 \\ Shimoedake & 2 & 2 & 60 & 19 & 2 & 60 & 19 & 0 & 0 \\ Shishaldin & 9 & 2 & 26 & 2 & 2 & 8 & 2 & 7 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of RASTer and AVA comparison performed by analyzing ASTER TIR data of 2000–2017 over Etna (Italy), Shinmoedake (Japan) and Shishaldin (AK, USA). Figure 8: Thermal anomalies detected by RASTer at Shinnoedake volcano on 25 March 2018.  
periods of Mt. Etna activity. An analysis is currently in progress to better assess hotspots flagged in periods of undocumented thermal volcanic activity (e.g., 27 July and 4 August 2012). Those features were probably associated with minor thermal activities (e.g., [12]) unreported by field reports, also confirming the relevance of satellite observations in providing information about subtle phases of unrest in well-monitored volcanic areas (e.g., [4]). Regarding the Shishaldin volcano--where, as for Shinnoodake, a long-quiescence period occurred, about 88.8% of detected thermal anomalies were ascribable to intra-crater thermal activities (e.g., May 2014-January 2015), and to lava effusions from volcano flanks (e.g., January 2020) reported by the Global Volcanism Program (GVP). Thermal anomaly detected by RASTer, ASTER scene of 12 August 2009, which was not confirmed by field reports, is consistent with MODIS observations of July-August 2009 [51]. Those results show that RASTer provided reliable information about volcanic thermal features of a different intensity level.