Global volcano monitoring through the Normalized Hotspot Indices (NHI) system

The Normalized Hotspot Indices (NHI) is a multi-channel algorithm developed to map thermal anomalies through the Multispectral Instrument onboard the Sentinel-2 satellite and the Operational Land Imager onboard the Landsat-8 satellite. The algorithm runs operationally under the Google Earth Engine platform and allows the analysis of volcanic thermal features (e.g. lava flows/lakes) through plots of the number of hot pixels, the total shortwave infrared radiance and the area of the hotspot. We present here the automated module of this tool: the NHI system. This system provides automated notifications about volcanic thermal anomalies detected at the global scale over the previous 48 h whenever the NHI web site (https://sites. google.com/view/nhi-tool) is accessed. The results of the first six months of operation are assessed through the analysis of satellite imagery and comparison with well-established programmes for global volcano monitoring. The low false positive rate (around 15%, including vegetation fires and data issues) and the successful identification of small, high-temperature features show that the NHI system may successfully integrate information from high temporal/low spatial resolution satellite data, despite some limitations (e.g. temporal sampling of the combined Sentinel-2 and Landsat-8 observations; delay of data ingestion in the Google Earth Engine platform). The recent ingestion of Landsat-9 data within the system has further extended the performance of the NHI system in supporting the surveillance of active volcanoes from space.