## 2 Methods
### Satellite Remote Sensing Monitoring  
Multispectral satellite images processing is increasingly demonstrating its potential in providing both timely event detection for volcanic effusive events and, in the case of eruption, extraction of parameters that can help tracking the lava flow [59]. Even if the volcanic features of interest are usually much smaller than the nominal pixel size of the satellite image, moderate spatial resolution sensors (~1 km) can detect emitted radiance in the mid-infrared (MIR) wavelengths, a spectral region in which high temperature events, such as active lava flows, vents, and domes, emit copious amounts of energy. Satellite infrared data represented a useful means to describe the evolution of the eruptive event occurred during 28 March-1 April at Stromboli volcano. In particular, we used the HOTSAT system [60,61, 62] to detect the presence of thermal anomalies through the analysis of multispectral infrared images acquired by a variety of satellite sensors with a revisit time of about 12 h per satellite, such as VIIRS (Visible Infrared Imaging Radiometer Suite), providing at-nadir pixel footprint of 375 m for I-bands, SLSTR (Sea and Land Surface Temperature Radiometer) on board of SENTINEL-3, and MODIS (Moderate Resolution Imaging Spectroradiometer), both providing 1 km pixels at-nadir. The combined use of sensors that differed for spatial characteristics (from 375 m to 1 km) and different acquisition times has proved to be a robust and reliable instrument for the thermal monitoring of active volcanoes [63, 64, 65]. The HOTSAT system locates the thermal anomalies (hotspot), computes the associated radiant heat flux summing up the contribute of each hotspot pixel, and, in the case of effusive eruption, provides the Time Averaged Discharge Rate (TADR) as proportional to the radiant heat flux [66]. The conversion from radiant heat flux to TADR was performed according to [PERSON] et al. [67] using: TADR = Q/(\(\rho\) (c\({}_{\rm{p}}\)\(\Delta\)T + c\({}_{\rm{L}}\)\(\Delta\)\(\Phi\))), where Q is the total thermal flux obtained summing up the radiative power computed for each hotspot pixel, \(\rho\) is the lava density (2600 kg m\({}^{-3}\)), c\({}_{\rm{p}}\) is the specific heat capacity (1150 J kg\({}^{-1}\) K\({}^{-1}\)), \(\Delta\)T is the eruption temperature minus temperature at which flow stops (100-200 K), c\({}_{\rm{L}}\) is the latent heat of crystallization (3.5 \(\times\) 10\({}^{5}\) J kg\({}^{-1}\), and \(\Delta\)\(\Phi\) is the volume percent of crystals that form while cooling through \(\Delta\)T (30-54%). The HOTSAT system was extended with a new module to process data acquired by Landsat 8 OLI and TIRS, Sentinel 2 MSI, and ASTER images in order to exploit higher spatial resolution multispectral images. Besides providing further information on the radiant heat flux, these data can be used to locate eruptive vents and describe the evolution of the lava flow field [68; 69; 70], based on the spatial and spectral resolution of the available bands and the phase and size of the eruption they catch. For example, in the case of an ongoing eruption, Sentinel-2 MSI, thanks to its bands in the SWIR (bands 11 and 12), can provide the position of an active vent and flow at the spatial resolution of 20 m, whereas Landsat-8 OLI, with its SWIR bands 6 and 7, can provide the same information at 30 m of spatial resolution. If a crusted lava flow is cooling, SWIR bands might not be able to detect it; on the other hand, it could still be visible in the thermal infrared TIR bands, i.e., Landsat 8 TIRS (bands 10 and 11) or ASTER (bands 10-14) at 100 and 90 m of spatial resolution, respectively. Due to the limited temporal resolution of these higher spatial resolution multispectral images, post-eruptive images occur more often than intra-eruptive ones and, in many cases, the flows cool too fast to be visible, even in the TIR bands.