Estimating Sub‐Pixel Temperatures During Volcanic Eruptions Using Dual‐Band Thermal Imaging Technique: Case Studies From Whakaari/White Island (New Zealand) and Tofua (Tonga)

This study presents the first application of a dual‐band satellite thermal monitoring technique in New Zealand and Tonga, using the 2019 eruption of Whakaari/White Island and the persistently active Lofia crater on Tofua volcano as case studies. At Whakaari/White Island, near‐synchronous Landsat‐8 and Sentinel‐2A data (16 min interval) were analysed using the Thermal Eruption Index (TEI) to constrain analyses to hotspot pixels with the strongest thermal signatures. Single‐band SWIR brightness temperatures (BT) produced peak values of 237–262 °C (Landsat‐8) and 244–287 °C (Sentinel‐2A). The dual‐band method significantly improved these estimates, producing temperatures of 418.7–477 °C (Landsat‐8 SWIR + TIRS) and 432.3 –540.8 °C (Sentinel‐2A SWIR + Landsat‐8 TIRS). Notably, Sentinel‐2A dual‐band results align more closely with airborne FLIR measurements collected 9 days earlier, which recorded maximum vent temperatures in the range of 410.2–589.2 °C. This improved agreement reflects the importance of finer 20 m SWIR spatial resolution of Sentinel‐2 in better resolving small, high‐temperature heat sources compared to Landsat‐8's 30 m resolution. At Tofua volcano (Tonga), three Landsat‐8 scenes spanning contrasting levels of thermal activity were analysed. For the non‐saturated May 2020 acquisitions, the maximum dual‐band temperature increased from 968.8 °C on 11 May 2020 to 1102.7 °C on 27 May 2020 following an eruptive episode, demonstrating the method's sensitivity to temporal changes in eruptive intensity. The September 2021 scene was characterised by SWIR‐band saturation within the central lava‐lake pixel, preventing direct retrieval of its brightness temperature; however, dual‐band estimates for the adjacent pixel reached up to 1187.3 °C, consistent with upper‐range basaltic lava lake temperatures. Overall, the dual‐band approach enhances quantification of sub‐pixel volcanic heat sources and demonstrates strong potential for operational thermal monitoring.