Thermal remote sensing for mapping the sub-Arctic permafrost and refining its southern limits

•A MODIS look-up-table converts daily thermal imagery to ground surface temperature.•An MAGT-based probability index is built to estimate permafrost continuity.•The southern limits of sub-Arctic permafrost are extracted at a km-scale.•Carbon release in the southern limits of permafrost deserve further investigation. Permafrost soils store more than one fourth of global soil organic carbon. This important carbon pool is threatened by carbon release from permafrost thawing. Especially in sub-Arctic transition zones, accurate mapping of permafrost is crucial for timely documentation of permafrost retreat and the consequent carbon uptake in its fragile ecosystems. Utilizing the 8-day Aqua/MODIS land surface temperature (LST) products in 2003–2020, this study extracts the southern limits of a sub-Arctic boreal mountain permafrost in the Amur River Basin bordering China, Russa, and Mongolia. A temporally dynamic MODIS Look-up-table is built to scale the MODIS LST to daily ground surface temperature. Using a thermal equilibrium model, the mean annual ground temperature (MAGT) on top of permafrost ground is estimated. The 18-year average MAGT = 0 °C defines the southern limits of the permafrost. Integrating the spatial distributions and temporal frequencies of annual MAGT