Testing the accuracy of solar-reflected radiation corrections applied during satellite shortwave infrared thermal analysis of active volcanoes

Shortwave infrared (SWIR) satellite imagery is frequently used to study the rmal emission from active volcanoes. When using daytime SWIR observations i t is necessary to first isolate the thermally emitted signal component, pri ncipally by subtracting contributions due to solar-reflected radiation. Two differing approaches have been used: (1) A "mean" approach based on the av erage SWIR reflectance of the nonthermally anomalous volcanic background, a nd (2) a "per-pixel" approach which estimates the SWIR reflectance of each pixel using a measure of their near infrared reflectance. We assess the acc uracy of these using near-coincident daytime and nighttime Landsat Thematic Mapper (TM) imagery of the active lava dome of Unzen Volcano. Using the ni ghttime data as a reference we find the most commonly applied mean daytime correction approach to be seriously inadequate, returning results for therm ally radiant area 81-94% smaller than the nighttime values. The per-pixel c orrection method appears somewhat more effective but still provides values 47-61% smaller than the reference data set. Daytime underestimation of tota l SWIR thermal spectral radiance is less severe than that of total thermall y radiant area because it is primarily pixels having low thermal signals th at pose a problem for the daytime approaches. Total thermal spectral radian ce is underestimated by 52-79% using the daytime data with the mean correct ion approach, and by 28-59% with the per-pixel approach. When applying the TM "dual-band" technique to determine subpixel hot spot temperatures from t he SWIR thermal signals, we find that surfaces cooler than 400 degreesC are poorly represented in the daytime retrievals. However, when using the per- pixel daytime data correction method the distribution of retrieved hot spot s >400 degreesC corresponds quite well to the pattern derived via nighttime imagery. Due to the significant parameter underestimation found to be inhe rent when using daytime data, this study further indicates the value of nig httime observations to volcanic SWIR thermal studies. If daytime data are u sed, then we recommend use of the per-pixel solar-reflected radiation corre ction method but would advise caution when quantitatively interpreting the derived thermal parameters, at least at andesitic and dacitic volcanoes. It is possible that daytime retrievals may be more robust at basaltic volcano es due to their characteristic higher magmatic temperatures and lower surfa ce reflectances.