A Long-term Coronal Dimming in High-Temperature Spectral Lines During an M-class Confined Flare

Coronal dimmings are observed as sudden and localized reductions in EUV and X-ray emission of the solar corona. Traditionally, significant dimmings at 1-2MK are regarded as robust indicators of coronal mass ejections (CMEs), reflecting the density depletion caused by plasma escaping into interplanetary space. Here we present a peculiar high-temperature dimming observed on 2012 July 5. Using Sun-as-a-star observations from SDO/EVE and GOES, we identified significant intensity drop in the Fe XVIII (6.5 MK) and Fe XX (9.3 MK) hot lines, with a maximum depth of over 20% observed in the GOES soft X-ray (SXR) flux. Spatially resolved analysis from SDO/AIA reveals that this signature originated from a failed eruption where the bulk of the plasma was constrained by the overlying magnetic loop system. This case demonstrates that deep coronal dimmings in hot lines can occur without actual mass loss, providing a critical caveat for the interpretation of stellar coronal dimmings used to find stellar CMEs.