Sensitivity Degradation of ISUAL Instruments and Its Impact on Observations

摘要: In the past 6 years, ISUAL payload onboard Taiwanese FORMOSAT-2 satellite has successfully carried out primary mission of performing a global survey transient luminous events (TLE) and lightning activities. The observation data have been used to construct first distribution map TLEs infer their occurrence rates. To register dim TLE emissions at night, employs electron-multiplying devices like photomultiplier, multi-anode array, microchannel plate intensify photons emitted by events. These devices, however, will degrade gradually with age. degradation rate performance change were monitored through periodic calibration observations are investigated carefully in order achieve precise photometry accurate statistics. annual rates up 14% identified for some sensor elements. This paper presents variations instrument efficiencies overall detection capabilities, before after applying gain-adjustment compensate degradation. resulting stable trend trigger suggests that bias from gain adjustments is minimal. point also supported near constant sprite halo deduced different statistical temporal intervals. Hence, anomalies elve 2008 ~ 2009 likely due El Nino Southern Oscillation (ENSO) Forecast based on indicates sensors sufficient sensitivity margins operate efficiently until end 2013, or even beyond.