Sensitivity degradation of ISUAL instruments and its impact on observations

Alfred Bing Chih Chen, Yen Jung Wu, Chih Yu Chiang, Yin Chieh Huang, Cheng Ling Kuo, Han Tzong Su, Rue Ron Hsu, Stephen B. Mende, Harald U. Frey, Stewart E. Harris, Yukihiro Takahashi, Lou Chuan Lee

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


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

Original languageEnglish
Pages (from-to)71-83
Number of pages13
JournalTerrestrial, Atmospheric and Oceanic Sciences
Issue number1
StatePublished - Feb 2012


  • Atmospheric electricity
  • Imager of sprites and upper atmospheric lightnings (isual)
  • Lightning
  • Space instrument
  • Transient luminous events (tles)


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