Abstract
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 Nino 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.
Highlights
A sprite, which is a brief flash occurring above the cloudtop of thunderstorms, and is one type of transient luminous events (TLEs), was first accidently discovered in 1989 (Franz et al 1990)
Imager of Sprites and Upper Atmospheric Lightnings (ISUAL) consists of three sensors including an intensified CCD imager that captured the transient appearances of TLE or lightning flashes, a spectrophotometer (SP hereafter) that recorded light variations within the field of view at 6 bands simultaneously, and an array photometer (AP hereafter) that obtained the light curve at different vertical heights in blue and red channels
The stable ISUAL trigger and lightning events suggest that the bias due to instrument degradation and the gain adjustment has been minimized empirically and successfully. This result implies that no additional correction for the variation of detection capability is needed for the statistical analyses of the TLE global occurrence rate and distribution reported by Chen et al (2008), Lee et al (2010), as well as the studies of the ISUAL recorded lightning (Chen et al 2007; Wu et al 2010)
Summary
A sprite, which is a brief flash occurring above the cloudtop of thunderstorms, and is one type of transient luminous events (TLEs), was first accidently discovered in 1989 (Franz et al 1990). To capture the dim transient light emitted from TLEs, the ISUAL sensors were equipped with several electron multiplying devices such as micro-channel plates and photomultiplier tubes. These devices are driven by high voltage power supplies to accelerate and to multiply the collected electrons; inevitably their sensitivities will degrade with use. Compared with conventional semiconductor detectors used in most space missions, the degradation of the electron multiplying devices driven by high voltage power supplies is expected to be significant. Over the past several years, special calibration observations were conducted periodically to monitor the sensitivity changes.
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