Isual Publications 2014

Isual Publications 2014

  • id
  • date time
    2014-04-08 14:38:00
  • 2009
  • 8
  • 張淑鈞(2009),‘福衛二號「高空大氣影像儀」酬載所記錄的遠紫外光訊號之分析’,國立成功大學物理研究所(碩士論文)
  • 福爾摩沙衛星二號的科學酬載-高空大氣閃電影像儀(The Imager of Sprites and Upper Atmospheric Lightning, ISUAL)進行高空放電現象的觀測已經超過五年,從影像、光譜以及空間上隨時間的變化等資訊已成功分辨出紅色精靈、淘氣精靈、精靈暈盤、巨大噴流、藍色噴流等事件,但是仍有一部份是目前沒有辦法定義及分類的事件。 
      根據前人的研究以及分析ISUAL資料的結果,已經確定紅色精靈、淘氣精靈和巨大噴流這三種高空放電現象具有遠紫外光訊號,不過有很多是影像中只有閃電,但是光譜儀卻記錄到遠紫外光訊號幾乎與閃電同時發生,在本文稱為「待解的事件」。我們推測這些待解的事件可能為影像中的閃電或是因為亮度較暗而無法在影像中成像的高空短暫發光現象所產生的。不過藉由模擬閃電光譜強度及考慮大氣吸收影像後,排除了閃電的可能性。而高空短暫發光現象中,只有淘氣精靈的遠紫外光訊號幾乎與閃電訊號同時,比較淘氣精靈的影像亮度以及遠紫外光強度後,發現兩者間具有高度線性相關性,且待解事件的遠紫外光強度都較小,因此認為待解事件中的遠紫外光訊號來自影像中隱藏的淘氣精靈。 
      另外,利用遠紫外光訊號統計可能的淘氣精靈數量增加倍率,發現實際觀測增加倍率與儀器偵測極限相符,因此實際上有淘氣精靈發生卻被ISUAL遺漏的數量,可藉由考慮儀器偵測極限或經由修正淘氣精靈的逸失比例來加以補救。

    Since FORMOSAT-2 launched in 2004, the scientific payload - ISUAL has recorded a large number of transient luminous events (TLEs). Based on the imager, the spectrophotometer (SP), and the array photometer (AP) data, we successfully categorized the recorded events into sprite, elves, halo, gigantic jet, and blue jet. But there are some events that their origins are not properly understood and therefore cannot be classified. 
    From previous studies of the ISUAL data analysis, it was found that sprite, elves and gigantic jet all have FUV (Far-Ultraviolet) emissions. However some events that were classified as lightning from the imager data were found to be accompanied by the SP FUV signals; we call these the “undefined events”. We conjectured that the FUV emission from these undefined events is either from lightning or from dim unseen TLEs. To resolve the source of FUV emission for these undefined events, we simulate the lightning optical spectrum and consider atmospheric absorption; it is found that FUV emission from the lightning channel is completely absorbed by the atmosphere and cannot possibly detect by the ISUAL sensors. The brightness of elves in imager and the FUV emission in SP is surprisingly found to be linearly proportional to each other. Furthermore, the SP FUV emission intensity of undefined events were found to be always lower than that from the elve events. Therefore, we conclude that the FUV emission of the undefined events is from the “dim elves” that having brightness below the detect level of the ISUAL imager. 
    Beside the SP FUV signal can be used as an elve indicator, we also found that the correction factor for the unseen elves is similar to the estimation based on the instrument detection limit. Hence, by considering the detection limit or by factoring in the under-counting factors, we can correctly find the elve occurrence rate from the ISUAL observed rate.

Sunday the 21st. ISUAL. All rights reserved.