巨大噴流的物理特性探討與理論模型的建立

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行政院國家科學委員會補助專題研究計畫

■成果報告

□期中進度報告

巨大噴流的物理特性探討與理論模型的建立

計畫類別： 個別型計畫 □ 整合型計畫

計畫編號：NSC 96－2112－M－006－003－MY3

執行期間： 96 年 8 月 1 日至 99 年 7 月 31 日

計畫主持人：許瑞榮

共同主持人：蘇漢宗

計畫參與人員：郭政靈，陳炳志，蔡禮聿，周容光，李立柔，張淑鈞，

吳宜軒，吳彥蓉│李宜蓁，楊國良。

成果報告類型(依經費核定清單規定繳交)：□精簡報告 ■完整報告

本成果報告包括以下應繳交之附件：

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、

列管計畫及下列情形者外，得立即公開查詢

□涉及專利或其他智慧財產權，□一年□二年後可公開查詢

執行單位：成功大學物理系

中華民國 100 年 2 月 15 日

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巨大噴流的物理特性探討與理論模型的建立-期中報告

NSC 96－2112－M－006－003－MY3

許瑞榮，蘇漢宗，郭政靈，陳炳志，蔡禮聿，周容光，李立柔，張淑鈞成功大學物理系 中文摘要： 在此計畫執行期間，我們完成了四項規劃目標如下：  改進現有的多波段觀測系統，以獲得巨大噴流的多波段影像，與衛星觀測結果比較。  使用高速攝影機觀測巨大噴流，以期能獲得巨大噴流的高速動態演歷，與衛星觀測結果比較。

 分析地面觀測資料與ISUAL 資料(包括Elves, sprite, gigantic jet, blue jet)與閃電及其他放電現象的關係。

 探索巨大噴流的物理機制，進而以數值方法模擬巨大噴流的噴發現象。 關鍵詞：

英文摘要：

In this project, four itemized goals had accomplished:

 Improve the existing multi-band observation system to obtain better ground images and facilitate the comparisons with the ISUAL images.

 Use the existing high speed intensified camera to investigate the spatial-temporal evolution of GJs and facilitate comparisons with those obtained by the ISUAL array-photometers.  Analyze ground and ISUAL data of TLEs to investigate their correlations with lightning and

other thundercloud-related discharges.

 Investigate the possible generating mechanisms of GJs through numerical modeling. Keywords: gigantic jets, lightning, multi-band observation system, intensified spectrograph, ISUAL data

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一、前言

過去十幾年來，觀察所謂的高空短暫發光現象（Transient Luminous Events, TLEs）(圖 1)已成為大氣物理、放電物理的重要課題。一系列 TLEs 被確認出來的有紅色精靈（red

sprite）、藍色噴流（blue jet）、淘氣精靈（elves）以及巨大噴流（gigantic jet），造成這些現

象的物理及化學機制，是一個有趣的研究主題。

圖 1. 雷雨雲與電離層之間的短暫發光現象已發現的有紅色精靈、淘氣精靈、藍色噴流、巨大噴流; adapted from Pasko (2003) & Lyons et al (2003)。

第一次在文獻上發表的紅色精靈觀測紀錄，是利用低光度攝影機拍攝到雷雨雲上有很亮的柱狀發光體。但是早在西元 1925 年諾貝爾獎得主 Wilson 就認為在雷雨雲上有可能發生這樣的現象。為什麼直到十幾年前才被發現呢？主要的原因是由於科技的發達，影像增強器及快速攝影機的問世，讓這些現象能夠以科學的儀器來完成記錄。以下觀測是高空向上閃電短暫發光現象的重要里程碑：  1989 年，紀錄到第一個紅色精靈的證據  1994 年，發現藍色噴流  1995 年，紀錄到紅色精靈的光譜（氮氣的第一正則譜線的確認）  1995 年，發現淘氣精靈  1998 年，台灣的太空計畫室(今稱為太空中心)選定成功大學物理系所提出「高空大氣向上閃電影像儀」(ISUAL)為「中華衛星二號」(今稱為福爾摩沙衛星二號)之科學酬載。

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 1999 年，ISUAL 團隊赴美學習紅色精靈的地面觀測。

 在 2001 年，Pasko 的研究群，也在波多黎各附近觀測到大型的藍色噴流。[Pasko, et. al . 2002]  2002 年，ISUAL 團隊發現巨大噴流  2003 年 1 月，哥倫比亞號太空梭從事 16 天的科學任務，其中有約 8 小時，由以色列太空人操作低光度攝影機，從太空中觀測高空短暫發光現象。不幸的是，哥倫比亞號太空梭於回航時爆炸，除了犧牲七位太空人之外，也遺失了部份的觀測資料。  2004 年 5 月 21 日，福爾摩沙衛星二號發射成功，成為世界上第一顆用來長期觀測高空短暫發光現象的衛星，ISUAL 科學酬載也成為第一個以觀測高空短暫發光現象為主要任務的科學酬載。並在 2004 年 7 月 4 日首次拍到紅色精靈。

 美國的學者在 2005 年已經使用了超高速低光度攝影機(Vision Research Phantom 7.1)，其時間解析度為 10,000frames/s。這些高速的動態影像，讓科學家可以了解紅色精靈中 streamer 發展情形以及在 streamer 前的 leader 傳播的情形

 美國猶他州立大學的團隊在 2005 年針對 TLEs 做一次多波段的觀測，其觀測波長範圍從 0.35-1.5micro，並首次觀測到紅色精靈的紅外線影像。所得的影像可與一般的攝影機所得資料相比，發現只有在 60 公里的高空以上，才有紅外線輻射，而且在 ELVEs 與 sprite halo 並無紅外線輻射。

 2005 年 3 月，福衛二號的 ISUAL 科學酬載，以首度從太空中辨認出，巨大噴流與藍色噴流。

 2008 年成大 ISUAL 團隊完成 TLE 的全球分佈

 2009 年成大 ISUAL 團隊完成巨大噴流衛星資料的分析。

 2010 年成大 ISUAL 團隊發現巨大噴流具有 Positive Streamer 與 Negative Streamer 二種類型。目前，除了美國有六個研究群與日本一個研究群，以及台灣 ISUAL 團隊，已在紅色精靈相關領域的研究做出相當的貢獻之外，全世界尚有德國、以色列、丹麥等國的研究團體正積極地投入相關的研究。近兩年來，巴西在紅色精靈的觀測上投入相當多的經費與人力，其中包括巴西的閃電偵測系統(BIT)，以及利用飛機載著儀器與人員從事 TLEs 觀測。由於巴西也是多雷雨的國家，有利於他們在 TLEs 的觀測，再加上最近巴西的國力漸強，可以預期他們將會是重要的競爭對手。另外，世界各國都已紛紛的發展自己的太空觀測計畫以及閃電偵測系統，對未來 TLE 與閃電的了解，將會有更大的進展。例如：  日本開始展開以高空氣球來觀測 TLE 的計畫，另外也有用微衛星來觀測的計畫。  丹麥也與 ESA 合作展開 ASIM 計畫，將在太空站(ISS)裝設儀器，觀測 TLE，重力波，

極光等現象。

 法國也有一個微衛星計畫 TARANIS，將用 ELFVLF 等儀器從太空觀測 TLE 及 TGF。  德國在歐洲中部建立一套類似美國 NLDN 的閃電偵測系統 LINET，但是他們比 NLDN

多一些功能，其中最明顯的是，可偵測到美國 NLDN 無法偵測到的雲間閃電(IC)的特性。

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他將可以驗證 Wilson 的假設，補足大域電流在雷雨雲上空的這一段未知的區域。「高空大氣閃電影像儀」(Imager of Sprites/ Upper Atmospheric Lightning, ISUAL)是世界上第一個可以長期從太空中，觀測各種高空短暫發光現象的衛星科學酬載[圖 2]。ISUAL 由

三個主要的偵測儀器所組成：(1)低光度「影像儀」，(2)六通道「光譜光度儀」，和(3)雙波段

「陣列光度儀」。這些儀器原始規畫的主要科學目的，是用來探測紅色精靈(sprite)、淘氣精

靈(elve)、精靈暈盤(halo)、藍色噴流(blue jet)與巨大噴流(gigantic jet)等高空短暫發光現象的全球分布情形；並提供高時間解析度的光譜光度以及具有空間解析度的雙波段光度資料，用以探討各種高空短暫發光現象的物理與化學特性。另外，具有六個不同波段濾鏡轉輪的影像儀，還可以用來從事大氣輝光與極光的研究。圖 2. 福衛二號科學酬載「高空大氣閃電影像儀 (ISUAL)」的三個主要儀器與衛星姿態及 ISUAL 酬載觀測區域之示意圖。本計畫是配合 ISUAL 計畫的執行，提供參與科學研究的研究生以及參與科學活動的經費，以期能夠協助 ISUAL 計畫順利的執行，並善用福衛二號科學酬載 ISUAL 的珍貴資料，培育國內相關的科學人才。

二、研究目的

本計畫的研究主題是巨大噴流的研究以及巨大噴流與閃電的關係。主要探索的科學目標包括有： (1) 改進現有的多波段觀測系統，以獲得巨大噴流的多波段影像，與衛星觀測結果比較。 (2) 以現有低光度光譜儀觀測巨大噴流，以期能獲得巨大噴流的光譜特性，與衛星觀測結果

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6 比較。 (3) 繼續改進現有的穿透式光譜儀成為反射式光譜儀，以期獲得更多藍光波段的光譜特徵，與衛星觀測結果比較。 (4) 使用高速攝影機觀測巨大噴流，以期能獲得巨大噴流的高速動態演歷，與衛星觀測結果比較。

(5) 分析地面觀測資料與ISUAL 資料(包括Elves, sprite, gigantic jet, blue jet)與閃電及其他放電現象的關係。

(6) 探索巨大噴流的物理機制，進而以數值方法模擬巨大噴流的噴發現象。 (7) 探索太陽系其他行星大氣的閃電與產生TLE 的可能性。

三、結果與討論

在本計劃三年的執行期間內，共發表論文 16 篇，另有 1 篇正在審查中：

[1]. Zhenggang Cheng, Steven A. Cummer, Han-Tzong Su and Rue-Ron Hsu, " Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses, " J. Geophys. Res. 112, A06318, doi:10.1029/2006JA011840 (2007). (8 pages, IF=2.953, Rank:12/137, Cited:8)

[2]. Cheng-Ling Kuo, A.B. Chen, Y.J. Lee, R.R. Hsu, H.T. Su, S.A. Cummer, L.C. Lee, H. Frey, S.B. Mende, Y. Takahashi, H. Fukunishi, " Modeling elves observed by FORMOSAT-2 satellite, " J. Geophys. Res. 112, A11312, doi:10.1029/2007JA012407 (2007). (18 pages, IF=2.953, Rank:12/137 (GEOSCIENCES, MULTIDISCIPLINARY), Cited:11)

[3]. H. U. Frey, S. B. Mende, S. A. Cummer, J. Li, T. Adachi, H. Fukunishi, Y. Takahashi, A. B. Chen, R.-R. Hsu, H.-T. Su, Y.-S. Chang, " Halos generated by negative cloud-to-ground lightning, " Geophys. Res. Lett. 34, L18801, doi:10.1029/2007GL030908 (2007). (5 pages, IF=2.744, Rank:14/137, Cited:10)

[4]. Enell, C.-F.; Arnone, E.; Adachi, T.; Chanrion, O.; Verronen, P. T.; Seppälä, A.; Neubert, T.; Ulich, T.; Turunen, E.; Takahashi, Y.; Hsu, R.-R.; " Parameterisation of the chemical effect of sprites in the middle atmosphere, " Ann. Geophy. 26, Issue 1, 2008, pp.13-27 (2008). (15 pages, IF=1.660, Rank:50/144(GEOSCIENCES, MULTIDISCIPLINARY), Cited:1)

[5]. Chen, Alfred B.; Kuo, Cheng-Ling; Lee, Yi-Jen; Su, Han-Tzong; Hsu, Rue-Ron; Chern, Jyh-Long; Frey, Harald U.; Mende, Stephen B.; Takahashi, Yukihiro; Fukunishi, Hiroshi; Chang, Yeou-Shin; Liu, Tie-Yue; Lee, Lou-Chuang, " Global distributions and occurrence rates of transient luminous events, " J. Geophys. Res. 113, A08306, doi: 10.1029/2008JA013101, (2008). (8 pages, IF=3.147,

Rank:11/144(GEOSCIENCES, MULTIDISCIPLINARY), Cited:18)

[6]. Kuo C L, Chen A B, Chou R K, Tsai L Y, Hsu R R, Su H T, Frey H U, Mende, S B, Takahashi Y, Lee L C, " Radiative emission and energy deposition in transient luminous events, " J.

Phys. D 41, 234014 (2008). (15 pages, IF=2.104, Rank:26/95(PHYSICS, APPLIED),

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[7]. Adachi, T., Hiraki Y., Yamamoto K., Takahashi Y., Fukunishi H., Hsu, R-R, Su, H-T, Chen, A. B., Mende, S. B., Frey, H.U., and Lee, L.C., " Electric fields and electron energies in sprites and temporal evolutions of lightning charge moment, " J. Phys. D 41, 234010 (2008). (12 pages, IF=2.104, Rank:26/95, Cited:11)

[8]. Kuo, Cheng-Ling, J. K. Chou, L. Y. Tsai, A. B. Chen, H. T. Su, R. R. Hsu*, L. C. Lee, S. A. Cummer, H. U. Frey, S. B. Mende, and Y. Takahashi, (2009), " Discharge processes, electric fields, and electron energy in ISUAL-recorded gigantic jets, " J. Geophys Res.114, A04314, doi:10.1029/2008JA013791 (10 pages, IF=3.082, Rank:18/155(GEOSCIENCES, MULTIDISCIPLINARY), Cited: 7)

[9]. Ningyu Liu, Victor P. Pasko, Harald U. Frey, Stephen B. Mende, Han-Tzong Su, Alfred B. Chen, Rue-Ron Hsu, Lou-Chuang Lee, " Assessment of Sprite Initiating Electric Fields and Quenching Altitude of $a^1\Pi_g$ State of N$_2$ Using Sprite Streamer Modeling and ISUAL Spectrophotometric Measurements, " J. Geophys Res., (2009). (12 pages, SCI, IF=3.082, Rank:18/155, Cited: 5)

[10]. T.-Y. Huang, C. Y. Chiang, C. L. Kuo, A. B. Chen, H. T. Su, and R. R. Hsu, "Investigations of Lightning-Induced Sudden Brightening in the OH Airglow Layer Observed By ISUAL Onboard FORMOSAT-II Satellite," in Coupling of Thunderstorms and Lightning Discharges to Near-Earth Space, P21-27, edited by N.B. Crosby, T.Y. Huang, and M.J. Rycroft, American Institute of Physics (2009).(Book)

[11]. Rue-Ron Hsu, Alfred B. Chen, Cheng-Ling Kuo, Han-Tzong Su, Harald Frey, Stephen Mende, Yukihiro Takahashi and Lou-Chung Lee, "On the Global Occurrence and Impacts of Transient Luminous Events (TLEs)" in Coupling of Thunderstorms and Lightning Discharges to Near-Earth Space, P99-107, edited by N.B. Crosby, T.-Y. Huang, and M.J. Rycroft, American Institute of Physics (2009).(Book)

[12]. P. K. Rajesh, J. Y. Liu, C. Y. Chiang, A. B. Chen, W. S. Chen, H. T. Su, R. R. Hsu, C. H. Lin, M.-L. Hsu and J. B. Nee, "First results of the limb imaging of 630.0 nm airglow using FORMOSAT-2/Imager of Sprites and Upper Atmospheric Lightnings," J. Geophys Res., 114, A10302, doi:10.1029/2009JA014087. (2009) (11 pages, IF=3.082, Rank:18/155, Cited:1) [13]. Toru Adachi, Steven A. Cummer, Jingbo Li, Yukihiro Takahashi, Rue-Ron Hsu, Han-Tzong

Su, Alfred B. Chen, Stephen B. Mende, Harald U. Frey, "Estimating lightning current moment waveforms from satellite optical measurements," Geophys. Res. Lett., 36, L18808, doi:10.1029/2009GL039911. (2009) (5 pages, IF=3.204, Rank:15/155, Cited:1)

[14]. Chou, J. K., C.-L. Kuo, L. Y. Tsai, A. B. Chen, H.-T. Su, R.-R. Hsu*, S. A. Cummer, J. Li, H. U. Frey, S. B. Mende, Y. Takahashi, and L.-C. Lee, “Gigantic jets with negative and positive polarity streamers”, (2010) J. Geophys. Res.,115, A00E45, doi:10.1029/2009JA014831. (13 pages, IF=3.082, Rank:18/155, Cited: 0)

[15]. Lee, L.-J., A. B. Chen, S.-C. Chang, C.-L. Kuo, H.-T. Su, R.-R. Hsu, C.-C. Wu, P.-H. Lin, H. U. Frey, S. B. Mende, Y. Takahashi, and L.-C. Lee (2010),”The controlling synoptic-scale factors for the distribution of the transient luminous events”, J. Geophys. Res.,115, A00E54, doi:10.1029/2009JA014823, (11 pages, IF=3.082, Rank:18/155, Cited: 0)

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Y. Takahashi, and L.-C. Lee, (2010), “ISUAL Far-Ultraviolet Events, Elves, and the Lightning Current”, J. Geophys. Res.,115, doi:10.1029/2009JA014861. (8 pages, IF=3.082, Rank:18/155, Cited: 2)

[17]. Chou, J. K., L. Y. Tsai, C. L. Kuo, Y. J. Lee, A. B. Chen, H. T. Su, R. R. Hsu, and L. C. Lee, Radiative emissions and behaviors of the electric jets observed in Taiwan TLE ground campaign, J. Geophys. Res. Paper #: 2010JA016162 (revised)

以下將針對與巨大噴流有直接相關的結果分別說明如下：

A.巨大噴流的光譜光度分析與可能的物理機制

巨大噴流產生機制的研究方面。目前已利用全球首次獲得GJ的高時間解析度光譜光度資料 (圖3)，首次獲得GJ發生時，高空中的折合電場(reduce E field)隨高度變化的情形(圖4)。分析ISAUL資料中的五個巨大噴流事件，可以估計出在fully developed jet的階段高空中的折合電場為400–655 Td，平均電子能量為8.5–12.3 eV，雖然比紅色精靈略高，但仍與 streamer model相符。另外，從影像、光譜光度訊號、陣列光度訊號與美國Duke大學的ELF/VLF的訊號，團隊結合雲對地閃電的模型以及動態的電離層的想法，提出巨大噴流的可能物理機制。參考圖5。這是本計劃的一個重要成果，已發表在JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A04314, 2009。未來將試著以數值模擬來繼續研究此一課題。參考附件一。

圖3. 全球首次的高時間解析度GJ光譜光度資料。可發現GJ在FDJ階段，除了有向上發展的情形，還具有從50 公里的高空向下回擊的訊號。

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9 圖4. 從SP2/SP3的比例可知1NN2+/2PN2隨高度的變化，進而估計出高空中的折合電場與平均電子能量。圖 5. 巨大噴流的可能物理過程。巨大噴流基本上是一個從雲頂向上放電的現象，與雲對地的放電現象，有部分的類似，但也有些不同之處。類似之處是：都有前導，回擊與連續電流的過程。不同之處是：由於高空大氣愈往上氣體密度愈稀薄，向上放電的巨大噴流，愈往上就擴散地愈廣；而且在 FDJ 之後，高空大氣的的解離使得電離層下降，回擊的過程是從 50km 的高空開始；展現連續電流的雲內閃光與後續噴流的光學特徵是清晰可見。

B. 巨大噴流的分類 (Chuo, et. al. JGR, 2010)

從 2004 年 7 月至 2009 年 4 月期間，FORMASAT-II/ISUAL 記錄了 32 個巨大噴流事件 (Gigantic jets, GJ)，挑選由影像儀(Imager)的波段 653-754 nm 所紀錄且單純發生 GJ 的事件共 20 個，由其型態及光譜特性分成三種類別。第一種類型(type I)的巨大噴流(圖 6)發展過程類似文獻記載地面觀測到的事件，分別為前導噴流(leading jet, LJ)、發展完整噴流 (fully-developed jet, FDJ)以及後續噴流(trailing jet, TJ)階段。顯著的光譜特徵是在 FDJ 階段有類似閃電回擊的過程(return-stroke-like process)，且於 TJ 階段雷雨雲內會有持續發光。由超低頻天電訊號(ultra low frequency, ULF, 0.1 Hz to 500 Hz)辨識 type I GJs 的放電為負電荷往電離層放電(negative discharge from cloud to ionosphere, -CIs)。第二種類型(type II)的巨大噴流(圖 7)則是由藍色噴流(blue jets, BJs)觸發，約 100 毫秒內慢慢往上完成至電離層的通道，相較 type I GJs 慢，且其高度約 50 km 至 90 km 的流束區(streamer zone)的亮度較 type I GJs 約暗~3.4 倍。Type II GJs 無明顯的 ULF 訊號，但有一個可能的事件對應+CI。由於帶正電荷的流束傳播的所需背景電場較帶負電荷的流束小約 2-3 倍，所以正流束較容易發展且對應發光亮度較低。且從正常雷雨雲電荷分佈的模型，BJs 為正電荷由雲頂往上放電，我們推論 type II GJs 對應 +CIs，電荷來源是由正常雷雨雲頂正電荷層，也可以解釋其電荷數不

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10

足以維持 TJ 階段的連續電流。第三種類型(type III)的巨大噴流(圖 8)則是由閃電觸發後往上發展成 GJ，影像的平均流束區亮度值則介於前兩種類型之間。Type III GJs 受先前觸發閃電所造成雲內電荷不平衡的影響，其放電的極性可能為+CIs 或-CIs。此結果發表於科學期刊 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A00E45, doi:10.1029/2009JA014831. 參考附件二。

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11 圖 7. Type II 巨大噴流的影像與光譜光度訊號

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12 圖 8. Type III 巨大噴流的影像與光譜光度訊號

C.台灣地面TLE觀測的噴流事件分析

由 2007 年的地面高空短暫發光事件 (transient luminous events, TLEs) 的觀測計畫，在 7 月 22 日於鹿林天文台(120°52' 25" E, 23° 28' 07" N; 高度 2862 m)拍攝到多個噴

流事件(包含 37 個 blue jets/starters 和 1 個 gigantic jet)與紅色精靈(圖 9)，經由多波段系統，其包含無加裝濾鏡(有效波段 400~780 nm)、加裝紅光濾鏡(有效波段 570~775 nm)以及藍光濾鏡(有效波段 430~500 nm) 的鏡頭，所拍攝到的影像中發現這些噴流事件的紅光佔有一定比例，藍光部分幾乎被大氣散射掉了。這和 ISUAL 的 Imager filter #1 (653-754 nm)所記錄到的噴流事件特性一致。這次觀測紀錄到一組序列：首先發生 blue starter，在 100 毫秒後、同一雲頂位置發生藍色噴流，更在50 毫秒之後發展成巨大噴流，此為第一個由地面拍攝到的 type II GJs(圖 10)。這些噴流事件沒有對應到極低頻(extremely low frequency, ELF,

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13 1-100 Hz)的訊號，無法確認放電極性，但由先前分析 ISUAL 的 type II GJs 猜測這類型的放電為+CI，於是在正常雷雨雲的假設下，即正電荷層在負電荷層的上方，利用連續記錄的影像探討噴流事件與前後閃電活躍性的關聯性，由定義的光學回擊(optical stroke)的分布圖以及累積噴流事件前後的光學回擊圖(圖 11)，我們猜測噴流事件會由先前同一區域的對地閃電或者鄰近的強閃電改變雲內電荷分布而觸發，也可能會影響之後閃電的發生。此結果已送至科學期刊 JOURNAL OF GEOPHYSICAL RESEARCH 審查中。參考附件三。

圖9.紅色精靈(a-c)與藍色噴流(d-f)的多波段影像

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14 圖11.從光學回擊來探討噴流事件與閃電的相關性

D. 熱帶氣旋上空TLEs的觀測

2005 年海棠颱風登陸台灣之際，ISUAL 在台灣附近 70 秒鐘左右的時間裡觀測到 ( 2005/07/18 14:55:53.834 ~ 14:57:00.257UTC)，颱風外圍發生了 8 個藍色噴流事件(blue jet)。這讓我們開始注意到 blue jets 事件雖然在所有 TLEs 中不屬於閃電、淘氣精靈(elves) 這類每天都會出現的事件類型，但可能在特殊的天氣條件下有密集產生的特性。2008 年 7 月 27 日的 ISAUL 資料中，又再一次更確認了我們的想法。時值 ISUAL 在鳳凰颱風登陸台灣之前，ISUAL 記錄到在它的外圍雨帶上，有非常密集的 blue jets 噴發。在約 200 秒的衛星通過時間裡(2008/07/27 15:04:55.111-15:08:18.439)，觀測到 107 個 blue jets，平均發生頻率高達每 2 秒鐘一個。相較於之前在鹿林山做地面觀測時，看到快速發生到令人目不暇給

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的藍色噴流事件，頻率也小於每 10 秒鐘一個。目前絕大多數文獻上的 blue jets 接是在地面觀測的結果，尚未有如 ISUAL 觀測到如此高的發生率。

於是，我們對 ISUAL 資料中較完整的部分(2005/1/1-2008/12/31 共 4 年的觀測時間)，所記錄到的 blue jets 事件進行了統計。在 1,264 個有清楚影像可提供確定發生位置的 blue jets 當中，共有 1,009 個事件是成群發生的，比例高達 79.83%，而在這接近八成的事件中，有 685 個事件是在衛星掃過同一區域(以 3 分鐘衛星通過時間，略等於 4.5 度的經緯度，即跨距 500 公里以內的中尺度氣象系統比對標準)中發生 5 個以上，意即平均發生率高於 40 秒鐘一個。在所有的 blue jets 事件當中，超過 50%的事件都是這樣密集地發生，但不論是在在一般地面資料，或是在 ISUAL 的衛星資料中，blue jets 都不是最常被觀測到的 TLE 類型，所以其中的機制相當值得探討。

與聯合颱風警報中心（Joint Typhoon Warning Center, JTWC）的 4 年路徑資料比對，得到的結果是幾乎最密集發生 blue jets 事件的氣象系統都是熱帶氣旋，甚至在雙颱出現的比例也很高。其他的部分，則是屬於本來就是全球閃電最集中的東南亞以及非洲一帶，有很高的雷雨雲活動頻率。參考圖 12。

圖 12：ISUAL blue jets 密集發生活動與熱帶氣旋、特殊區域的關聯性

另外，我們也同時提出熱帶氣旋的水平風切可以將一般為dipole的雷雨雲結構，吹成 bi-pole的結構，使得雲頂電場變成較大電場，有利於BJ 形成，參考圖13。相關的結果，正在撰寫論文中。

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16 圖13.熱帶氣旋上空較易產生BJ的可能模型。熱帶氣旋的水平風切可以將一般為dipole的雷雨雲結構，吹成 monopole的結構，使得雲頂電場變成較大的電場，有利於BJ 形成。 E. 巨大噴流的高速動態演歷 在計畫執行中，地面觀測以高速攝影為主，希望能觀測到GJ的高速動態變化，然而在計畫的執行期內2007/08/01-2010/07/31的努力，雖然獲得一些紅色精靈的高速動態演歷情形，參考圖14，但仍未觀測到GJ。直到，2010年8月，才獲得一些巨大噴流的高速影像。然而，由於時間解析度並不夠高，無法分辨出巨大噴流的回擊過程。圖14. 以高速攝影機所拍攝的紅色精靈影像。每一個畫面的曝光時間為4ms。

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17 四、自評 以下是執行此計畫的自評：計畫目標執行情形 (1) 改進現有的多波段觀測系統，以獲得巨大噴流的多波段影像，與衛星觀測結果比較。執行完成，結果優良。論文投稿中。 (2) 以現有低光度光譜儀觀測巨大噴流，以期能獲得巨大噴流的光譜特性，與衛星觀測結果比較。已進行多次觀測，但光譜儀效果不好，故暫緩執行。 (3) 繼續改進現有的穿透式光譜儀成為反射式光譜儀，以期獲得更多藍光波段的光譜特徵，與衛星觀測結果比較。由於研究生蔡禮聿與周容光都專心投入颱風上空TLEs與ISUAL的GJ觀測資料的分析，因此光譜儀的改進計畫就暫緩執行。 (4) 使用高速攝影機觀測巨大噴流，以期能獲得巨大噴流的高速動態演歷，與衛星觀測結果比較。已有部分的結果，資料分析中。 (5) 分析地面觀測資料與ISUAL 資料(包括 Elves, sprite, gigantic jet, blue jet)與閃電及其他放電現象的關係。已完成，成效優良。已有論文已發表，上有其他成果正在撰寫論文中。 (6) 探索巨大噴流的物理機制，進而以數值方法模擬巨大噴流的噴發現象。已提出巨大噴流的可能模型。論文已發表。 (7) 探索太陽系其他行星大氣的閃電與產生 TLE 的可能性。經由成大天文台40公分的望遠鏡三個月的測試觀測，目前仍無所獲；也就是從成大天文台觀測金星上空是否存在TLE的研究，成效不佳。

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18

國科會補助專題研究計畫成果報告自評表

請就研究內容與原計畫相符程度、達成預期目標情況、研究成果之學術或應

用價值（簡要敘述成果所代表之意義、價值、影響或進一步發展之可能性）、

是否適合在學術期刊發表或申請專利、主要發現或其他有關價值等，作一綜

合評估。

1. 請就研究內容與原計畫相符程度、達成預期目標情況作一綜合評估

■ 達成目標

□ 未達成目標（請說明，以 100 字為限）

□ 實驗失敗

□ 因故實驗中斷

□ 其他原因

說明：

2. 研究成果在學術期刊發表或申請專利等情形：

論文：■已發表 □未發表之文稿 □撰寫中 □無

專利：□已獲得 □申請中 □無

技轉：□已技轉 □洽談中 □無

其他：

（以 100 字為限）

3. 請依學術成就、技術創新、社會影響等方面，評估研究成果之學術或應用

價值（簡要敘述成果所代表之意義、價值、影響或進一步發展之可能性）

（以 500 字為限）

由於福衛二號科學酬載 ISUAL 的唯一性以及台灣地理環境的特殊性，ISUAL 團隊所發表都是相當重要的成果，絕大部分都發表在該領域的重要期刊 JGR 或 GRL，甚受國際矚目。這些成果，也將有助於高空短暫發光現象此一領域的發展。

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Discharge processes, electric field, and electron energy in

ISUAL-recorded gigantic jets

Cheng-Ling Kuo,1,2 J. K. Chou,1 L. Y. Tsai,1A. B. Chen,1,3 H. T. Su,1,4 R. R. Hsu,1,3,4 S. A. Cummer,5H. U. Frey,6 S. B. Mende,6 Y. Takahashi,7 and L. C. Lee2

Received 1 October 2008; revised 23 January 2009; accepted 3 February 2009; published 23 April 2009.

[1] This article reports the first high time resolution measurements of gigantic jets from

the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment. The velocity of the upward propagating fully developed jet stage of the gigantic jets was 107m s1, which is similar to that observed for downward sprite streamers. Analysis of spectral ratios for the fully developed jet emissions gives a reduced E field of 400 –655 Td and average electron energy of 8.5 – 12.3 eV. These values are higher than those in the sprites but are similar to those predicted by streamer models, which implies the existence of streamer tips in fully developed jets. The gigantic jets studied here all contained two distinct photometric peaks. The first peak is from the fully developed jet, which steadily propagates from the cloud top (20 km) to the lower ionosphere at 90 km. We suggest that the second photometric peak, which occurs1 ms after the first peak, is from a current wave or potential wave –enhanced emissions that originate at an altitude of 50 km and extend toward the cloud top. We propose that the fully developed jet serves as an extension of the local ionosphere and produces a lowered ionosphere boundary. As the attachment processes remove the charges, the boundary of the local ionosphere moves up. The current in the channel persists and its contact point with the

ionosphere moves upward, which produces the upward surging trailing jets. Imager and photometer data indicate that the lightning activity associated with the gigantic jets likely is in-cloud, and thus the initiation of the gigantic jets is not directly associated with cloud-to-ground discharges.

Citation: Kuo, C.-L., et al. (2009), Discharge processes, electric field, and electron energy in ISUAL-recorded gigantic jets, J. Geophys. Res., 114, A04314, doi:10.1029/2008JA013791.

1. Introduction

[2] Gigantic jets (GJs), large-scale discharges from the

cloud top (16 – 18 km) to the lower ionosphere, have been reported by several ground campaigns [Pasko et al., 2002; Su et al., 2003; van der Velde et al., 2007] and a satellite experiment [Hsu et al., 2005; Su et al., 2005]. On the basis of the monochrome images with a time resolution of 16.7 ms, the temporal optical evolution of the GJs typically contains three stages: the leading jet, the fully developed jet (FDJ)

and the trailing jet (TJ) [Su et al., 2003]. The upward propagating leading jet may be the initial stage of the FDJ, playing a role similar to that of a stepped leader in conventional lightning. In the FDJ stage, the GJ optically links the cloud top and the lower ionosphere. The trailing jet has features similar to those of the blue jets (BJs) and propagates from the cloud tops up to60 km altitude. The optical emission of the trailing jet lasts for more than 0.3 s, and the overall duration of the GJs is 0.5 s. No related strong lightning activity has been associated with the reported GJs. In some cases ELF signals with a positive polarity have been observed with the GJs that imply that negative charge from the cloud top is transferred to the bottom of the ionosphere [Su et al., 2003]. In other cases, however, no clear ELF signature was observed [van der Velde et al., 2007].

[3] Another category of upward propagating discharges

from the cloud top is the blue jets [Wescott et al., 1995, 2001; Lyons et al., 2003], which has a lower terminal altitude compared to GJs. Petrov and Petrova [1999] was the first to propose that the blue jet develops from the leader streamer zone, and hence is filled with branching streamer channels. On the basis of the work of Petrov and Petrova [1999], Pasko and George [2002] performed a

three-JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A04314, doi:10.1029/2008JA013791, 2009

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for Full Article

1_{Department of Physics, National Cheng Kung University, Tainan,}

Taiwan.

2_{Institute of Space Science, National Central University, Jhongli,}

Taiwan.

3_{Institute of Space, Astrophysical and Plasma Sciences, National Cheng}

Kung University, Tainan, Taiwan.

4_{Earth Dynamic System Research Center, National Cheng Kung}

University, Tainan, Taiwan.

5

Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina, USA.

6

Space Sciences Laboratory, University of California, Berkeley, California, USA.

7

0148-0227/09/2008JA013791$09.00

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dimensional fractal modeling to simulate the morphology of blue jets and blue starters. Result from their numerical work agrees well with those for the observed jet phenomena [Wescott et al., 1995, 2001; Pasko et al., 2002]. Recently, Raizer et al. [2006, 2007] pointed out that the stem of a blue jet is a leader, which is a hot plasma channel with a temperature of 1500 – 5000 K [Raizer, 1991, p. 365; Bondiou and Gallimberti, 1994; Aleksandrov et al., 1997]. A leader is capped at its top side by the streamer zone [Raizer et al., 2006, 2007]. The streamers are relatively cold filaments of plasma, and the front of the streamers is a nonlinear ionization wave where air is ionized by the electron impact processes [Raizer, 1991, p. 327, and refer-ences therein]. The current in the BJs is supplied by the leader. The gas temperature in the leader channel can be as high as 5000 K from joule heating. At such a high temper-ature, the associative ionization (O + N! NO+ + e) and detachment processes (O + O ! e + O2) increases the

electron density [Aleksandrov et al., 1997] and enhances the conductivity of the leader. The high conductivity in leader transmits the high electric potential from the charge region in thunderstorm, which generates the streamers at the cap of the leaders [Raizer et al., 2006, 2007]. The streamer-leader phenomena in the BJs may also present in the GJs, and the disparities between them may only in their terminal altitudes and the initiating points inside the thunderstorms.

[4] Recently, a unified model of various types of upward

discharges, including BJs and GJs, has been proposed by Krehbiel et al. [2008]. They suggested that BJs can be triggered by the charge imbalance established by the cloud-to-ground (CG) or intracloud (IC) lightning in the storms. On the basis of the proposed model, a BJ begins as electrical breakdown between the upper charge reservoir in the thundercloud and the screening charge at the cloud top. In contrast, a GJ starts as a normal IC discharge inside the cloud, but propagates upward and escapes the cloud just as a ‘‘bolt from the blue’’ does. However, past observations of GJs [Pasko et al., 2002; Su et al., 2003; van der Velde et al., 2007] were based on video-rate monochrome cameras. Thus the image data cannot resolve the electric discharges oc-curred inside the thunderclouds. The observations also provided no spectroscopic information and temporal reso-lution to discriminate the dynamic behaviors in the dis-charge channels that span the cloud top and the lower ionosphere.

[5] In this paper, selected gigantic jets recorded using a

spaceborne instrument called Imager of Sprites and Upper Atmospheric Lightning (ISUAL) are analyzed and reported. These GJ events all featured an upward and a downward propagating photometric peak that separated in time by

1 ms. Evidence supports the interpretation that one peak is associated with the fully developed jet and the other is from a return-stroke-like process will be presented and discussed. Using the ISUAL photometric data, we are also able to deduce the propagating velocities, the magnitude of the reduced E field and the average electron energy in the FDJ of the GJs. The reduced E field is defined as E/N, where E is the E field strength and N is the neutral density. The derived values support the existence of streamer breakdown regimes in the FDJ. From the GJ-associated radio emissions recorded by Duke University ULF/ELF system, the electric nature of the FDJ is confirmed and the origin of the ELF emissions is proposed.

2. Instrumentation and Observation

[6] The ISUAL payload on the FORMOSAT-2 satellite

consists of an ICCD imager (Imager), a six-channel spec-trophotometer (SP) and a dual-band array photometer (AP). The images reported here were obtained through a 623 – 750 nm filter with an image frame integration time of 29 ms. The key SP data are from channel 2 (centered at 337 nm; bandwidth 5.6 nm) and channel 3 (centered at 391.4 nm; bandwidth 4.2 nm), respectively, from N2 secondary

posi-tive band (2PN2, 0 – 0) and N2 +

first negative band (1NN2 +

, 0 – 0). Other SP channels include the SP1 (150 – 280 nm), the SP4 (624 – 750 nm), the SP5 (centered at 777.4 nm), and the SP6 (244 – 392 nm). The SP1 channel detects photons from N2Lyman-Birge-Hopfield (LBH) band, the SP4

chan-nel measures N2first positive band (1PN2) emissions, the

SP6 channel is for sensing the 2PN2 and 1NN2 +

emissions and the SP5 channel is for the detection of the lightning 777.4 nm OI emissions. The bandwidths of the different SP filters are summarized in Table 1. The major emission bands in molecular nitrogen detected by ISUAL SP are listed in Table 2. The ISUAL AP contains blue (370 – 450 nm) and red (530 – 650 nm) band multiple-anode photometers. Each AP module has 16 vertically stacked PMTs with a combined field of view (FOV) of 22 deg (H) 3.6 deg (V) [Chern et al., 2003; Mende et al., 2005], and senses temporal and spatial variations of emissions along the vertical direction. For an event occurring near the Earth limb 3300 km away, the vertical spatial resolution of the individual AP channel is 12 km. The ISUAL Imager, SP and AP are coaligned at the center of their views. The ISUAL Imager and SP are bore-sighted, and their FOV is approximately 20 deg (H) 5 deg (V).

[7] An onboard routine is initiated to process an event

when the changes of the brightness sensed by the ISUAL SP exceed preset threshold values. The SP sends an event flag to the ISUAL auxiliary electronic units to move the data in the circular buffers into the hardware memory. For each event flag, ISUAL stores six consecutive frames of images, typically one frame before the trigger time and 5 frames after. The ISUAL SP samples at a constant rate 10 kHz rate at all times. For each event, ISUAL saves 25 ms of SP data before the trigger point and an additional 205 ms data after the trigger. The ISUAL AP usually samples at a rate of 20 kHz. However, 20 ms after receiving a trigger, the AP rate slows down to 2 kHz and maintains at this rate for 212 ms. The total length of AP data is 240 ms (8 ms to 232 ms).

Table 1. Band Passes of the ISUAL Spectrophotometer and the Major Emissions in the Passing Band

SP Channel Center Wavelength (nm) Band Width at 50% (nm) Major Emission Band SP1 220 140 N2LBH SP2 337 5.6 2PN2(0 – 0) 337 nm SP3 391.4 4.2 1NN2+(0 – 0) 391.4 nm SP4 687 126.7 1PN2 SP5 779 7.9 O I 777.4 nm SP6 317.6 148.2 2PN2

A04314 KUO ET AL.: ISUAL-RECORDED GIGANTIC JETS

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[8] FORMOSAT-2 is a Sun-synchronous satellite with

14 daily revisiting orbits. The ISUAL payload globally surveys transient luminous events (TLEs) and other lumi-nous emissions in the upper atmosphere with a side-looking view. The region covered by ISUAL FOV is3 106km2, and is scanned from south to north in each orbit [Chern et al., 2003; Mende et al., 2005]. Figure 1 shows the obser-vational geometry for an ISUAL gigantic jet recorded on 1 October 2005 1122:23.898 UT. The cloud top is assumed to be16 km [Su et al., 2003], only 2 km lower than the typical starting height (18 km) of blue jets [Wescott et al., 1995, 2001]. Ray SC connected the ISUAL Imager (S; altitude 891 km) and the geometric center of the cloud emission C. Using the satellite orbital information, the latitude and longitude of the point C can be calculated by intersecting the ray SC and the curve at an altitude of the assumed cloud top 16 km (the dotted line in Figure 1). The distance SC is estimated as 3000 km. The geographic location of the point C is calculated to be 10.2 deg S, 177.5 deg E.

3. Representative GJ Event on 1 October 2005 1122:23.898 UT

[9] The image sequence of a GJ recorded on 1 October

2005 1122:23.898 UT and the associated SP signals are shown in Figure 2. Figure 2a and Figures 2b – 2f are recognized as the FDJ and the TJ features, respectively, of the GJ as reported by Su et al. [2003]. In the first 29 ms of

the FDJ in Figure 2a, the horn-shaped GJ had a full width at half maximum (FWHM) diameter of5 km below 60 km, and fanned out above 60 km altitude. As measured from the ISUAL ICCD imager, the average brightness of this gigan-tic jet at altitudes of40 – 80 km, at the time of Figure 2a, was0.4 –0.8 mega-Rayleigh (MR) over the 29 ms expo-sure time with the brightest part at an altitude of70 km. The Rayleigh is the unit for the integrated light emission rate in TLEs along the line of sight from each pixel of the ISUAL CCD imager. One Rayleigh is equivalent to 106 photons cm2-column s1[Baker and Romick, 1976].

[10] In Figures 2g – 2l, the first and the second SP data

peaks after the event trigger are labeled as P1 and P2, respectively. The first peak is present in the SP1 (LBH N2),

SP2 (2PN2), SP3 (1NN2+), SP4 (1PN2) and SP6 (2PN2,

1NN2+), but not in the SP5 (OI 777.4 nm). The SP records

the GJ emissions integrated over the entire FOV. Hence, we utilize the AP data to resolve the altitude evolution profile of the radiative emission as shown in Figure 3a. In Figure 3b, the expected altitude for each pixel in the imager frame can be estimated from the observational geometry of Figure 1, assuming a cloud top altitude of 16 km. The left and right vertical axes of Figure 3b indicate the altitude in units of km and the respective AP channel numbers 8 – 16. Figure 3c is the expanded view of the AP signal traces between 1.5 and 4 ms (the dashed region in Figure 3a). In Figure 3c, the blue and the red emission curves represent the AP recorded brightness with blue and red filters, respectively, in arbitrary linear units for

Table 2. Salient Parameters of the Major Emission Band Systems for Molecular Nitrogen

Emission Band System Upper and Lower States Transition Wavelength (nm) Lifetime Quenching Altitudes (km) 1PN2 N2(B3Pg)! N2(A3Su+) 478 – 2531a 6msb 67c 2PN2 N2(C3Pu)! N2(B3Pg) 268 – 546a 50 nsd 30e LBH N2 N2(a1Pg)! N2(X1Sg+) 100 – 260a 56msb 84f 1NN2 + N2 + (B2Su + )! N2 + (X2Sg + ) 286 – 587a 63 nsb 48e a

The wavelength range of the band emissions is taken from Lofthus and Krupenie [1977].

b

The middle values of the lifetime, inverse of the Einstein coefficient (Ak), for the major vibrational levels of the upper state of the band system [Gilmore

et al., 1992] are adopted.

c

At this quenching altitude, the collisional quenching rates of 1PN2with molecular nitrogen and oxygen are 1.6 1011and 1.5 1010, respectively

[Piper, 1992].

d

The lifetime of 2PN2is from Vallance-Jones [1974]. e_{The collisional quenching rates with N}

2and O2for 2PN2are 1.12 1011and 2.85 1010; they are 4.53 1010and 7.36 1010for 1NN2+

[Pancheshnyi et al., 1997].

f_{For LBH N}

2, the collisional quenching rates with N2and O2are 2.2 1011and 4.3 1010[Marinelli et al., 1989, Table 1].

Figure 1. ISUAL observational geometry for the gigantic jet recorded on 1 October 2005 1122:23.898 UT, where the FORMOSAT-2 satellite S at altitude hs 890 km and the geometric center C of cloud top at

16 km are labeled.

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each AP channels. Evidently, the first SP photometric peak (P1) in Figure 2 can be further resolved by the ISUAL AP and is the upward propagating wave in Figure 3c. The AP data indicate that the luminous pulse traveled from30 km to 90 km. Approximately one millisecond after P1, the AP signals also indicate that the second SP photometric peak (P2) is a downward propagating wave traveled from 50 km to 20 km.

[11] Figures 3d and 3e show a detailed view of the

altitude profile of the blue-filtered and the red-filtered AP data between 1.1 and 0.7 ms relative to the GJ trigger time. The vertical resolution of each AP channel is12 km at the distance of 3000 km between the GJ and the satellite. A background signal obtained by averaging AP data before the time range of interest has been subtracted from each signal. For this GJ, we choose the time range between 4 and 3.75 ms to obtain the background. In Figure 3d, between 1.05 and 0.9 ms, a weak but distinguishable luminous emission peak at altitudes of 27 – 39 km can be found in the AP blue channel 10. Between 0.9 and 0.3 ms, the blue luminous pulse shows up at altitudes of 39 – 51 km in the AP blue channel 11. At a time of0.9 ms, labeled by ‘‘B1,’’ the flat signal in two subsequent channels implies that the luminous emission peak is crossing from AP blue channel 10 to 11. The estimated error on altitude is half the channel height,6 km. Hence we can identify the AP blue signals with the arrival time 0.9 ms, 0.3 ms, 0.05 ms, respectively labeled by B1, B2 and B3, were from the altitudes of 39, 51 and 63 km.

[12] For the AP red module (Figure 3e), before0.6 ms,

we can discern the red emission peak at altitudes of 39 – 51 km in the AP red channel 11. The AP red emissions at the time of0.35, 0.025, and 0.35 ms (labeled by ‘‘B2,’’ ‘‘B3’’ and ‘‘B4’’) were from the altitudes of 51, 63 and 75 km. The

AP red signals preceded the AP blue signals by0.05 and 0.025 ms for emissions from altitudes ‘‘B2’’ and ‘‘B3.’’ The estimated timing error is 0.05 ms. Hence, we can identify the emissions with the arrival times0.9, 0.3, 0.05 and 0.35 ms were from altitudes of 39, 51, 63 and 75 km and respectively labeled them as ‘‘B1,’’ ‘‘B2,’’ ‘‘B3’’ and ‘‘B4.’’ On the basis of these data, the propagation velocities were deduced to be 1.7, 3.6 and 4.2 107m s1, respectively. The luminous emission peak accelerates as it approaches the bottom of the ionosphere with an estimated acceleration between 2 and 4 1010_{m s}2_.

[13] As indicated by the blue photometric traces in Figure 3c,

1 ms after peak P1, a luminous pulse (P2) appeared to originate at altitudes of 40 – 50 km and traveled downward. However, because of the significant quenching under 60 km altitude for the 1PN2 emission, the corresponding signal

train was absent from the AP red channels. The velocity of the downward propagating pulse can be derived from the AP blue module data and is found to be 3 107_{– 1}₁₀8_m

s1, which is slightly higher than the velocity of the FDJs. Also within 150 ms following the photometric peak P2, a trailing jet with a mushroom-like cap appeared to propagate upward as depicted in the imager data, Figures 2b – 2f. The FWHM diameter of this cap is8 km, as estimated from Figure 2b. From Figure 2b, the average brightness of the TJ at altitudes in 27 – 62 km was 2 MR, with the brightest portion having 2.8 MR brightness in the altitude region 39 – 51 km. At the time of the peak P2 in Figure 2, the SP data show nearly no red emission (SP4) and thus the emissions are blue-dominated. However, after the peak P2, a slow-varying emission curve (denoted as C3 in Figure 2 and Figure 3) in SP2, SP3, SP4 and SP6 at an estimated altitude of <27 km started to rise, as shown in Figure 2. ISUAL GJ on 1 October 2005, 1122:23.898 UT. Figures 2a – 2f are the time sequence images

from the ISUAL Imager, and Figures 2g – 2l are signal traces from the six ISUAL SP channels, where the first peak (P1) and the second peak (P2) were from the fully developed jet and a return-stroke-like process in the GJ, respectively.

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Figure 3a. This indicates that most of the long-persisting emissions were from the cloud-deck level.

4. Electric Field and Electron Energy in the GJs [14] Compared with previous GJ observations [Pasko et

al., 2002; Su et al., 2003; Hsu et al., 2005; Su et al., 2005; van der Velde et al., 2007], ISUAL provides high temporal resolution (0.05 ms for the AP), detailed spectroscopic information, and the ability to observe blue emissions due to a higher atmospheric transmittance for satellite observa-tions. Previous work has shown that the E field magnitudes and the average electron energies derived from the ratio SP2/SP3 ratio are consistent with the predictions of the sprite streamer model [Kuo et al., 2005; Liu et al., 2006]. Below we assume that most of the GJ luminous emissions

are from streamers, and analyze them using the methods discussed by Kuo et al. [2005].

[15] The relative response functions, R(l), for all of the

ISUAL instruments have been calibrated in the preflight tests [Mende et al., 2005]. Here we consider the major band emissions (1PN2, 2PN2, N2LBH and 1NN2

+

) of molecular nitrogen [Kuo et al., 2007, and references therein]. The percentage of the total band emission into an ISUAL sensor unit is defined as the band percentage Bk(h) from the kth

band. Bk(h) also is a function of the altitude h and can be

expressed as Bkð Þ ¼h X l Ikð ÞT l; hl ð ÞR lð Þqkð ÞDlh X l Ikð ÞDll ð1Þ

Figure 3. (a) The associated AP photometric data for the GJ in Figure 1. Data from the red and the blue modules are indicated by the red and the blue lines, respectively. (b) The first frame of the ISUAL Imager data for this GJ. (c) Expanded view of the AP signal traces between 1.5 and 4 ms (the dashed rectangular in Figure 3a). The background-subtracted AP data with (d) blue filter and (e) red filter for the time ranging between1.1 and 0.7 ms.

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where Ik(l) is the intensity of emission lines in the kth band

emission as a function of wavelength l, T(l, h) is the atmospheric transmittance, R(l) is the instrument response, and qk(h) is the quenching ratio. The major attenuation

mechanisms include O2 absorption, O3 absorption and

molecular Rayleigh scattering [Kuo et al., 2007, and references therein]. The quenching factor qk(h) is defined

by [Vallance-Jones, 1974]

1

1þ kq;N2NN2ð Þ þ kh q;O2NO2ð Þh

=Ak

where kq,N2and kq,O2are the collisional quenching rates for molecular nitrogen and oxygen; NN2(h) and NO2(h) are the number densities of molecular nitrogen and molecular oxygen as a function of altitude h; Ak is the Einstein

coefficient for the kth band emission. The collisional quenching rates and Einstein coefficients for the various band emissions are given in Kuo et al. [2007, and references therein]. The number densities of molecular nitrogen and oxygen are calculated using the MSIS model [Hedin, 1991]. [16] We use the known band percentages of ISUAL SP2

and SP3 to infer the total band emissions in 2PN2and 1NN2+

[Kuo et al., 2005, 2008]. The ratio 1NN2+/2PN2reflects the

relative ratio of the ionization rate for 1NN2+to the

excita-tion rate for 2PN2. Compared with the projected rate ratio of

1NN2+to 2PN2calculated using the ELENDIF code [Kuo et

al., 2007], the same ratio derived from the ISUAL observed events can be used to deduce the reduced E field and the average electron energy [Kuo et al., 2005]. The validity of calculating the emission intensity ratio 1NN2+/2PN2is

jus-tified under the steady state conditions.

[17] The altitude range of the major emissions in the

streamer region of the GJ precursor is about the height of one or two AP channels (12 – 24 km) between altitudes of 40 to 80 km. The altitude uncertainty for the upward propagating luminous emissions in the fully developed stage of the GJ is about half of an AP channel, 6 km. Taking the possible error due to the altitude of the cloud top into account, we estimate the error on altitudes to be ±10 km. Using the time- and altitude-resolving ISUAL AP data shown in Figure 3, the time in the SP readings (Figure 2) can be converted into the occurrence height.

The altitude-varying ratio of 1NN2+/2PN2is computed and

shown in Figure 4a. Figures 4b and 4c present the inferred reduced E field and the average electron energy in the representative GJ, computed using the methods reported by Kuo et al. [2005]. The dashed lines in Figure 4 mark the maximum and minimum values of the derived quantities, after taking the possible error of ±10 km on altitudes into account. In the streamer region of this GJ event, the ratio (1NN2+/2PN2) is0.07. The reduced E field

is thus 394 Td (1 Townsend = 1021 V-m2) and the average electron energy is8.5 eV.

[18] During the first 3-year survey (2004 – 2007), 13 GJs

were identified from the ISUAL recorded events [Chen et al., 2008]. We selected 5 GJs with clear AP and SP signals, as shown in Table 3, for detailed analyses. The observed and the deduced luminous characteristics of the selected ISUAL GJs are listed in Table 3. The brightness of the FDJ varies from 0.35 – 2 MR at the altitude of the cloud tops (20 km) to the bottom of the ionosphere (90 km). The measured brightness of the TJ is weaker (0.2 – 0.8 MR) in four of the five GJs. The FDJ velocity is measured from the AP data with an altitude resolution of12 km and a time resolution of 0.05 ms. The GJ velocities in Table 3 are all on the order of 107m s1. The ratio of 1NN2+to 2PN2emission

is 0.07 – 0.27, corresponding to a reduced E field of 400 – 655 Td. This in turn yields the average electron energy in the FDJ streamer region to be 8.5 – 12.3 eV. The reduced E field and the average electron energy are significantly higher than those in the ISUAL sprites as reported by Kuo et al. [2005]. However, the results for the GJs are similar to those obtained from a streamer model [Liu and Pasko, 2004; Liu et al., 2006]. It is a good indication that the high E field exists in the streamer tip and the magnitude of E field can exceed 3Ek, where Ek is the conventional

breakdown threshold field [Raizer, 1991, p. 135].

[19] The velocity of the observed FDJs is on the order of

107m s1, two orders of magnitude faster than that of the blue jets (105m s1) [Wescott et al., 1995] but comparable to the typical downward and upward sprite streamer veloc-ities (107_{m s}1_{) measured by 10,000 fps imagery [McHarg}

et al., 2007] and a multianode photometer array [McHarg et al., 2002]. The highest upward sprite streamer velocity was 1.4 108 _{m s}1_{, nearly half of the light speed. Similar}

Figure 4. Altitudinal variation of (a) the 1NN2+to 2PN2emission ratio, (b) the inferred reduced E field

(E/N), where E is the E field strength and N is the neutral density, and (c) the deduced average electron energy for the GJ on 1 October 2005 1122:23.898 UT. The dashed lines represent the lower and the upper bounds of these physical quantities.

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downward streamer velocities measured by 10,000 fps imagery and photometer array were also reported in Stenbaek-Nielsen and McHarg [2008]. The acceleration of the FDJ is on the order of 1010 m s2 at altitudes of 40 – 70 km, which is also consistent with observed sprite streamer accelerations; 1.8 1010 _{m s}2 _{for the upward}

streamers and 0.5 1010 _{m s}2 _{for the downward}

streamers. [McHarg et al., 2007]. Numerical streamer simulations show the propagating velocities to be around 106 m s1, the same as that for laboratory streamers [Briels et al., 2008]. However, in the fast expansion and acceleration phase of overvoltage streamers initiated in 1.1Ek

field, the speed of the streamers can reach 2.2 107_{m s}1_as

reported in numerical simulations [Liu and Pasko, 2004; Liu et al., 2006].

[20] In Table 3, three of the five GJs were found to have

associated radio emissions with a positive polarity (i.e., positive charge moving downward inside the cloud) as seen in the data recorded by the Duke University extremely low/ ultralow frequency (ULF/ELF) radio system. The detected polarity ULF/ELF sferics is positive at the time of peak P1. No strong lightning emission in SP5 (lightning 777.4 nm) infer no strong stroke, which contributes no significant signal of sferics inside the cloud. It implies that currents in the FDJ stage of the GJs generate the ULF/ELF emis-sions. This would be the negative cloud-to-ionosphere (CI) as proposed by Su et al. [2003]. In the CI scenario, the negative streamers carry negative charges and propagate upwardly from the cloud top to the lower ionosphere in the FDJ stage of GJs.

[21] It should be noted that the apparent saturation of the

images, Figures 2a – 2f and Figure 3b, from the emissions associated with the lightning activity at the cloud deck level is a processing artifact. To bring out the detailed structures of the GJs, we have to set a very narrow intensity range for these images. Since even the dimmest lightning illumination substantially outshines the brightest GJs, a proper setting for the GJs will make the lightning emissions in the same image frame appear to saturate. In reality, the lightning emissions at the cloud deck level for the GJ presented in Figure 2 are weak, as it can be discerned from the associated 777.4 nm emissions in ISUAL SP channel 5.

[22] To summarize, the inferred reduced E field and the

average electron energy in the fully developed jets are 560 ± 110 Td and 10.9 ± 1.7 eV, respectively. These values are

substantially higher than those in sprites, but are similar to those predicted by the streamer models [Liu et al., 2006]. Thus, these results imply the existence of streamer tips in the FDJ stage of the GJs. The speed of upward moving fully developed jets is the same order as that of downward sprite streamers. The upward propagating luminous emissions in the FDJs (P1 in Figure 3c) are thus from streamer tips that extend about1 – 2 AP channels (12 – 24 km), as shown in Figures 3d and 3e. Sferics associated with three of the five GJs listed in Table 3 exhibited positive polarity, which supports the existence of upward moving negative streamers in the FDJ stage of GJs.

5. Discussion

[23] Starting as a lightning leader but escaping from the

cloud top [Krehbiel et al., 2008], the fully developed jet behaves similarly to the streamer-leader phase of a long spark [Bazelyan and Raizer, 2000, pp. 27 – 89; Bondiou and Gallimberti, 1994; Mazur et al., 2000] but with very different length and time scales. The reduced E field derived from the detected photometric signal in the FDJ confirms the existence of the streamer region in the GJs. This suggests that the escaped lightning leader above the cloud top produces streamers that extend from30 km to 90 km; a length which is substantially longer than the several meters for the laboratory streamers [Bazelyan and Raizer, 2000, p. 86]. Also the luminous duration of the fully developed jet, represented by the P1 photometric peak in Figures 2 and 3c, can last for several milliseconds whereas it is only tens of microsecond for the laboratory leaders [Bazelyan and Raizer, 2000, p. 86]. The associated positive polarity sferics at the time of the peak P1 also suggests that streamers carry negative charges, and thus its leader should also be negative.

[24] In contrast to an arc setting in after a laboratory spark

reaches the opposite electrode in the final jump phase, the FDJ streamers of the negative leader exhibit a different behavior when they reach the ionosphere. For the GJ events in the present work, a return-stroke-like process seems to occur after the FDJ has bridged the cloud top and the lower ionosphere. From the temporal and the spatial evolution of the P2 photometric peak, Figure 2 and Figure 3c, the luminosity appeared to originate at 50 km altitude and propagated toward the cloud top. As the FDJ faded away, a

Table 3. Observed and Derived Physical Quantities for the Five Selected GJs

Trigger Timea

Geolocationa ULF/ELFb FDJc TJd FDJe

Lon Lat Polarity H (km) Brightness H (km) Brightness H (km) V (x1E7 m s1) 1NN2+/2PN2 eV E/N

21 Mar 2005 1950:33.999 54.97 12.32 N/A 25 – 87 0.35 – 1.0 25 – 61 0.3 – 0.5 50 – 87 2.7 – 5.1 N/A N/A N/A 1 Oct 2005 1122:23.898 177.54 10.22 N/A 27 – 87 0.4 – 0.8 27 – 62 2 39 – 87 1.7 – 4.2 0.07 8.5 400 13 Dec 2005 1305:54.688 158.42 11.11 + 23 – 85 0.4 – 1.0 23 – 63 0.2 – 0.8 33 – 73 1.5 – 4.3 0.20 11.7 600 28 Feb 2006 0435:52.993 72.68 4.43 + 23 – 90 0.5 – 2.3 23 – 63 0.2 – 0.6 35 – 50 2.8 0.20 11.2 585 14 Mar 2006 1633:00.609 111.40 16.79 + 26 – 95 0.5 – 1.0 26 – 68 0.12 38 – 68 2.4 – 4.5 0.27 12.3 655

a

The geolocation (longitude and latitude) means the ground projection of the GJ events, which were calculated using the observational geometry shown in Figure 1.

b

Source polarity of the sferics was inferred from the data recorded by the Duke University extremely low/ultralow frequency (ULF/ELF) radio system. The plus symbol (+) indicates that the seferics has positive polarity.

c

FDJ denotes the fully developed stage of the GJ, and its estimated altitudes and brightness are in units of km and mega-Rayleigh, respectively.

d

Same as footnote c, and TJ stands for the trailing jet of the GJs.

e

The reduced E field and the average electron energy were calculated using the streamer optical model [Kuo et al., 2005, 2007].

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trailing jet slowly surged upward. These characteristics all seem to point to a scenario depicted in Figure 5. When the FDJ connected to the lower ionosphere, the ionized gas in the electric path serves as the extension of the local ionosphere and resulted in a lowered local ionosphere boundary. Then positive charges flew from this lowered boundary toward the cloud top or a potential wave propa-gated backward along the original leader channel and produced the ‘‘return-stroke-like’’ phenomenon. This addi-tional current or the strong electric field associated with the potential wave produces additional ionization along the original channel, and further heats the leader channel. Since the lower-altitude region has faster charge attachment rates, the boundary of the ionosphere gradually moves upward and eventually returns to the normal height. In this scenario, the trailing jet actually can be treated as a continuous current that connects the lower edge of the local ionosphere and the cloud top. As the attachment process moves the effective lower edge of the ionosphere upward and the continuous current persists, the contact point also moves up and the continuous current appears as an upsurging trailing jet. Since the image integration time is 29 ms and during this period the tip of the trailing jet clearly moved up, so the charge relaxation time at the 50 – 70 km altitude is longer than 1 ms but certainly shorter than 1 s [Pasko et al., 1998; Sentman et al., 2008].

[25] The trailing jet in Figures 2b – 2f likely is composed

of hot leader channels. The photon flux from the continuous emissions in the leader channel was below the detection limit of the AP and cannot be discerned in Figure 3a. The mushroom-like cap of the TJ may be the fan-out streamer region [Raizer et al., 2006]. The leader channel is heated by the joule heating of the injected streamer currents. Because of the dominant associative ionization and the detachment processes above the critical temperature, the emission from the hot leader channels persists for at least five ISUAL Imager frames (>150 ms). The TJ feature of GJs shares some interesting features with the secondary TLEs follow-ing the primary sprites [Marshall and Inan, 2007]. They are

both blue-dominated at altitudes <60 km. The luminous period of the trailing jets can be up to 0.5 s for the event reported by Su et al. [2003] and was >0.3 s for the ISUAL GJs studied here. For the secondary TLEs, the entire luminous duration lasts less than 0.5 s [Marshall and Inan, 2007]. The proposed generating mechanism of the second-ary TLE also needs the preceding sprite to ionize the air in the altitudinal region of 50 – 90 km and lowers the iono-spheric boundary to40 – 50 km.

[26] For the processes that produce the P2 pulse, an

alternative possibility is the backward streamer-leader from the negative leader FDJ [Mazur et al., 2000 and references therein]. If this was to happen, then the P2 pulse should be from the positive branch of the space leader [Bazelyan and Raizer, 2000, p. 85]. Therefore, the occurrence of the P2 pulse is expected to overlap in time with the P1 pulse associated with the FDJ. Since the P2 pulse trailed the P1 pulse by 1 ms, this implies that if the space leader did appear it must have occurred right below the original ionospheric boundary. Only below 50 km altitude, the luminous emissions from the backward propagating streamer-leader were intense enough and became visible to the ISUAL sensors. The backward streamer-leader fuses with the original negative leader, and together they form the new hot leader channel [Pasko, 2008, and references therein]. In laboratory studies, the negative long spark leader contains three to five steps between a 6 m rod-plane gap [Rakov and Uman, 2003, and references therein]. However, for the ISUAL recorded GJs studied in the article, at most there is only a single stepping process that radiated the P2 peak, which may imply it is a failed step formation [Pasko, 2008]. [27] The accompanying cloud emissions (C3 in Figure 2)

were from the lightning activity inside or below the cloud. From Figures 2a – 2f, the cloud luminous emissions for this event had complicated shapes and distinct layers. This is entirely unlike the ISUAL-observed cloud optical emissions for sprite-producing CGs [Kuo et al., 2005], which have compact, symmetrical shapes since the primary emission source, the cloud to ground channel, is deeply embedded Figure 5. During the fully developed jet stage of the GJ, the local ionosphere boundary could be located

at a much lower altitude because of the presence of the ionized channel in the FDJ. A return-stroke-like process could start at the local ionosphere boundary and extend toward the cloud top. The charge attachment process depletes the low-altitude free electrons faster, causes the ionosphere boundary to move upward, and produces the upward movement of the trailing jet.

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