Isual Publications 2014

Isual Publications 2014

  • id
  • date time
    2014-04-08 14:38:00
  • 2007
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  • 郭政靈(2007),‘福衛二號科學酬載所觀測到的sprites以及elves的分析’,國立成功大學物理研究所(博士論文)
  • 福衛二號是世界上第一個觀測高空短暫發光現象的酬載衛星。衛星上的科學酬載ISUAL包含了影像儀(Imager)、光譜光度儀(Spectrophotometer)以及陣列式光度儀(Array Photometer)等三套儀器,其主要的任務是從太空觀測高空閃電的全球分佈與高時間解析度的光譜光度資料,用以探索產生高空短暫發光現象在地表之分佈與事件內部的物理狀態。本文是分析ISUAL所記錄的紅色精靈與淘氣精靈等兩種高空短暫發光現象,與理論模型的模擬結果相比較,用以檢驗理論的有效性,進而反演出原生閃電的特性。 

    紅色精靈通常出現在雷雨雲上方,是一種伴隨著誘發閃電之後幾毫秒之內,發生在地表上空40到90公里高處的發光現象。在紅色精靈中,紅光部份主要是來自氮氣(N2)的第一正則譜系(1PN2),藍光部份則包含氮氣的第二正則譜系(2PN2)及游離氮氣的第一負則譜系(1NN2+)。我們利用理論模型,計算在不同外加電場下的低度游離氣體內,電子激發氮氣各個譜系的激發率,進而得到光譜光度儀中的337.0 nm(1NN2+)通道與391.4 nm (2PN2)通道所量到的強度比相對於外加折合電場的關係。再和衛星觀測數據所推算的比率擬合,反推出紅色精靈內部的折合電場與電子的平均能量和特徵能量分別為~2-3 Ek (Ek為breakdown field=118.5 Td) , 6.2-9.2 eV和4.5-6.5 eV。比Morrill 等人在2002年利用飛機觀測所得的數值高2-3倍,但是卻符合Streamer理論模型所預測的值 (Pasko et al., 1998; Liu and Pasko, 2004;2005;2006)。 

    淘氣精靈是閃電的電磁脈衝激發高度87公里左右(電離層的底部)的氮氣,因而產生的發光現象。在此,因為ISAUL可以觀測高空短暫發光現象的紫外線波段,我們延伸前人所發展的淘氣精靈模型(Inan et al., 1996; Veronis et al., 1999,Barrington-Leigh and Inan, 1999),使之可以涵蓋紫外線到近紅外線波段(185-800 nm)的淘氣精靈光譜;同時在計算激發態密度函數時,除了電子激發效應,氣體分子的自發輻射與碰撞退火(collisional quenching)效應之外,也加入影響較小的cascade項,以期得到較完整且精確的結果。另外,我們也考慮大氣中氧氣(O2)與臭氧(O3)的吸收,以及氣體分子的散射(Rayleigh scattering)對ISUAL觀測資料的影響。理論模擬的結果顯示,我們淘氣精靈模型所預測的光譜儀讀值與ISUAL光譜光度儀的觀測值非常接近。在影像的模擬方面,我們考慮地球的曲面的影響,計算出理論上影像儀所應觀測到的淘氣精靈幾何形態,結果也與觀測相當吻合。陣列式光度儀的模擬結果也與ISUAL的觀測結果相當一致。最後,我們在淘氣精靈模型中,給定不同的閃電瞬間電流,以計算淘氣精靈的光度值,再利用觀測到的淘氣精靈光度,反演出產生淘氣精靈的閃電電流峰值。與美國國家閃電探測網(NLDN)所量測的閃電電流峰值比對後,我們發現其中只有兩個事件有相對應的資料,但是由ISUAL所反演出的閃電電流峰值與NLDN所測的電流峰值,相差在25%以內,驗證了理論模擬的可靠性。利用此一驗證的結果,我們可以使用所觀測的淘氣精靈亮度,反演出所對應的原生閃電的電流峰值。結果顯示,我們所選取的淘氣精靈是較亮的事件,其原生閃電的電流峰值是介於160kA與400kA之間;而可以被ISUAL量測的淘氣精靈,其對應的原生閃電的最低電流峰值約為80kA,也就是ISUAL紀錄的淘氣精靈皆來自於大電流閃電。由於衛星的觀測範圍,幾乎涵蓋全球,因此,我們可利用福衛二號的淘氣精靈觀測,用以探討大電流閃電的全球分布情形。

    FORMOSAT-2 is the first satellite mission that features a payload dedicating for the survey of upper atmospheric transient luminous phenomena/events (TLEs). The payload consists of three sensor packages including an intensified CCD imager, a six-channel spectrophotometer and a dual-band array photometer. The main mission goals are to obtain the global distribution and the high time resolution spectral information of TLEs. Through these data, the global occurrence of TLEs and the physical conditions in TLEs can be deduced. This thesis work gives detailed analyses of the ISUAL recorded sprites and elves. Theoretical works based on electromagnetic field-driven weakly ionized gases are carried out to elucidate the generating mechanisms and the salient properties of TLEs. The ISUAL (Imager of sprites and upper atmospheric lightning) observational data are used to validate the theoretical results. Reversely, the theoretical results can couple with the experimental data to deduce the peak current of the elve parent lightning. 
    Sprites often appear above active thunderstorms following their parent lightning within a few milliseconds, and their luminous bodies span the region between 40 to 90 km altitudes. The reddish glow in sprites is mostly from the N2 first positive group (1PN2), while the bluish glow in the dendritic region comes from N2 second positive group (2PN2) and N2+ first negative group (1NN2+). We numerically simulate the responses of a weakly ionized gas to an externally applied DC E-field to obtain the excitation rates of 2PN2 and 1NN2+ and the ratio of ISUAL SP2 (337.0 nm from 1NN2+) to SP3 (391.4 nm from 2PN2) intensities as a function of the reduced E-field. By fitting the observed ISUAL and the theoretical SP2/SP3 ratios, average electron energy, characteristic electron energy and reduced E-field strength in sprites are derived. The deduced average electron energy is 6.2-9.2 eV and the reduced E-field is ~2-3 times of Ek (Ek is the breakdown field of 118.5 Td) in sprites are higher than those from ground or aircraft campaigns, which reported an average electron energy of ~2.2 eV and an E-field comparable to the breakdown field in strength. However our results are consistent with the predictions from the theoretical streamer model for sprites (Pasko et al., 1998; Liu and Pasko, 2004;2005;2006). 
    Elve is a transient luminous glow induced by the lighting-radiated electromagnetic pulse (EMP) at the lower boundary of the nighttime ionosphere at 87 km altitude. Due to the much reduced absorption of the ultraviolet emissions in the rarified upper atmosphere, ISUAL instruments are able to observe the ultraviolet emissions in elves, and thus cover the elve emissions ranging from 185 nm (Far UV) up to 800 nm (Near IR). Hence we extend the lightning EMP model developed by Barrington-Leigh and Inan (1999) to study the ISUAL recorded elves. Moreover, in computing the density of states for the excitation states, the spontaneous emissions, the collisional quenching effects and the contributions of the minor cascading pathways are accounted for. To take into account the effect of atmospheric attenuation, three major attenuation mechanisms: O2, O3 and molecular Rayleigh scattering are considered. The finite curvature of the Earth's surface is also considered while computing the forms of elves. Excellent agreements are found between the modeled emissions and morphologies and those observed by the ISUAL instruments. To validate the elve EMP model, we look for the associated ground lightning data for the ISUAL elves studied in this thesis. Two associated NLDN CG events were found for the 105 behind-the-limb elves. For these two elves, the difference between the photon fluxes measured by the ISUAL Imager and those computed by the EMP model using the NLDN derived peak currents are found to be less than 25%. Finally, the peak currents of the elve-causing cloud-to-ground lightning are derived based on the modeled and the ISUAL observed photometric intensities. For the behind-the-limb elves studied in this thesis, the inferred peak current of the causative CG was 160-400 kA at β=0.5. The relatively high elves initiation current was due to the high pre-set trigger threshold (corresponding to the elve flux induced by an 80 kA CG) of the ISUAL SP and the long event distances of greater than 3700 km. Therefore, ISUAL can be viewed as a space probe of elves and the elve-producing intense lightning.

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