Analyzing the Sprite Emission Ratio from FORMOSAT-2 ISUAL Array Photometer Data

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題名:	Analyzing the Sprite Emission Ratio from FORMOSAT-2 ISUAL Array Photometer Data
作者:	陳沛羽;Chen, Pei-Yu
貢獻者:	太空科學研究所
關鍵詞:	紅色精靈;福爾摩沙衛星二號;高空大氣閃電影像儀;陣列式光度計;Sprite;FORMOSAT-2;ISUAL;Array Photometer
日期:	2018-07-24
上傳時間:	2018-08-31 11:34:29 (UTC+8)
出版者:	國立中央大學
摘要:	「高空大氣閃電影像儀」 (ISUAL, Imager of Sprite and Upper Atmospheric Lightning)是福爾摩沙衛星二號(FORMOSAT-2)上的科學酬載儀器，其任務目標不只是調查紅色精靈(Sprites)在全球的發生率分佈，也同時觀測紅色精靈(Sprites)所發出的可見光。在過去的研究中，Adachi et al. (2006, 2008)指出，利用ISUAL的「陣列式光度計」 (AP, Array Photometer)來測量出Sprites所發出的N2 2P (Second positive band)及N2 1P (First positive band)亮光，而計算出Streamer Peak的Reduced Electric Field為0.24~1.24 E_k(上方區域)及0.81~3.16 E_k(下方區域)。在這篇論文裡的Sprite Emission Ratio可以被視為了解驅動Sprites產生電漿化學反應電場的重要參數之一。此篇論文使用ISUAL AP於2004年7月4日至2008年1月22日的資料及2009年1月18日至2013年1月24日含有ULF (Ultra Low Frequency)的資料，從中挑選28個事件使用Sum、Temporal Filter及Peak三種不同方法來進行Sprite Emission Ratio計算。與利用Photon Yield所計算出的Sprite Emission Ratio比較後，結果顯示在高度較高的時候，Sum、Temporal Filter及Peak這三種方法所計算出的Sprite Emission Ratio差異較小，在高度低的時候差異較大。Adachi et al. (2006, 2008)指出，這些差異變化是來自於高度預測誤差加上Quench項k_q [M]的指數增加所造成。此篇文章結果建議應加入一個一次指數函數〖ae〗^hx到氣體密度[M]，使[M]值隨高度遞減而下降，並且低於原使用的[M]值。在此篇文章中我們也將所分析計算出的Sprite Emission Ratio與利用電漿化學模型計算的結果做比較，結果顯示在5 E_k的Reduced Electric Field下所得到的N2 2P及N2 1P的發光率分別為5.6×〖10〗^16 photon/s及4.86×〖10〗^16 photon/s。模型所預估的N2 2P及N2 1P發光亮度分別為9.33×〖10〗^3 Rayleigh及8.1×〖10〗^3 Rayleigh。(1 Rayleigh=106 photon/cm2-column/s) 在使用三種不同方式計算結果與電漿化學模型結果比較下，利用Temporal Filter方法所計算出的平均Sprite Emission Ratio (1.17)與模型所計算出的Sprite Emission Ratio (1.15)最為接近。(Sum: 1.09；Peak: 1.93) 另外，利用ISUAL AP隨時間變化的訊號，可以計算出向上及向下Streamer移動速度約為107至108 m/s。在28個選出的Sprites事件中有15個事件有相對應的ULF資料，在這15個Sprite事件中，大部分的iCMC (impulse Charge Moment Change)都比最小能產生Sprite的iCMC值(>200 C-km)還要高，且有一個事件的Peak Current達到133 kA。未來可以透過調整氣體密度[M]、增加在較低(~40 km)及較高(~90 km)高度的模擬結果，以及增加分析事件數量來更進一步了解Sprite Emission Ratio、模型結果與三種分析方法的關係。;The ISUAL (Imager of Sprites and Upper Atmosphere Lightnings) is a scientific payload of FORMOSAT-2 satellite. The ISUAL project not only conducts the survey of the global occurring rate of sprites, but also investigates the optical emissions associated with sprites. In past studies, Adachi et al. (2006, 2008) reported that the sprite emission ratio of N2 2P (Second positive band) to N2 1P (First positive band) obtained from the ISUAL AP (Array Photometer) data can be used to derive the reduced electric field in the peak of streamers of recorded sprites. Adachi et al. (2006, 2008) also found that the reduced electric field is 0.24~1.24 E_k in the upper-diffuse region and 0.81~3.16 E_k in the lower-structured region of sprites. In this work, the sprite emission ratio can be one of the critical parameters to reveal the driving electric field in the plasma chemistry in sprites. We used 28 selected sprite events from July 4th, 2004 to January 22nd, 2008 and January 18th, 2009 to January 24th, 2013 with ULF (Ultra Low Frequency) data from ISUAL array photometer (AP) on FORMOSAT-2 satellite to analyze the sprite emission ratio using the sum, temporal filter and peak three different methods. Compared the sprite emission ratios calculated from photon yield, the results show that the difference in the sprite emission ratio between the sum, temporal filter and peak methods are smaller at higher altitudes and larger at lower altitudes. This has been pointed out by Adachi et al. (2006, 2008) that the difference between different methods is a result of the errors in altitude estimation and the exponential increase of the quenching term k_q [M]. Our results suggest that with altitude decrease, a one-term exponential parameter 〖ae〗^hx should let quencher density [M] become lower than the original [M] in central Africa. In this work, we also compared the sprite emission ratios with plasma chemistry model. The result shows that the sprite emission rates 5.6×〖10〗^16 photon/s for N2 2P and 4.86×〖10〗^16 photon/s for N2 1P at the reduced electric field of 5 E_k. The estimated brightness of N2 2P and N2 1P calculated by the model are 9.33×〖10〗^3 Rayleigh and 8.1×〖10〗^3 Rayleigh, respectively (1 Rayleigh = 106 photon/cm2-column/s). Compared with the sprite emission ratio estimated from the model (1.15), the average sprite emission ratio calculated using the temporal filter method at 70 km height (1.17) has the most similar value among three methods (the sum method: 1.09; the peak method: 1.93). In addition, our data also show the speeds of the downward and upward moving streamers were calculated using time-series signals of ISUAL AP, and it gives a range of ~107 to ~108 m/s. Among the 28 selected sprite events, 15 events have ULF data. Within these 15 sprite events, most of the impulse charge moment changes (iCMCs) are higher than the threshold of producing sprites (>200 C-km) and one of the sprite events shows a relatively stronger peak current of 133 kA. In the future, we can modify the quencher [M], add the model results at lower (~40 km) or higher (~90 km) altitudes and increase the samples of sprite events to understand of the relations of the sprite emission ratio between the three methods and the model results.
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