Electrodynamical processes in sprites derived from FORMOSAT-2/ISUAL measurements
Electrodynamical processes in sprites derived from FORMOSAT-2/ISUAL measurements
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源自 FORMOSAT-2/ISUAL 测量的子画面中的电动力学过程
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发表时间：
2007
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通讯作者：
足立 透
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作者：
足立 透
最近发现的出现在活跃雷暴上空的精灵，为对流层和中高层大气之间的电动力耦合提供了一个新的视角。为了阐明精灵的电动力学过程，我们分析了新开发的FORMOSAT-2卫星上的ISUAL/阵列光度计观测到的光学数据。阵列光度计由两个光学系统组成，测量340-480 nm和510-750 nm两个波长范围，垂直分辨率约14 km，时间分辨率为50微秒。从2004年7月4日到2006年6月25日，ISUAL共观测到482个精灵事件。我们根据它们的形态将这些精灵事件分为三类：光晕事件、光晕流光事件和流光事件。通过使用阵列光度计观测到的蓝/红发射比，我们估计了精灵中的时空分辨电场强度。我们发现在~ 75 km高度有明显的转变，这对应于精灵的上漫射区和下流光区之间的形态转变。扩散区的电场强度为0.6-0.8 Ek，其中Ek为常规击穿场，这支持了理论预期，即在没有明显电离过程的情况下可以产生扩散发射。另一方面，流带区域的流场为1 ~ 2 Ek，比模拟的单流带预测场小几倍。我们认为，这种差异是由于持久的成分，如向上的分支和头部结构的较低部分的精灵。此外，结合阵列光度计数据和极低频磁场数据，我们估计了各精灵类别闪电电荷矩变化的时间演变。在时间尺度较短（≈1 ms）的情况下，发现放电电荷矩值为~ 400 C-km的闪电放电会产生一个精灵晕，而放电电荷矩值为~ 1000 C-km的闪电放电除了晕还会产生精灵飘带。另一方面，长时间尺度（~ 10 ms）和大电荷矩值（~ 1000 C-km）的闪电放电产生无明显光晕的飘带。所得到的结果既考虑了形成流光所必需的随时间不变的常规击穿场，也考虑了由于电子附着过程而随时间增加的光晕光学发射所必需的临界电场。
Recent discovery of sprites occurring above active thunderstorms has visualized a new aspect of the electrodynamic coupling between the troposphere and the middle/upper atmosphere. In order to clarify electrodynamical processes in sprites, we have analyzed optical data observed with the newly developed ISUAL/array photometer onboard the FORMOSAT-2 satellite. The array photometer consists of two optical systems measuring two wavelength ranges of 340-480 nm and 510-750 nm with a vertical resolution of ˜14 km and a temporal resolution of 50 microseconds. During the period from July 4, 2004 to June 25, 2006, the ISUAL has observed 482 sprite events. We have classified these sprite events into three categories based on their morphology: halo events, halo-streamer events and streamer events. By using blue/red emission ratios observed with the array photometer, we have estimated spatiotemporal-resolved electric field intensities in sprites. We have found a distinct transition at an altitude of ˜75 km, which corresponds to the morphological transition between the upper-diffuse region and the lower-streamer region of sprites. The magnitudes of electric fields in the diffuse region are 0.6-0.8 Ek where Ek is the conventional breakdown field, supporting theoretical expectation that diffuse emissions can be produced without significant ionization process. On the other hand, those in the streamer region are 1-2 Ek, which is a few times smaller than the predicted fields in the modeled single streamer. We have suggested that this discrepancy is due to the long-lasting components such as the lower portions of the upward branches and bead structures in sprites. Furthermore, by combining the array photometer data with the ELF magnetic field data, we have estimated the temporal evolutions of lightning charge moment changes in each sprite category. In the case that time scale is short (˜1 ms), it is found that the lightning discharge with a charge moment value of ˜400 C-km produces a sprite halo while the lightning discharge with ˜1000 C-km produces sprite streamers besides a halo. On the other hand, the lightning discharge with a long time scale (˜10 ms) and a large charge moment value (˜1000 C-km) is found to produce streamers without discernible halo. The obtained results are interpreted considering both the conventional breakdown field necessary for the formation of streamers which is invariable with time and the critical electric field necessary for the optical emissions of halos which increases with time due to the electron attachment process.