太陽日冕中高能電子的無碰撞傳輸; Collisionless Transport of Energetic Electrons in the Solar Corona

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题名:	太陽日冕中高能電子的無碰撞傳輸;Collisionless Transport of Energetic Electrons in the Solar Corona
作者:	李光武;Kuang-Wu Lee
贡献者:	太空科學研究所
关键词:	無碰撞傳輸;電子加速;太陽閃焰;模擬;electron acceleration;plasma;collisionless transport;simulation;solar flare
日期:	2008-06-24
上传时间:	2009-09-22 09:47:27 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	在無碰撞的太陽日冕電漿環境中有許多的瞬間高能現象發生。由於日冕中極低的粒子密度，以及瞬間高能現象的時間尺度遠小於碰撞週期，因此古典的粒子彈性碰撞在此環境中並不適用。在此環境下非碰撞的傳輸效應對於電漿分布的演化有決定性的影響。由最新的太陽閃焰觀測中，我們常可發現所謂的”不連續冪次分佈”，其代表著無碰撞電漿傳輸效應的強大影響力。在太陽日冕物理中，另一個重要的主題是關於太陽閃焰前磁重聯的產生機制。在強磁化的無碰撞電漿中，帶電粒子是被束縛於特定的磁力線上。當磁重聯產生時，由於強電流產生的非碰撞的傳輸效應，帶電粒子的”磁凍結效應”會被打破。而此非碰撞傳輸效應也就是所謂的”異常電阻”。本論文中，我們將首先以線性估算的方式，探討太陽日冕的磁場環境下可產生的異常電阻。對於電漿，我們以多流體的方式來描述系統中產生的電漿波與不穩定性。由先前的模擬結果我們知道，區域性的電漿靜電結構對於電漿傳輸特性有很大的影響。線性頻散分析結果顯示出，兩種靜電不穩定性在此電流環境下成長，他們分別是 Buneman instability 與 modified two stream instability (MTSI)。其頻率以及波數的頻譜也在此分析。為了推估不穩定性在飽和條件下所產生的異常電阻，我們與先前的 Vlasov 模擬結果做比較。有別於傳統上認為 Buneman 不穩定性提供了大部分異常電阻的產生，我們發現斜向傳播的 MTSI 也可產生可觀的異常電阻。在觀測中，通常太陽閃焰所推估的高能電子約是幾十至幾百 keV，然而若以日冕區域的磁重聯出流 Alfvén 速度推估，電子的動能最多只有幾個 keV。為了解決此能量上的不一致，觀測上所看到的日冕環頂 Hard X Ray 來源，因此被推測是一個快速震波。快速震波是一個很有效率的粒子加速機制，許多的二維日冕磁流體模擬也顯示出日冕環頂快速震波的存在。然而因為磁流體所須符合的電中性，以及電子離子等溫條件，磁流體模擬結果並無法解釋所觀測到的能量不一致。對於此一問題，我們以理論推估的磁重聯出流 Alfvén 速度，以及一個反射邊界條件，對日冕環頂快速震波做一維電磁粒子模擬 (1D EM PIC)。結果顯示原本大約 7 keV 的電子經過快速震波後大約可以加速到 120 keV，也就是到達了日冕環頂 Hard X Ray 的能量範圍。另外，對於沿著日冕環頂向色球層傳輸的高能電子流會造成日冕區域違反電中性，反向電流因此被提出來解決此一問題。在此條件下，一個背景質子與兩個對向流動的電子物種存在於系統中。線性分析結果顯示出兩種不同的靜電不穩定性存在，分別是 Buneman 與 electron-electron two stream 不穩定性。以Vlasov模擬在周期性邊界條件下，靜電 Double Layers (electrostatic DLs) 的產生可以耗散反向運動的電子流，而電漿的加熱效應在模擬的飽和階段是自然的結果。在此系統中 DLs 扮演了能量轉換者的角色。此一靜電結構在發展初期將電子流的動能轉換為電漿波的能量，並且在結構發展的飽和階段又將電漿波的靜電場能轉換為電漿的熱能。另一個值得注意的模擬結論是，原本由日冕環頂向下傳輸的高斯分佈電漿，在經過與 DLs 交互作用後逐漸演化成”不連續冪次分佈”。此結果顯示出不連續冪次分佈可能是電漿與日冕環中靜電 Double Layers 的自然結果。 Many energetic and transient plasma phenomena take place in collisionless solar coronal environment. The classical binary collision in the corona takes relatively longer time than the usual time scale of the phenomena, and the mean free path of plasma is around 1 AU in coronal condition. Therefore the collisionless transport dominates the plasma evolution in this environment. From recent observations of solar flares, the spectrum shows a clear broken-power-law distribution, which indicates a strong influence of collisionless plasma transport effect. Another important topic in solar corona physics is the magnetic reconnection which is assumed to take place prior to the solar flare. To break the frozen-in condition in strongly magnetized collisionless plasma, some resistive effect should take place to reduce the curl-B generated current, i.e. the anomalous resistivity causes the current reduction in central current sheet. In this thesis the anomalous resistivity is first studied from a linear estimation in a current carrying system. A multi-fluid description of plasma is used to study the waves and instabilities with coronal plasma parameters. As we know the localized electrostatic structures influence significantly on plasma transport in current carrying system. Two instabilities, the Buneman instability and the modified two stream instability (MTSI), are identified and their frequency wave-number (ω-k) spectra are analyzed. To estimate the resulting anomalous resistivity in the instability saturation stage, we compare with the previous Vlasov simulation results. We found, in contrast to the general understanding that Buneman mode contributes primarily to the generated anomalous resistivity, an obliquely propagating modified two stream instability also generates a non-negligible anomalous resistivity. Because of the energy gap between the coronal reconnection outflow and the observed chromospheric hard X ray (HXR) emission, an existence of loop-top fast shock was proposed for a secondary acceleration of electrons. We performed a one-dimensional electromagnetic particle-in-cell simulation with a reflective boundary, which mimics the coronal soft X ray (SXR) loop. Because of the accumulation of magnetic field, a clear fast mode magnetosonic shock formed on upstream side of the reflective boundary. Electron and ion heating are shown in the downstream of the fast shock. An original reconnection outflow with 7 keV kinetic energy can be finally accelerated to around 120 keV, which is the typical energy level of the observed flare footpoint emissions. As to the frequently observed flare broken-power-law spectrum, which is best fitted by two different distribution index, we are interesting in the generation mechanism of this specific spectrum. For high energy electrons traveling downward to chromosphere in the coronal loops, a return current model is proposed to satisfy the charge neutrality condition in corona and also the Faraday’s induction law in flaring loops. With a stationary background ion species and two counter-streaming electron species, Buneman and electron-electron two stream instability take place in the current-free system. In the Vlasov simulation with periodic boundary condition, electrostatic double layers (DLs) are generated to dissipate the drifts of counter-streaming electrons. Plasma heating is a natural consequence in the final stage of plasma evolution. In the system the electrostatic double layers take the role as an energy converter. DLs first accumulate kinetic energy from electron drifts for its growth of electric field energy, and in the late stage of DLs evolution the wave energy is further converted into thermal energy of plasma. Interestingly the spectrum of downward propagating electron finally shows a broken-power-law distribution, which corresponds to the frequent observed footpoint emissions. The result shows that a broken-power-law spectrum on flare footpoints is possibly a consequence of collisionless plasma transport with electrostatic DLs.
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