摘 要

太陽風中存在有多種不連續結構，如震波、切向不連續(tangential discontinuity，TD)與旋轉不連續(rotational discontinuity，RD)。Knetter and Neubauer (2001)運用triangulation方法，分析2001年四個衛星群觀測到的行星際空間之方向不連續結構（包含切向不連續及旋轉不連續），另外也用了較普遍使用的Minimum Variance Analysis (MVA)方法以做比對。結果發現約在Bn/B0＜0.4時才有不連續結構被觀測到，而觀測到的方向不連續結構則大部份為Bn/B0≦0.2，其中B0是RD上游的磁場強度，Bn是不連續面上磁場的法線分量。為什麼Bn/B0＞0.4的旋轉不連續沒有被衛星群所觀測到？本文將探討這個問題。
首先以數值模擬的結果來研究RD存在的條件。本研究採用ㄧ維之混合粒子碼(hybrid code)，以實際太陽風的粒子組成來探討在行星際空間中RD發生之情形。我們討論不同磁場法線夾角θ=5o，15o，25o，35o，…85o （cosθ＝Bn/B0）時，RD隨時間發展的情形。模擬的結果顯示，在0＜Bn/B0＜1時皆存有穩定的RD結構。
我們認為問題的核心並非在於Bn/B0 > 0.4時RD是否能存在，而是RD的發生機制會不會產生出Bn/B0 > 0.4的RD結構。在一般的認知中，太陽風中RD的產生有二個機制：其一是由太陽表面附近的磁場重聯過程中產生(Lee et al.,1995)，其二為直接由太陽風中Alfvén波模擾動的非線性演化而來(Cohen and Kulsrud ,1974)。根據磁場重聯理論與觀測：磁場重聯的重聯速率為V1/VA1 (= Bn/B0)，其值通常≦0.2，因此產生的RD結構亦約略為Bn/B0≦0.2。而由Alfvén波模的非線性演化，得出的結論為：不論Bn/B0值之值為何，皆可有RD結構產生。依照我們的模擬結果，在0＜Bn/B0＜1的RD結構皆是穩定的，因此磁場重聯應是產生太陽風中觀測到的RD的主要來源。

Abstract

There are various discontinuities in the solar wind, such as shock waves, tangential discontinuities (TDs) and rotational discontinuities (RDs). Knetter and Neubarner (2001) reported a statistical study for the observed discontinuity structures by the four cluster spacecraft in the interplanetary space. The result shows the directional discontinuities (either TDs or RDs) were observed only when Bn/B0＜0.4 and most of them occur when Bn/B0≦0.2, where B0 and Bn denote the upstream and normal magnetic fields of a discontinuity, respectively. It challenge us to examine why the RDs are rarely observed in the environment with Bn/B0＞0.2. In this study, we first examine the stability of RD in the one-dimensional hybrid simulations. We analyze the evolution of RDs with different rotation angles of tangential magnetic field () and normal angles ( = cos1(Bn/B0)). Our results show that the rotational discontinuity can stably exist in the solar wind with 0＜Bn/B0＜1. We suggest that RDs observed with Bn/B0≦0.4 are mainly related to the generation condition of RDs, but not on their stability. The interplanetary RDs can be generated from either magnetic reconnections near the solar surface (Lee et al., 1995) or the nonlinear evolution of Alfvén waves in the solar wind (Cohen and Kulsrud, 1974). The theory, simulations and observations show the reconnection rate, V1/VA1 (= Bn/B0), is usually less than 0.2, and the associated RDs could have Bn/B0 < 0.2. On the other hand, the nonlinear Alfvén waves exit without a restriction on Bn/B0. Therefore we suggest that the magnetic reconnection is the main source for the generation of interplanetary rotational discontinuities examined by Knetter and Neubarner(2001).

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