非對稱電流片磁重聯產生之不連續面與膨脹波

Formation of discontinuities and expansion waves in the outflow region of magnetic reconnection in an asymmetric current sheet

徐彭伯堯(Boryau Hsupeng)

指導教授 ： 李羅權

國立中央大學/地球科學學院/太空科學研究所/碩士(2015年)

摘要

在太陽風、磁層頂以及夜側電漿片所觀察到的電流片時為非對稱的，在這類電流片裡兩側的電漿密度與磁場大小並不一樣。我們利用混合粒子碼模擬非對稱電流片磁重聯產生的MHD不連續結構與膨脹波。在對稱的電流片中，我們找到了四種複合結構：(1)RD-SS複合結構：慢震波(slow shock，SS)連在旋轉不連續(rotational discontinuity，RD)的下游；(2)SS-RD複合結構：旋轉不連續接連在慢震波之後；(3)SS-RD-SS複合結構：旋轉不連續被困在慢震波裡；以及(4)關閉慢震波(switch-off slow shock，SSS)。在非對稱的電流片中，我們發現在較高密度側的旋轉不連續的磁場旋轉角度通常比低電漿密度側的來得大。在非對稱電流片的模擬中，單獨的旋轉不連續、單獨的慢震波、或是一對各自孤立的旋轉不連續以及慢震波，可產生於電流片的其中一側。在更不對稱的電流片裡，低電漿密度側內的旋轉不連續有可能不存在。在非常不對稱的電流片裡，我們如預期的(Levy et al. [1964])在電漿密度非常高的那一側找到旋轉不連續與其後方的膨脹慢波(slow expansion wave，SE)。

關鍵字

不連續面 ； 膨脹波 ； 電流片 ； 磁重聯

並列摘要

The current sheets observed in the solar wind, magnetopause, and nightside plasma sheet can be asymmetric, in which the plasma densities and/or magnetic field magnitudes on the two sides of the current sheet are not equal. A hybrid code is used to simulate the 1-D Riemann problem for the generation and evolution of MHD discontinuities and expansion waves in the outflow region of magnetic reconnection in an asymmetric current sheet. In a symmetric current sheet, four types of compound structures are found: (a) RD-SS compound structure: slow shock (SS) is attached to the downstream of rotational discontinuity (RD), (b) SS-RD: SS is followed by an adjacent RD, (c) SS-RD-SS: RD is trapped inside SS, and (d) switch-off slow shock (SSS). In the asymmetric current sheet, the rotational angle of magnetic field across an RD on the side with a higher plasma density is usually larger than that with a lower plasma density. In the asymmetric cases, only a pure RD, a single SS, or a pair of separated RD and SS may appear on one side of the current sheet. When the asymmetry is further increased, RD may become absent in the low density side. For a highly asymmetric current sheet, a slow expansion wave (SE) is formed behind the rotational discontinuity (RD) on the side with a very high plasma density, as expected from Levy et al. [1964].

並列關鍵字

discontinuity ； expansion wave ； current sheet ； magnetic reconnection

參考文獻

[1] Landau, L. D. and Lifshitz, E. M., “Electrodynamics of continuous media”, Course of theoretical physics, Oxford: Pergamon Press, 1960.

[2] Lin, Y. and Lee, L. C., “Structure of Reconnection Layers in the Magnetosphere”, Space Science Reviews, Volume 65, Issue 1-2, pp. 59-179, March 1993.

[3] Chao, J. K. and Goldstein, B., “Modification of the Rankine-Hugoniot relations for shocks in space”, Journal of Geophysical Research, Volume 77, Issue 28, p. 5455, 1972.

[4] Burlaga, L. F. and Ness, N. F., “Tangential Discontinuities in the Solar Wind”, Solar Physics, Volume 9, Issue 2, pp.467-477, Octobers 1969.

[5] Burlaga, L. F., “Nature and origin of directional discontinuities in the solar wind”, Journal of Geophysical Research, Volume 76, Issue 19, p. 4360, 1971.