The Plasma and Magnetic Field Characteristics of a Double Discontinuity in Interplanetary Space
The Plasma and Magnetic Field Characteristics of a Double Discontinuity in Interplanetary Space
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DOI：
10.1007/s11207-007-0278-7
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发表时间：
2007-03
期刊：
影响因子：
2.8
通讯作者：
P. Zuo;Xueshang Feng
中科院分区：
文献类型：
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作者：
P. Zuo;Xueshang Feng
双间断是一种罕见的复合结构，它由一个慢激波层和一个相邻的下游旋转间断层组成。本文报道了1997年5月15日Wind探测到的双间断的观测结果。这种双间断被认为是磁云边界层的前边界。利用Wind卫星的高分辨率等离子体和磁场资料，严格识别了激波层和旋转不连续层。所观测到的慢激波层上下游区的跳跃条件与朗肯-许贡纽关系符合得很好。激波系中的流速Un <VAcosθ B，在慢激波层两侧。在上游区域，慢马赫数Ms 1 = Un 1/Vs 1为1.95（大于1），在下游区域，慢马赫数Ms 2 = Un 2/Vs 2为0.31（小于1）。其中VA和Vs分别代表阿尔文速度和局部慢磁声速，θ B是磁场方向和激波法线之间的夹角。48分钟后，当航天器位于磁层弓形激波之外时，Geotail也探测到了Wind观测到的磁云边界层。然而，Geotail的观测表明，它的前边界不再是一个双间断，旋转间断层消失，这表明这种双间断在从Wind传播到Geotail时是不稳定的。
A double discontinuity is a rarely observed compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer in the downstream region. In this paper, we report the observations of a double discontinuity detected by Wind on May 15, 1997. This double discontinuity is found to be the front boundary of a magnetic cloud boundary layer. We strictly identify the shock layer and the rotational discontinuity layer by using the high-resolution plasma and magnetic field data from Wind. The observed jump conditions of the upstream and downstream region of the slow shock layer are in good agreement with the Rankine – Hugoniot relations. The flow speeds in the shock frameUn<VAcosθBnon both sides of the slow shock layer. In the upstream region, the slow Mach numberMs1=Un1/Vs1is 1.95 (above unity), and in the downstream region, the slow Mach numberMs2=Un2/Vs2is 0.31 (below unity). HereVAandVsrepresent the Alfvén speed and the local slow magnetosonic speed, respectively, andθBnis the angle between the direction of the magnetic field and the shock normal. The magnetic cloud boundary layer observed by Wind was also detected by Geotail 48 min later when the spacecraft was located outside the bow shock of the magnetosphere. However, Geotail observations showed that its front boundary was no longer a double discontinuity and the rotational discontinuity layer disappeared, indicating that this double discontinuity was unstable when propagating from Wind to Geotail.