Magnetic cloud fit by uniform-twist toroidal flux ropes
Magnetic cloud fit by uniform-twist toroidal flux ropes
复制标题
通过均匀扭转环形磁通绳拟合磁云
DOI：
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
E. Romashets
中科院分区：
文献类型：
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作者：
M. Vandas;E. Romashets
上下文近年来对太阳风磁云观测的详细研究表明，磁云是一种低扭曲的行星际通量绳。通常，它们的磁场由圆柱体中的轴对称线性无力场（伦德奎斯特场）拟合，相反，其具有朝向通量绳的边界的强且增加的扭曲。因此，另一个领域，轴对称均匀扭曲的无力场在一个圆柱（黄金霍伊尔场）已成为用来分析磁云。目标。磁云是弯曲的，对于某些观测，需要环形而不是圆柱形的通量绳来近似云场。因此，我们试图推导出一个轴对称的均匀扭曲的无力场在一个环，无论是准确的，或近似的，并比较它与观察。方法.从螺线管和力自由的条件下，在环曲面柱坐标方程解析求解。磁云的磁场和速度观测结果与使用非线性最小二乘法获得的解进行了比较。结果得到了环中（近）均匀扭转磁场的三种解。它们都是完全螺线管形的，在高纵横比的极限下，它们倾向于Gold-Hoyle场。第一个解具有完全均匀的扭转，另外两个解具有近似均匀的扭转和近似无力场。磁云观测的分析表明，这些场与观测场的拟合程度与已知的近似线性无力（米勒-特纳）场相同，但它也表明，当使用（几乎）均匀扭转模型场时，环的几何参数可能无法通过拟合可靠地确定。参数集在环形的大小和纵横比方面有很大的不同，产生了相当质量的拟合。
Context. Detailed studies of magnetic cloud observations in the solar wind in recent years indicate that magnetic clouds are interplanetary flux ropes with a low twist. Commonly, their magnetic fields are fit by the axially symmetric linear force-free field in a cylinder (Lundquist field), which in contrast has a strong and increasing twist toward the boundary of the flux rope. Therefore another field, the axially symmetric uniform-twist force-free field in a cylinder (Gold-Hoyle field) has become employed to analyze magnetic clouds. Aims. Magnetic clouds are bent, and for some observations, a toroidal rather than a cylindrical flux rope is needed for a local approximation of the cloud fields. We therefore try to derive an axially symmetric uniform-twist force-free field in a toroid, either exactly, or approximately, and to compare it with observations. Methods. Equations following from the conditions of solenoidality and force-freeness in toroidally curved cylindrical coordinates were solved analytically. The magnetic field and velocity observations of a magnetic cloud were compared with solutions obtained using a nonlinear least-squares method. Results. Three solutions of (nearly) uniform-twist magnetic fields in a toroid were obtained. All are exactly solenoidal, and in the limit of high aspect ratios, they tend to the Gold-Hoyle field. The first solution has an exactly uniform twist, the other two solutions have a nearly uniform twist and approximate force-free fields. The analysis of a magnetic cloud observation showed that these fields may fit the observed field equally well as the already known approximately linear force-free (Miller-Turner) field, but it also revealed that the geometric parameters of the toroid might not be reliably determined from fits, when (nearly) uniform-twist model fields are used. Sets of parameters largely differing in the size of the toroid and its aspect ratio yield fits of a comparable quality.