On the value of 'αAR' from vector magnetograph data -: I.: Methods and caveats
On the value of 'αAR' from vector magnetograph data -: I.: Methods and caveats
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DOI：
10.1023/a:1005108632671
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发表时间：
1999-08-01
期刊：
影响因子：
2.8
通讯作者：
Skumanich, A
中科院分区：
文献类型：
--
作者：
Leka, KD;Skumanich, A
本研究的重点是通过参数α(通常定义为（del x B-h）(z)/B-z = mu（0)J(z)/B-z）量化磁扭，并从矢量磁图数据中推导出它。该参数可以在测量向量B的每个空间点上进行评估，但也可以计算单个alpha值来描述整个活动区域，这里称为“alpha（AR）”。我们测试了三种方法来计算这样一个参数，检查数据噪声对结果的影响，并讨论了与分配这样一个数量相关的限制。讨论的三种方法是：(1)使用α (x,y)分布的矩来参数化α (x,y)的分布，(2)使用最小二乘拟合来确定函数J(z)(x,y) = α (AR)B(z)（x,y）的斜率，以及(3)确定α的值，其中常数α无力解的水平场与观测到的水平磁场最接近。结果在质量上是令人鼓舞的：在不同的方法之间，alpha（AR）参数的结果值通常在不确定度范围内是一致的，即使结果alpha（AR）可能在大小上不同，在某些情况下也可能在符号上不同。当对数据采用最小噪声阈值时，最严重的差异发生。当计算仅限于3西格玛或更好的检测时，实际上，三种方法之间在数量上是相当一致的。然而，使用不同的方法直接比较不同的活动区域必须谨慎进行。讨论了不同方法的差异、一致性和总体稳健性。论文II （Leka, 1999）讨论了仪器限制（空间分辨率和受限视场）对活动区域alpha（AR）的影响，以及量化alpha（AR）的有效性。
This investigation centers upon the quantifying magnetic twist by the parameter alpha, commonly defined as (del x B-h)(z)/B-z = mu(0)J(z)/B-z, and its derivation from vector magnetograph data. This parameter can be evaluated at each spatial point where the vector B is measured, but one may also calculate a single value of alpha to describe the active region as a whole, here called 'alpha(AR)'. We test three methods to calculate such a parameter, examine the influence of data noise on the results, and discuss the limitations associated with assigning such a quantity. The three methods discussed are (1) to parameterize the distribution of alpha(x,y) using moments of its distribution, (2) to determine the slope of the function J(z)(x,y) = alpha(AR)B(z)(x,y) using a least-squares fit and (3) to determine the value of alpha for which the horizontal field from a constant-alpha force-free solution most closely matches the observed horizontal magnetic field. The results are qualitatively encouraging: between methods, the resulting value of the alpha(AR) parameter is often consistent to within the uncertainties, even though the resulting alpha(AR) can differ in magnitude, and in some cases in sign as well. The worst discrepancies occur when a minimal noise threshold is adopted for the data. When the calculations are restricted to detections of 3 sigma or better, there is, in fact, fair quantitative agreement between the three methods. Still, direct comparison of different active regions using disparate methods must be carried out with caution. The discrepancies, agreements, and overall robustness of the different methods are discussed. The effects of instrumental limitations (spatial resolution and a restricted field-of-view) on an active-region alpha(AR), and quantifying the validity of alpha(AR), are addressed in Paper II (Leka, 1999).