Physics
Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999soph..188....3l&link_type=abstract
Solar Physics, v. 188, Issue 1, p. 3-19 (1999).
Physics
44
Scientific paper
This investigation centers upon the quantifying magnetic twist by the parameter α, commonly defined as (∇xB_h)_z/B_z=μ_0J_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 α to describe the active region as a whole, here called `α_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 α(x,y) using moments of its distribution, (2) to determine the slope of the function J_z(x,y)=α_ARB_z(x,y) using a least-squares fit and (3) to determine the value of α for which the horizontal field from a constant-α force-free solution most closely matches the observed horizontal magnetic field. The results are qualitatively encouraging: between methods, the resulting value of the α_AR parameter is often consistent to within the uncertainties, even though the resulting α_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 3σ 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 α_AR, and quantifying the validity of α_AR, are addressed in Paper II (Leka, 1999).
Leka Kimberly Dawn
Skumanich Andrew
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