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Resolving the 180◦ Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions  

K D Leka   Submitted: 2009-08-11 18:08

The objective testing of algorithms for performing ambiguity resolution in vector magnetic field data is continued, with an examination of the effects of noise in the data. Through the use of analytic magnetic field models, two types of noise are "added" prior to resolving: noise to simulate Poisson photon noise in the observed polarization spectra, and a spatial binning to simulate the effects of unresolved structure. The results are compared through the use of quantitative metrics and performance maps. We find that while no algorithm severely propagates the effects of Poisson noise beyond very local influences, some algorithms are more robust against high photon-noise levels than others. In the case of limited spatial resolution, loss of information regarding fine-scale structure can easily result in erroneous solutions. Our tests imply that photon noise and limited spatial resolution can act so as to make assumptions used in some ambiguity resolution algorithms no longer consistent with the observed magnetogram.We confirm a finding of the earlier comparison study, that results can be very sensitive to the details of the treatment of the observed boundary and the assumptions governing that treatment. We discuss the implications of these findings, given the relative sensitivities of the algorithms to the two sources of noise tested here. We also touch on further implications for interpreting observational vector magnetic field data for general solar physics research.

Authors: K.D. Leka ? Graham Barnes ? A.D. Crouch ? Thomas R. Metcalf ? G. Allen Gary ? Ju Jing ? Y. Liu
Projects: Hinode/SOT

Publication Status: Solar Physics (accepted)
Last Modified: 2009-08-12 07:25
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Photospheric Magnetic Field Properties of Flaring Versus Flare-Quiet Active Regions. IV. A Statistically Significant Sample  

K D Leka   Submitted: 2006-12-04 10:54

Statistical tests based on linear discriminant analysis are applied to numerous photospheric magnetic parameters, continuing toward the goal of identifying properties important for the production of solar flares. For this study, the vector field data are U. Hawai?i Imaging Vector Magnetograph daily magnetograms obtained between 2001-2004. Over 1,200 separate magnetograms of 496 numbered active regions comprise the dataset. At the Soft X-Ray C1.0 level, 359 magnetograms are considered "flare productive" in the 24 hours post-observation. Considering multiple photospheric variables simultaneously indicates that combinations of only a few familiar variables encompass the majority of the predictive power available. However, the choice of which few variables is not unique, due to strong correlations among photospheric quantities such as total magnetic flux and total vertical current, two of the most powerful predictors. The best discriminant functions result from combining one of these with additional uncorrelated variables, such as measures of the magnetic shear, and successfully classify over 80% of the regions. By comparison, a success rate of approximately 70% is achieved by simply classifying all regions as "flare quiet". Redefining flare-productive at the M1.0 level, parameterizations of excess photospheric magnetic energy outperform other variables. However, the uniform flare-quiet classification rate is approximately 90%, while incorporating photospheric magnetic field information results in at most a 93% success rate. We demonstrate using nonparametric discriminant analysis that the results are quite robust. Thus we conclude that the state of the photospheric magnetic field at any given time has limited bearing on whether that region will be flare productive.

Authors: K D Leka and G. Barnes
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2006-12-04 11:09
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Abstracts by Author
Resolving the 180◦ Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions
Photospheric Magnetic Field Properties of Flaring Versus Flare-Quiet Active Regions. IV. A Statistically Significant Sample

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