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Fig. 2.

image

Processing steps performed to calculate the best threshold to compute EUV activity masks. Data are from September 22, 2013. (a): HMI magnetogram. (b): magnetogram after resizing and applying Gaussian smoothing over the absolute values. (c): final magnetogram mask, using 25 G as the threshold for feature determinations. (d): square root of the STEREO/SDO composite image from the same solar region. (e): region of the STEREO/SDO image after applying the square-root transformation, resizing, and applying Gaussian smoothing. (f): section of the final EUV mask, using the best threshold determined by the date-threshold fit.

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