Probing the method of correcting Faraday rotation in vector magnetograms

By analyzing the vector magnetograms of Huairou Solar Observing Station (HSOS) taken at the line center (0.0 angstrom) and the line wing (-0.12 angstrom) of FeI lambda 5324.19 angstrom, we make an estimate of the measured errors in transversal azimuths (60) caused by Faraday rotation. Since many factors, such as the magnetic saturation and scattered light, can affect the measurement accuracy of the longitudinal magnetic field in the umbrae of sunspots, we limit our study in the region vertical bar B-z vertical bar < 800 G. The main mean azimuth rotations are about 4 degrees, 6 degrees, 7 degrees and 9 degrees, while vertical bar B-z vertical bar are in the ranges of 400-500 G, 500-600 G, 600-700 G and 700-800 G, respectively. Moreover, we find there is also an azimuth rotation of about 8 degrees at the wavelength offset -0. 12 angstrom of the line compared against a previous numerical simulation. We calculate the average electric currents (J(z)) of 393 magnetograms in the region 500 G < vertical bar B-z vertical bar < 800 G. After the average correction to azimuths with the above estimations, the mean value of current in active regions < J(z)> shows an obvious return-to-zero tendency: changing from the original 0.234 x 10(-2) Am-2 to 0.5 x 10(-4) Am-2. We also present the mean electric current in the region 20 G < vertical bar B-z vertical bar < 500 G, which is -0.6 x 10(-4) Am-2, and nearly the same magnitude as the above corrected value (0.5 x 10(-4) Am-2). Both are around zero. In addition, we calculate J(z) in the region vertical bar B-z vertical bar > 500 G (no the upper limit). At 0.5 x 10(-5) Am-2, the < J(z >) presents little difference with the value in the region 500 G < vertical bar B-z vertical bar < 800 G, which shows those pixels of vertical bar B-z vertical bar > 800 G are few in most of the magnetograms and give little contribution to J(z) The correction is promising and Faraday rotations in the region 500 G < vertical bar B-z vertical bar < 800 G are main interference effect on the determination of J(z) in the vector magnetograms at HSOS. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.