Probing the method of correcting Faraday rotation in vector magnetograms Original Research Article

By analyzing the vector magnetograms of Huairou Solar Observing Station (HSOS) taken at the line center (0.0 Å) and the line wing (−0.12 Å) of FeI λ5324.19 Å, we make an estimate of the measured errors in transversal azimuths (δϕ) 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 ∣Bz∣ < 800 G. The main mean azimuth rotations are about 4°, 6°, 7° and 9°, while ∣Bz∣ 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° at the wavelength offset −0.12 Å of the line compared against a previous numerical simulation. We calculate the average electric currents (Jz) of 393 magnetograms in the region 500 G < ∣Bz∣ < 800 G. After the average correction to azimuths with the above estimations, the mean value of current in active regions 〈Jz〉 shows an obvious return-to-zero tendency: changing from the original 0.234 × 10−2 Am−2 to 0.5 × 10−4 Am−2. We also present the mean electric current in the region 20 G < ∣Bz∣ < 500 G, which is −0.6 × 10−4 Am−2, and nearly the same magnitude as the above corrected value (0.5 × 10−4 Am−2). Both are around zero. In addition, we calculate Jz in the region ∣Bz∣ > 500 G (no the upper limit). At 0.5 × 10−5 Am−2, the 〈Jz〉 presents little difference with the value in the region 500 G < ∣Bz∣ < 800 G, which shows those pixels of ∣Bz∣ > 800 G are few in most of the magnetograms and give little contribution to Jz. The correction is promising and Faraday rotations in the region 500 G < ∣Bz∣ < 800 G are main interference effect on the determination of Jz in the vector magnetograms at HSOS.