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Vector Magnetic Field Synoptic Charts from the Helioseismic and Magnetic Imager (HMI)

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Vector magnetic field synoptic charts from the Helioseismic and Magnetic Imager (HMI) are now available for each Carrington Rotation (CR) starting from CR 2097 in May 2010. Synoptic charts are produced using 720-second cadence full-disk vector magnetograms remapped to Carrington coordinates. The vector field is derived from the Stokes parameters (\(I, Q, U, V\)) using a Milne–Eddington-based inversion model. The \(180^{\circ}\) azimuth ambiguity is resolved using the minimum energy algorithm for pixels in active regions and for strong-field pixels (the field is greater than about 150 G) in quiet-Sun regions. Three other methods are used for the rest of the pixels: the potential-field method, the radial acute-angle method, and the random method. The vector field synoptic charts computed using these three disambiguation methods are evaluated. The noise in the three components of the vector magnetic field is generally much higher in the potential-field method charts. The component noise levels are significantly different in the radial-acute charts. However, the noise levels in the random-method charts are lower and comparable. The assumptions used in the potential-field and radial-acute methods to disambiguate the weak transverse field introduce bias that propagates differently into the three vector-field components, leading to unreasonable pattern and artifacts, whereas the random method appears not to introduce any systematic bias. The current sheet on the source surface, computed using the potential-field source-surface model applied to random-method charts, agrees with the best solution (the result computed from the synoptic charts with the minimum energy algorithm applied to each and every pixel in the vector magnetograms) much better than the other two. Differences in the synoptic charts determined with the best method and the random method are much smaller than those from the best method and the other two. This comparison indicates that the random method is better for vector field synoptic maps computed from near-central meridian data. The vector field synoptic charts provided by the Joint Science Operations Center (JSOC) are therefore produced with the random method.

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We thank the numerous team members who have contributed to the success of the SDO mission and particularly to the HMI instrument. We thank the anonymous referee for the suggestion of analyzing the difference between the best synoptic charts (using the minimum method) and the three types of synoptic charts, which made the comparison more effective. The code for calculating the potential field from the LoS field with the FFT method was developed by K.D. Leka and G. Barnes at Northwest Research Associates, Inc., Boulder. This work was supported by NASA Contract NAS5-02139 (HMI) to Stanford University.

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Correspondence to Yang Liu.

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Liu, Y., Hoeksema, J.T., Sun, X. et al. Vector Magnetic Field Synoptic Charts from the Helioseismic and Magnetic Imager (HMI). Sol Phys 292, 29 (2017). https://doi.org/10.1007/s11207-017-1056-9

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  • Magnetic fields, photosphere
  • HMI: vector magnetic field
  • Synoptic chart