Comparison of polar magnetic fields derived from MILOS and MERLIN inversions for Hinode/SOT-SP data

The detailed investigation of the polar magnetic field and its time evolution is one of the major achievements of Hinode. Precise measurements of the polar magnetic field are essential for understanding the 11-year solar cycle, and they provide important constraints for identifying the source regions of the solar wind. In this study, we use the MILOS inversion code to derive the polar magnetic field in the same way as Shiota et al. (2012), and compare it with the Hinode/SOT-SP Level-2 data derived by the HAO MERLIN inversion code. Both MILOS and MERLIN are Milne-Eddington inversion codes. The method of converting the magnetic field vector to the local vertical and the method of disambiguating the magnetic field azimuth are also applied in the same way to the results with the two inversions. We found that the radial magnetic flux density (the magnetic flux density with respect to the local vertical) tends to be about 1.2 times larger in the results with the MERLIN inversion than in those with the MILOS inversion. This tendency is common to the polar magnetic fields observed at different times of the solar cycle. When we run MILOS with the magnetic filling factor fixed to be equal to the MERLIN values, the radial magnetic flux density derived from the MILOS and HAO inversion codes is almost the same. This suggests that the discrepancy in the radial magnetic flux density is caused by the difference in the magnetic filling factor between the MILOS and MERLIN inversions. The result with the MERLIN inversion tends to have a larger magnetic filling factor and it can be explained by the different assumptions of the scattered light profile in these two inversions. The relationship between the radial magnetic flux density and the magnetic filling factor becomes complex in the polar (limb) observations due to the contribution of the transverse magnetic field component to the estimation of the radial magnetic flux density.