On the Hemispheric Bias Seen in Vector Magnetic Field Data

The east–west component of the magnetic field, B ϕ , as observed in solar magnetograms containing quiet regions, is found to change its sign when the regions cross the central meridian. It is seen in both HMI/SDO and VSM/SOLIS full disk vector magnetograms. A mismatch between the calibrated line-of-sight and transverse fields is the reason for this hemispheric bias problem. Here mismatch means that one of the fields is either over-estimated or under-estimated. For HMI data, the transverse field is over-estimated. This mismatch is caused ultimately by a filling factor that is not precisely determined when unresolved structures are present. An updated inversion procedure for HMI observations, developed recently, is able to derive the filling factor with reasonable accuracy. The new data show that the hemispheric bias problem has been mitigated substantially.