Improvements of the Longitudinal Magnetic Field Measurement from the Solar Magnetic Field Telescope at the Huairou Solar Observing Station

The weak-field approximation, implying a linear relationship between Stokes- V/I and the longitudinal magnetic field [B-parallel to], often suffers from saturation observed in strong magneticfield regions such as sunspot umbrae. In this work, we seek to improve the magnetic-field observations carried out by the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station, China. We propose using a non-linear relationship between Stokes-V/I and B-parallel to to derive the magnetic field. To determine the form of the relationship, we perform a cross-calibration of the observed SMFT data and magnetograms provided by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The algorithm for the magnetic-field derivation is described in detail. We show that using a non-linear relationship between Stokes-V/I and B-parallel to allows us to eliminate magnetic-field saturation inside the sunspot umbra. The proposed technique enables one to enhance the reliability of the SMFT magnetic-field data obtained even long before the space-based instrumentation era, since 1987.

Automated summary

This study aims to improve magnetic field observations made by the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station in China, proposing the use of a non-linear relationship for deriving the magnetic field and cross-calibrating observations with magnetograms from the Solar Dynamics Observatory. The non-linear relationship effectively eliminates magnetic field saturation within sunspot umbrae.