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

• Published in: Solar physics Vol. 296; no. 11
• Main Authors: Plotnikov, Andrei, Kutsenko, Alexander, Yang, Shangbin, Xu, Haiquing, Bai, Xianyong, Zhang, Hongqi, Kuzanyan, Kirill
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.11.2021; Springer Nature B.V
• Subjects: Algorithms; Astrophysics and Astroparticles; Atmospheric Sciences; Instrumentation; Magnetic fields; Magnetic measurement; Physics; Physics and Astronomy; Saturation; Solar activity; Solar magnetic field; Solar observatories; Solar physics; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Sunspot umbrae; Sunspots; Magnetic fields, photosphere; Instrumentation and data management; Instrumental effects
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-021-01901-6

The weak-field approximation, implying a linear relationship between Stokes- V / I and the longitudinal magnetic field [ B ∥ ], often suffers from saturation observed in strong magnetic-field 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 ∥ 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 ∥ 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.