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

TL;DR

This paper introduces a non-linear calibration method for the Solar Magnetic Field Telescope data that reduces saturation effects in magnetic field measurements, especially in sunspot umbrae, improving data reliability since 1987.

Contribution

It proposes a novel non-linear relationship calibration for SMFT data, enhancing magnetic field measurement accuracy in strong field regions.

Findings

Eliminates magnetic saturation in sunspot umbrae.

Improves the reliability of long-term magnetic field data.

Enables better analysis of solar magnetic activity.

Abstract

The weak-field approximation implying linear relationship between Stokes $V/I$ and longitudinal magnetic field, $B_{\Vert}$, often suffers from saturation observed in strong magnetic field regions such as sunspot umbra. In this work, we intend to improve the magnetic field observations carried out by the \textit{Solar Magnetic Field Telescope} (SMFT) at Huairou Solar Observing Station, China. We propose using non-linear relationship between Stokes $V/I$ and $B_{\Vert}$ 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 \textit{Helioseismic and Magnetic Imager} on board the \textit{Solar Dynamics Observatory}. The algorithm of the magnetic field derivation is described in details. We show that using non-linear relationship between Stokes $V/I$ and $B_{\Vert}$ allows us to eliminate magnetic field saturation inside 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.