Long-Term Variations in Background Bias and Magnetic Field Noise in HSOS/SMFT Observations

The Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS) has conducted continuous observations of solar vector magnetic fields for nearly four decades, and while the primary optical system remains unchanged, critical components—including filters, polarizers, and detectors—have undergone multiple upgrades and replacements. Maintaining data consistency is essential for reliable long-term studies of magnetic field evolution and solar activity, as well as current helicity. In this study, we systematically analyze background bias and noise levels in SMFT observations from 1988 to 2019. Our dataset comprises 12,281 vector magnetograms of 1484 active regions. To quantify background bias, we computed mean values of Stokes Q/I, U/I and V/I over each entire magnetogram. The background bias of Stokes V/I is small for the whole dataset. The background biases of Stokes Q/I and U/I fluctuate around zero during 1988–2000. From 2001 to 2011, however, the fluctuations in the background bias of both Q/I and U/I become significantly larger, exhibiting mixed positive and negative values. Between 2012 and 2019, the background biases shift to predominantly positive values for both Stokes Q/I and U/I parameters. To address this issue, we propose a potential method for removing the background bias and further discuss its impact on the estimation of current helicity. For each magnetogram, we quantify measurement noise by calculating the standard deviation (σ) of the longitudinal (Bl) and transverse (Bt) magnetic field components within a quiet-Sun region. The noise levels for Bl and Bt components were approximately 15 Gauss (G) and 87 G, respectively, during 1988–2011. Since 2012, these values decreased significantly to ∼6 G for Bl and ∼55 G for Bt, likely due to the installation of a new filter.