Vector magnetic field measurement and helicity of active regions

The Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of the National Astronomical Observatory of China has been obtaining photospheric vector magnetograms on a daily basis for more than 20 years. In this brief review, I will first introduce our magnetograph, together with some efforts on correcting for a few known problems in filter-type magnetograph measurement. Then, I will focus on what we can learn from these vector magnetic field measurements with an emphasis on the helicity of active regions. It is generally believed that magnetic helicity is produced by solar dynamo in the convection zone and its accumulation in the corona will influence coronal dynamics. Our observations of helicity in active regions can thus be used to constrain dynamo models and suggest favorable conditions for coronal eruptions