Quantifying solar superactive regions with vector magnetic field observations

Context. The vector magnetic field characteristics of superactive regions (SARs) hold the key for understanding why SARs are extremely active and provide the guidance in space weather prediction. Aims. We aim to quantify the characteristics of SARs using the vector magnetograms taken by the Solar Magnetic Field Telescope at Huairou Solar Observatory Station. Methods. The vector magnetic field characteristics of 14 SARs in solar cycles 22 and 23 were analyzed using the following four parameters: 1) the magnetic flux imbalance between opposite polarities; 2) the total photospheric free magnetic energy; 3) the length of the magnetic neutral line with its steep horizontal magnetic gradient; and 4) the area with strong magnetic shear. Furthermore, we selected another eight large and inactive active regions (ARs), which are called fallow ARs (FARs), to compare them with the SARs. Results. We found that most of the SARs have a net magnetic flux higher than 7.0x10(21) Mx, a total photospheric free magnetic energy higher than 1.0x10(24) erg cm(-1), a magnetic neutral line with a steep horizontal magnetic gradient (>= 300 GMm(-1)) longer than 30 Mm, and an area with strong magnetic shear (shear angle >= 80 degrees) greater than 100 Mm(2). In contrast, the values of these parameters for the FARs are mostly very low. The Pearson chi(2) test was used to examine the significance of the difference between the SARs and FARs, and the results indicate that these two types of ARs can be fairly distinguished by each of these parameters. The significance levels are 99.55%, 99.98%, 99.98%, and 99.96%, respectively. However, no single parameter can distinguish them perfectly. Therefore we propose a composite index based on these parameters, and find that the distinction between the two types of ARs is also significant with a significance level of 99.96%. These results are useful for a better physical understanding of the SAR and FAR.