Quantifying solar superactive regions with vector magnetic field observations

Abstract

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 7.0×10 21 Mx, a total photospheric free magnetic energy higher than 1.0 × 10 24 erg cm −1 , a magnetic neutral line with a steep horizontal magnetic gradient (≥300 G Mm −1 ) longer than 30 Mm, and an area with strong magnetic shear (shear angle ≥80• ) greater than 100 Mm 2 . In contrast, the values of these parameters for the FARs are mostly very low. The Pearson χ 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.

The magnetic field in the solar atmosphere

“…It is worth noting that, such analyses often suffer the problem of projection effects which is noticeable in the degree of flux imbalance systematically increasing with distance from the disk center (Green et al 2003). Chen and Wang (2012) recently revisited the importance of flux imbalance for the eruptive nature of ARs; they only considered ARs that fulfilled certain selection criteria, including being observed within ±30 • from the central meridian and being associated to major flaring activity. They analyzed 14 ARs and found a significant imbalance of magnetic flux: more than half of the ARs showed a flux imbalance of 20 % (see also Tian et al 2002;Romano and Zuccarello 2007).…”

Influence of Photospheric Magnetic Conditions on the Catastrophic Behaviors of Flux Ropes in Solar Active Regions

“…What can be observed is the photospheric magnetic conditions. Determining the influence of the photospheric magnetic conditions on the catastrophe of a flux rope system could not only help us better understand the decisive factors for catastrophe, but also shed light on the flare/ CME productivity of active regions (e.g., Romano & Zuccarello 2007;Schrijver 2007;Wang & Zhang 2008;Chen & Wang 2012;Liu et al 2016). By numerical simulations in spherical coordinates, Sun et al (2007) found that if the global photospheric flux is concentrated too close to the magnetic neutral line, the system loses its catastrophic behavior.…”

Space Weather Related to Solar Eruptions With the ASO-S Mission

“…Chen et al [96] selected 45 SARs in cycles 21-23 according to the following four criteria, i.e., maximum sunspot area, soft X-ray flare index, the 10.7 cm radio peak flux, and short-term total solar irradiance decrease. Chen and Wang [97] studied the vector magnetic field properties of 14 SARs and eight large but inactive ARs, which were called fallow ARs (FARs), in cycles 22 and 23. It revealed that SARs and FARs had significantly different vector magnetic field properties.…”