Based mainly on the magnetic field data from the Helioseismic and Magnetic Imager (HMI) on board the Soar Dynamic Observatory (SDO) and the high-resolution data obtained by the one-meter New Vacuum Solar Telescope (NVST) at the Fuxian Solar Observatory of Yunnan Observatories of Chinese Academy of Sciences (CAS), Dr. WANG Jincheng et al. investigate the naked emergence of an anti-Hale active region. The result was published recently in Astronomy and Astrophysics.
Active regions (ARs) are the most visible manifestations of magnetic fields on the Sun, lasting for days or even weeks. It is generally believed that active regions are birthed of the convergence of the emerging magnetic flux. As new magnetic field emerges into the solar atmosphere from subsurface, it would interact either with the ambient magnetic fields and plasma or with itself, resulting in many visible signals in the atmosphere at various atmospheric heights and temperature regimes such as coronal mass ejections (CMEs), flares, eruptions of the filaments, jets, and small-scale bursts.
According to the numerical simulations, a twisted flux tube can emerge from the convection zone into the corona, passing through the photosphere and chromosphere as a result of the magnetic buoyancy or Parker instability, which leads to the formation of an Ω-shaped flux tube. However, some simulation suggested that the weakly twisted flux tube could hardly emerge into the photosphere due to the influence of the horizontal expansion.
Observationally, it is rare to directly observe a high twisted flux tube emerging from the subsurface. The discrepancy between the numerical simulations and the observations raises a significant question: how much twist the flux tube needs for its emergence?
Devoting to understand the above issue, the researchers investigate the emerging process and magnetic properties of a naked anti-Hale active region NOAA 12720 during the period from August 24 to 25, 2018.
In this work, the research team calculates different evolving parameters and magnetic parameters during the emergence of the active region. They found that this active region showed a two-step emergence pattern with a relatively gradual emergence followed by a relatively sudden change to more rapid emergence. It is also found that the pole separation has a power-law dependence on the magnetic flux during the emergence.
Most importantly, the research team uses two different methods which are based on magnetic helicity calculation and force-free parameter to estimate the twist in this anti-Hale active region. They derive that the twist number of the whole active region is on the order of 0.1 turns, meaning that the whole emerging active region was a lowly twisted structure.
Therefore, they conclude that the simple active region does not need too much initial twist in the intrinsic magnetic field for its emergence.
Contact:
WANG Jincheng, YNAO, CAS
wangjincheng@ynao.ac.cn
