Naked emergence of an anti-Hale active region

I. Overall evolution and magnetic properties^⋆

¹ Yunnan Observatories, Chinese Academy of Sciences, Kunming Yunnan 650216, PR China
e-mail: wangjincheng@ynao.ac.cn
² CAS Key Laboratory of Solar Activity, National Astronomical Observatories, Beijing 100012, PR China
³ Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
⁴ University of Chinese Academy of Sciences, Yuquan Road, Shijingshan Block, Beijing 100049, PR China
⁵ Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife, Spain
⁶ Universidad de La Laguna, Dpto. Astrofisica, 38206 La Laguna, Tenerife, Spain

Received: 1 March 2021
Accepted: 10 May 2021

Abstract

Aims. In order to understand the emergence of the active region, we investigate the emerging process and magnetic properties of a naked anti-Hale active region during the period between August 24 to 25, 2018.

Methods. Using the data from Helioseismic and Magnetic Imager on board the Soar Dynamic Observatory and the New Vacuum Solar Telescope, we calculated different evolving parameters (such as pole separation, tilt angle) and magnetic parameters (such as vertical electric current, force-free parameter, relative magnetic helicity) during the emergence of the active region. With these calculated parameters and some reasonable assumptions, we use two different methods to estimate the twist of the active region.

Results. The magnetic flux and pole separation continue increasing while the tilt angle exhibits a decreasing pattern during the emergence of the active region. The increase of the pole separation is mainly contributed as a result of the enhancement in the longitude direction. A power-law relationship between pole separation and total flux is found during the emergence of the active region. On the other hand, it is found that both the positive and negative electric currents increased equivalently and the average flux-weighted force-free parameter α̃ remains almost consistently positive, on the order of ∼10⁻⁸ m⁻¹. The relative magnetic helicity is mainly contributed by the shear term, while the relative magnetic helicity injection flux of the shear term changes its sign at the latter stage of the emergence. The twist number of the whole active region remains on the order of 10⁻¹ turns during the emergence of the active region.

Conclusions. We find that the magnetic flux tube with low twist also could emerge into the solar atmosphere.