Magnetic helicity and eruptivity in active region 12673

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: kmorait@phys.uoa.gr
² Institute for Astronomy, University of Hawaii at Manoa, Pukalani, HI 96768-8288, USA

Received: 11 May 2019
Accepted: 13 July 2019

Abstract

Context. In September 2017, the largest X-class flare of solar cycle 24 occurred from the most active region (AR) of this cycle, AR 12673. This AR attracted much interest because of its unique morphological and evolution characteristics. Among the parameters that were examined in the AR was magnetic helicity, but either only approximately, or intermittently, or both.

Aims. We here study the evolution of the relative magnetic helicity and of the two components of its decomposition, the non-potential, and the volume-threading one, in the time interval around the highest activity of AR 12673. We especially focus on the ratio of the non-potential to total helicity, which has recently been proposed as an indicator of AR eruptivity.

Methods. We first approximated the coronal magnetic field of the AR with two different optimization-based extrapolation procedures, and chose the method that produced the most reliable helicity value at each instant. Moreover, in one of these methods, we weighted the optimization by the uncertainty estimates derived from the Helioseismic and Magnetic Imager (HMI) instrument for the first time. We then followed an accurate method to compute all quantities of interest.

Results. The first observational determination of the evolution of the non-potential to total helicity ratio seems to confirm the quality it has in indicating eruptivity. This ratio increased before the major flares of AR 12673 and afterwards relaxed to lower values. Additionally, we discuss the evolution patterns of the various helicity and energy budgets of AR 12673 and compare them with results from other works.