A Two-step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673

Abstract

Solar flares are often associated with coronal eruptions, but there are confined ones without eruption, even for some X-class flares. How such large flares occurred and why they are confined are still not well understood. Here we studied a confined X2.2 flare in NOAA 12673 on 2017 September 6. It exhibits two episodes of flare brightening with rather complex, atypical ribbons. Based on topology analysis of extrapolated coronal magnetic field, we revealed that there is a two-step magnetic reconnection process during the flare. Prior to the flare, there is a magnetic flux rope (MFR) with one leg rooted in a rotating sunspot. Neighboring to the leg is a magnetic null-point structure. The sunspot drives the MFR to expand, pushing magnetic flux to the null point, and reconnection is first triggered there. The disturbance from the null-point reconnection triggers the second reconnection, i.e., a tether-cutting reconnection below the rope. However, these two reconnections failed to produce an eruption, because the rope is firmly held by its strapping flux. Furthermore, we compared this flare with an eruptive X9.3 flare in the same region with 2 hours later, which has a similar MFR configuration. The key difference between them is that, for the confined flare, the MFR is fully below the threshold of torus instability, while for the eruptive one, the MFR reaches entirely above the threshold. This study provides a good evidence supporting that reconnection alone may not be able to trigger eruption, rather, MHD instability plays a more important role.

A Study of Pre-flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

“…1. In consistence with many previous case studies (e.g., Rust 2003;Gibson et al 2006;Canou et al 2009;Green & Kliem 2009;Yeates & Mackay 2009;Canou & Amari 2010;Savcheva et al 2015;Su et al 2015;Yurchyshyn et al 2015;Zou et al 2019), MFRs generally exists prior to major solar flares. With a rigorous definition, there are over 90% of the studied events have well-defined MFRs in the flare site, i.e., a coherent group of magnetic field lines with twist above one turn and the field line possessing the peak value of twist as being the rope axis.…”

Time-dependent data-driven coronal simulations of AR 12673 from emergence to eruption

“…This active region and its eruptions have been the subject of a number of studies (e.g. Yang et al 2017;Chertok et al 2018;Hou et al 2018;Inoue et al 2018;Liu et al 2018;Verma 2018;Yan et al 2018;Morosan et al 2019;Romano et al 2019;Zou et al 2019). During its transit across the solar disc, this AR produced four GOES X-class flares and numerous weaker flares.…”

“…it occurred without a CME eruption), while the other one was associated with a CME (e.g. Liu et al 2018;Zou et al 2019). This was a halo CME and had a plane-of-the-sky speed of approximately 1500 km s −1 .…”

“…In addition, Yan et al (2018) presented a detailed study of two solar flares (including the X9.3 flare) on 6 September from AR 12673 and also suggested that the shearing motions and rotations of the sunspots were important contributors to the formation and eruption of the flux ropes. Zou et al (2019) also performed a topological analysis of the coronal magnetic field extrapolated with an NLFFF model. This study suggested that reconnection occurred during both flares and again emphasised the role of sunspot rotation in driving the expansion of the flux rope that subsequently triggers the reconnection.…”

Ideal MHD instabilities for coronal mass ejections: interacting current channels and particle acceleration

“…3 The current sheet details, obtained by a finite beta, isothermal MHD simulation from [56], underneath the kink-unstable flux rope evolution. [94]: the purple flux rope only erupts much later, despite successive reconnections in its vicinity. This is consistent with a failure to exceed the torus instability treshold.…”