The magnetic field environment of active region 12673 that produced the energetic particle events of September 2017

TL;DR

This study examines the magnetic environment of active region 12673 to understand its role in producing solar energetic particle events, focusing on magnetic configurations, CME behavior, and magnetic connectivity to Earth.

Contribution

It provides a detailed analysis of the magnetic field environment of AR 12673 and its influence on SEP production, highlighting the importance of magnetic topology and CME expansion.

Findings

SEP-productive events involve null point reconnection or CME expansion into open field

Fast CMEs (>1500 km/s) are associated with SEP events

Magnetic connectivity and CME propagation influence SEP escape and detection

Abstract

Forecasting solar energetic particles (SEPs), and identifying flare/CMEs from active regions (ARs) that will produce SEP events in advance is extremely challenging. We investigate the magnetic field environment of AR 12673, including the AR's magnetic configuration, the surrounding field configuration in the vicinity of the AR, the decay index profile, and the footpoints of Earth-connected magnetic field, around the time of four eruptive events. Two of the eruptive events are SEP-productive (2017 September 4 at 20:00~UT and September 6 at 11:56~UT), while two are not (September 4 at 18:05~UT and September 7 at 14:33~UT). We analysed a range of EUV and white-light coronagraph observations along with potential field extrapolations and find that the CMEs associated with the SEP-productive events either trigger null point reconnection that redirects flare-accelerated particles from the flare site to the Earth-connected field and/or have a significant expansion (and shock formation) into the open Earth-connected field. The rate of change of the decay index with height indicates that the region could produce a fast CME ($v >$ 1500~km~s$^{-1}$), which it did during events two and three. The AR's magnetic field environment, including sites of open magnetic field and null points along with the magnetic field connectivity and propagation direction of the CMEs play an important role in the escape and arrival of SEPs at Earth. Other SEP-productive ARs should be investigated to determine whether their magnetic field environment and CME propagation direction are significant in the escape and arrival of SEPs at Earth.