The Role of Twist in Kinked Flux Rope Emergence and Delta-spot Formation

Abstract

It has been observationally well established that the magnetic configurations most favorable for producing energetic flaring events reside in δ-spots, a class of sunspots defined as having opposite polarity umbrae sharing a common penumbra. They are frequently characterized by extreme compactness, strong rotation and anti-Hale orientation. Numerous studies have shown that nearly all of the largest solar flares originate in δ-spots, making the understanding of these structures a fundamental step in predicting space weather. Despite their important influence on the space environment, surprisingly little is understood about the origin and behavior of δ-spots. In this paper, we perform a systematic study of the behavior of emerging flux ropes to test a theoretical model for the formation of δ-spots: the kink instability of emerging flux ropes. We simulated the emergence of highly twisted, kink-unstable flux ropes from the convection zone into the corona, and compared their photospheric properties to those of emerged weakly twisted, kink-stable flux ropes. We show that the photospheric manifestations of the emergence of highly twisted flux ropes closely match the observed properties of δ-spots, and we discuss the resulting implications for observations. Our results strongly support and extend previous theoretical work that suggested that the kink instability of emerging flux ropes is a promising candidate to explain δ-spot formation, as it reproduces their key characteristics very well.

Signatures of Magnetic Flux Ropes in the Low Solar Atmosphere Observed in High Resolution

“…that as magnetic fields are advected, stretched, and compressed during their emergence, a highly sheared magnetic polarity inversion line (PIL) of the δ spot is formed with an elongated, alternating pattern of both polarities, which is greatly reminiscent of magnetic channels. The simulation results of Knizhnik et al (2018) corroborate the kink-unstable emerging flux ropes as a promising mechanism of δ spot formation, and their results also showed the development of elongated, secondary oppositepolarity regions between primary polarities, which are similar to the observed magnetic channels. Therefore, it is appealing to suggest that magnetic channels are a photospheric signature of emerging flux ropes.…”

Electric Current Neutralization in Solar Active Regions and Its Relation to Eruptive Activity

“…This behavior can also be seen in AR 11072, which is discussed in Liu et al (2017). There is some indication of this behavior in Török et al (2014) and Knizhnik et al (2018). However, none of these simulations indicate that currents should become neutralized alongside flux emergence.…”

Flare-productive active regions

“…Later, Knizhnik et al. ( 2018 ) surveyed the evolution of kink-unstable tubes with varying the twist intensity. They revealed, for example, that the separation of both polarities on the surface becomes smaller (i.e., more compact) with increasing the twist, which underpins the kink instability as a promising candidate for explaining -spot formation.…”