Flare-productive active regions

Abstract

Strong solar flares and coronal mass ejections, here defined not only as the bursts of electromagnetic radiation but as the entire process in which magnetic energy is released through magnetic reconnection and plasma instability, emanate from active regions (ARs) in which high magnetic non-potentiality resides in a wide variety of forms. This review focuses on the formation and evolution of flare-productive ARs from both observational and theoretical points of view. Starting from a general introduction of the genesis of ARs and solar flares, we give an overview of the key observational features during the long-term evolution in the pre-flare state, the rapid changes in the magnetic field associated with the flare occurrence, and the physical mechanisms behind these phenomena. Our picture of flare-productive ARs is summarized as follows: subject to the turbulent convection, the rising magnetic flux in the interior deforms into a complex structure and gains high non-potentiality; as the flux appears on the surface, an AR with large free magnetic energy and helicity is built, which is represented by -sunspots, sheared polarity inversion lines, magnetic flux ropes, etc; the flare occurs when sufficient magnetic energy has accumulated, and the drastic coronal evolution affects magnetic fields even in the photosphere. We show that the improvement of observational instruments and modeling capabilities has significantly advanced our understanding in the last decades. Finally, we discuss the outstanding issues and future perspective and further broaden our scope to the possible applications of our knowledge to space-weather forecasting, extreme events in history, and corresponding stellar activities. Electronic supplementary material The online version of this article (10.1007/s41116-019-0019-7) contains supplementary material, which is available to authorized users.

Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms Around the Carrington Event in 1859

“…In theory, the white-light brightenings, which are the foot points of strong electron beaming from the reconnection site, should be located on both sides of the polarity inversion line. If this is the case, the polarity inversion line crosses the middle of this spot group, indicating a delta configuration, the most flare-productive category of the sunspots (see Toriumi et al, 2017;Toriumi & Wang, 2019;Zirin & Liggett, 1987).…”

Temporal Evolution of Spatially Resolved Individual Star Spots on a Planet-hosting Solar-type Star: Kepler-17

“…In the Kepler 17, we cannot find any stellar flares in the light curve, although the stellar brightness variations indicate the existence of large star spots that have a potential to produce a superflare (> per one year (Maehara et al 2017), so such stars without superflares during Kepler's observational period can be classified to have relatively less flare-productive spots. One possibility is that the large spots without any flares, like spots on Kepler 17, can have a simple polarity shape (e.g., α-type or β-type spots), which is known to rarely produce extreme flares in the case of the sunspots (e.g., Toriumi & Wang 2019). On sunspots, complex spots show relatively fast decay, so that lifetimes of star spots can be an indicator of spot complexity and flare productivity.…”

“…Investigations on the temporal evolution of individual star spots are at present a major challenge but quite important for several kinds of fields. (i) Spots are essential for the occurrence of flares, not only on the Sun (see, Toriumi & Wang 2019, and references therein) but also on the stars (e.g., Shibata et al 2013), thus the flux emergence on stellar surface is a key to understand why and how stellar superflares occur. (ii) The decay pattern of spots is thought to be related to the convective diffusion, which can constrain the stellar diffusion coefficient parameters on performing numerical modeling of the stellar dynamo.…”

Спектр Мощности Магнитного Поля В Невозмущенной Фотосфере Солнца

“…Сильные локальные магнитные поля сосредоточены в группах солнечных пятен, т. е. в активных областях, и ими обусловлена вспышечная активность Солнца. Такие поля подвергаются интенсивным исследованиям как в прошлом, так и в настоящее время (см., например, недавний обзор Toriumi and Wang, 2019). Однако солнечная фотосфера лишь не более чем на 20 % покрыта активными областями (АО), а остальная ее часть занята так называемыми невозмущенными областями, где также присутствует магнитное поле.…”