Can Superflares Occur on the Sun? A View from Dynamo Theory

Abstract

Recent data from the Kepler mission has revealed the occurrence of superflares in sun-like stars which exceed by far any observed solar flares in release of energy. Radionuclides data do not provide evidences for occurrence of superflares on the Sun over the past eleven millennia. Stellar data for a subgroup of superflaring Kepler stars are analysed in an attempt to find possible progenitors of their abnormal magnetic activity. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo-mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation is also possible for excitation. For comparable conditions, magnetic field strength of second configuration is much larger rather of the first one just because dynamo do not spend its efforts for periodic magnetic field inversions but use its for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. Starting from the recent paper [1]), we find that anti-solar differential rotation or anti-solar sign of the mirrorasymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.

Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

“…Furthermore, this study along with previous observational results and global simulations suggest that the slowly rotating stars possess strong large-scale magnetic fields and possibly polarity reversals and cycles. These slowly rotating solar-like stars may also be prone to produce superflares (Maehara et al 2012), which was also suggested by Katsova et al (2018).…”

New estimation of non-thermal electron energetics in the giant solar flare on 28 October 2003 based on Mars Odyssey observations

“…Even our largest energy estimation resulting from the thick target model and fixed E c = 10 keV gives an energy (≈ 5.7 × 10 33 ergs) slightly lower than the limit of Aulanier et al (2013), which means that purely HXR observations (and non-thermal electron energetics derived from them) of this particular flare cannot remove the energetic gap between the strongest solar flares and flares on the Suntype stars. Probably some other dynamo mechanism should operate on stars with superflares (see, e.g., Katsova et al 2018;Brandenburg and Giampapa 2018), but this topic is outside the scope of the present work.…”

Retrospective analysis of GLEs and estimates of radiation risks

“…Undoubtedly, above doubts and disputes have serious reasons, and it seems to be not accidental that a principal debate on methodological topics is still continuing (e.g., Smart et al, 2014Smart et al, , 2016Duderstadt et al, 2014Duderstadt et al, , 2016aMelott et al, 2016;Wolff et al, 2016;Mekhaldi et al, 2017;Sukhodolov et al, 2017;Katsova et al, 2018;. In particular, Smart et al (2014) did not accept the ''closure'' of the event AD 1859 by Wolff et al (2012) and brought new arguments in favor of their nitrate method.…”

“…As it turned out, these debates are not only of methodological interest, but are of a fundamental nature. According to the current thinking, dominating at present, the nitrate method is not suitable at all for studying past SEP events (e.g., Wolff et al, 2012Wolff et al, , 2016Duderstadt et al, 2014Duderstadt et al, , 2016aMekhaldi et al, 2015Mekhaldi et al, , 2017Sukhodolov et al, 2017;Katsova et al, 2018). In their opinion, this is partly due to equivocal detection of nitrate spikes in single ice cores and possible alternative sources, such as biomass burning plumes, especially in the case of AD1859 event (Wolff et al, 2012).…”