Solar and stellar flares: frequency, active regions and stellar dynamo

TL;DR

This paper explores how flare frequency relates to active region size and magnetic fields, suggesting a unified model for solar and stellar flares that hinges on dynamo efficiency differences.

Contribution

It demonstrates that flare occurrence depends on active region size and magnetic field strength, proposing a unified model for solar and stellar superflares based on dynamo effectiveness.

Findings

Weak flares show little dependence on spottedness.

Large flares are more common in large active regions.

Solar and stellar flare distributions can be described by a single model.

Abstract

We demonstrate that for weak flares the dependence on spottedness can be rather weak. The fact is that such flares can occur both in small and large active regions. At the same time, powerful large flares of classes M and X occur much more often in large active regions. In energy estimates, the mean magnetic field in starspots can also be assumed equal to the mean field in the sunspot umbra. So the effective mean magnetic field is 900 Mx/cm$^2$ in sunspots and 2000 Mx/cm$^2$ in starspots. Moreover, the height of the energy storage cannot be strictly proportional to A$^{1/2}$. For stars, the fitting factor is an order of magnitude smaller. The analysis of the occurrence rate of powerful solar X-ray flares of class M and X and superflares on stars shows that, with allowance for the difference in the spottedness and compactness of active regions, both sets can be described by a single model. Thus, the problem of superflares on stars and their absence on the Sun is reduced to the problem of difference in the effectiveness of the dynamo mechanisms.