Leonid L. Kitchatinov

Magnetic fields in fully convective M-dwarfs: oscillatory dynamos versus bistability

M-dwarfs demonstrate two types of activity: (1) strong (kilogauss) almost axisymmetric poloidal magnetic fields; and (2) considerably weaker non-axisymmetric fields, sometimes including a substantial toroidal component. Dynamo bistability has been proposed as an explanation. However, it is not straightforward to obtain such a bistability in dynamo models. On the other hand, the solar magnetic dipole at times of magnetic field inversion becomes transverse to the rotation axis, while the magnetic field becomes weaker at times far from that of inversion. Thus, the Sun resembles a star with the second type of activity. We suggest that M-dwarfs can have magnetic cycles, and that M-dwarfs with the second type of activity can just be stars observed at times of magnetic field inversion. Then the relative number of M-dwarfs with the second type of activity can be used in the framework of this model to determine parameters of stellar convection near the surface.

Reversals of the solar dipole

Context. During a solar magnetic field reversal the magnetic dipole moment does not vanish, but migrates between poles, in contradiction to the predictions of mean-field dynamo theory. Aims. We try to explain this as a consequence of magnetic fluctuations. Methods. We used the statistics of fluctuations to estimate observable signatures. Results. Simple statistical estimates, taken with results from mean-field dynamo theory, suggest that a non-zero dipole moment may persist through a global field reversal. Conclusions. Fluctuations in the solar magnetic field may play a key role in explaining reversals of the solar dipole.

Towards understanding dynamo action in M dwarfs

Recent progress in observational studies of magnetic activity in M dwarfs urgently requires support from ideas of stellar dynamo theory. We propose a strategy to connect observational and theoretical studies. In particular, we suggest four magnetic configurations that appear relevant to dwarfs from the viewpoint of the most conservative version of dynamo theory, and discuss observational tests to identify the configurations observationally. As expected, any such identification contains substantial uncertainties. However the situation in general looks less pessimistic than might be expected. Several identifications between the phenomenology of individual stars and dynamo models are suggested. Remarkably, all models discussed predict substantial surface magnetic activity at rather high stellar latitudes. This prediction looks unexpected from the viewpoint of our experience observing the Sun (which of course differs in some fundamental ways from these late-type dwarfs). We stress that a fuller understanding of the topic requires a long-term (at least 15 years) monitoring of M dwarfs by Zeeman-Doppler imaging.

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

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 released energy. Radionuclide data do not provide evidence 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 can also be excited. For comparable conditions, magnetic field of second configuration is much stronger than that of the first one just because dynamo does not spend its energy for periodic magnetic field inversions but uses it 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. As suggested in a recent paper [1], we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry 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.

Superflares on Giant Stars

The Kepler mission has identified huge flares on various stars, including some solar-type stars. These events are substantially more energetic than solar flares, and are referred to as superflares. Even a low probability of such a superflare occurring on the Sun would be a menace to modern society. A flare comparable in energy to that of superflares was observed on September 24 and 25, 1989 on the binary HK Lac. Unlike the Kepler stars, observations of differential rotation are available for HK Lac. This differential rotation appears to be anti-solar. In the case of anti-solar differential rotation, dynamo models can producemagnetic-activity waves with dipolare symmetry, as well as quasi-stationary magnetic configurations with quadrupolar symmetry. The magnetic energy of such stationary configurations is usually about two orders of magnitude higher than the energy associated with activity waves. We believe that this mechanism could provide sufficient energy to produce superflares on late-type stars. Some simple models in support of this idea are presented.