Role of longitudinal activity complexes for solar and stellar dynamos

Abstract

In this paper we first discuss observational evidence of longitudinal concentrations of magnetic activity in the Sun and rapidly rotating late-type stars with outer convective envelopes. Scenarios arising from the idea of rotationally influenced anisotropic convective turbulence being the key physical process generating these structures are then presented and discussed - such effects include the turbulent dynamo mechanism, negative effective magnetic pressure instability (NEMPI) and hydrodynamical vortex instability. Finally, we discuss non-axisymmetric stellar mean-field dynamo models, the results obtained with them, and compare those with the observational information gathered up so far. We also present results from a pure alpha-squared mean-field dynamo model, which show that time-dependent behavior of the dynamo solutions can occur both in the form of an azimuthal dynamo wave and/or oscillatory behavior related to the alternating energy levels of the active longitudes.Comment: 12 pages, 6 figures, to appear in the IAUS 294 proceeding

Multiperiodicity, modulations and flip-flops in variable star light curves

“…Krause & Rädler 1980;Küker & Rüdiger 1999;Moss et al 1995;Tuominen et al 2002;Mantere et al 2013) show stable drift periods, which makes it equally hard to explain the results obtained in this work with the azimuthal dynamo wave scenario. We note, however, that the recent models of Mantere et al (2013) indicate that the oscillations in the magnetic activity level are probably connected to the cycle length of the drift of the non-axisymmetric structures. Using the drift period P spot obtained in this work, an oscillation in the magnetic energy level with a period of…”

“…Because of the small differential rotation, the α 2 or α 2 Ω dynamo scenarios that are widely used to model these objects can still be judged to be perfectly valid tools; these models consistently show azimuthal dynamo waves both in the kinematic (velocity field given, see e.g. Mantere et al 2013) and non-linear regimes (velocity field and differential rotation profile are solved for, see e.g. Tuominen et al 2002).…”

“…In addition to the dynamo wave changing its position in the phase-time plot, the solutions may exhibit oscillatory behavior concerning the energy level of the mean magnetic field, reminiscent of a stellar cycle. In a recent study of Mantere et al (2013), the modeling showed that the drift cycle length (i.e., the time required for the non-axisymmetric magnetic structure to make a full rotation in the rotational frame of reference) and the oscillation period of the magnetic energy were always connected. It has to be noted that in this particular study, only a very small fraction of the parameter space was mapped, within which solutions for which the drift cycle length and oscillation of the magnetic energy levels coincided were found to be preferred.…”

Multiperiodicity, modulations, and flip-flops in variable star light curves

“…The phase behavior, if not due to differential rotation nor latitudinal dynamo waves, could also be a manifestation of an azimuthal dynamo wave, predicted to be excited in rapid rotators (e.g., Krause & Raedler 1980), verified from mean-field dynamo models (e.g., Moss et al 1995;Küker & Rüdiger 1999;Mantere et al 2013), and also now found from direct numerical simulations (Cole et al 2014b). These dynamo waves most often behave as if detached from the overall rotation of the object, moving with a different speed than the stellar surface.…”

Doppler images of DI Piscium during 2004–2006

“…This can be understood as an azimuthal dynamo wave of the non-axisymmetric field (see e.g. Mantere et al 2013, and references therein). Similar behaviour, but with a less systematic drift, has been reported in the single giant FK Com (Korhonen et al 2007;Hackman et al 2013).…”