Active longitudes, nonaxisymmetric dynamos and phase mixing

Abstract

We discuss the problem of solar active longitudes from the viewpoint of dynamo theory. We start from a recent observational analysis of the problem undertaken by Berdyugina & Usoskin (2003, A&A, 405, 1121 and Usoskin et al. (2005, A&A, 441, 347) who demonstrated from a study of sunspot data that solar active longitudes rotate differentially, with a small but significant asynchrony between northern and southern hemispheres. We suggest two concepts by which the underlying magnetic structure could lead to the observed phenomenologythe true differential rotation of a nonaxisymmetric magnetic structure and a stroboscopic effect. In the latter case, a solid body rotation of nonaxisymmetric magnetic structure is illuminated by an activity wave propagating from middle latitudes to the solar equator, and so mimics a differential rotation. We then discuss several mechanisms which could in principle lead to the excitation of active longitudes. In particular, we consider dynamo excitation of nonaxisymmetric magnetic modes, nonaxisymmetric structures as a manifestation of a relic magnetic field in the solar core, nonaxisymmetric solar hydrodynamics and nonlinear instabilities that lack axial symmetry. We conclude that these mechanisms all provide ways to explain the phenomenology, provided the stroboscopic interpretation is accepted. Of course, a quantitative explanation in the framework of any scenario requires ultimately a detailed numerical simulation. The interpretation of the available observations as a true differential rotation appears to provide a much more severe challenge for theorists. We are unable to suggest a plausible mechanism of this kind; however we can not exclude in principle such an explanation. We relate the phenomenon of solar active longitudes to the information available concerning stellar active longitudes, and also consider evidence from other tracers of solar activity.

Rotational periodicities in sunspot relative numbers

“…This result was doubted by Pelt et al (2005). After refining their methods, Usoskin et al (2005) and Berdyugina et al (2006) confirmed the previous result. Usoskin et al (2005) introduced a dynamic reference frame which rotates with the mean rate of the differential rotation with respect to the phase of the solar cycle.…”

Global analysis of active longitudes of sunspots

“…Berdyugina et al (2006) and Usoskin et al (2007) summarized the dynamo modes related to the occurrence of preferred longitudes in solar and stellar activity and discussed the interpretation of the existence of active longitudes within several solar dynamo related and other scenarios (see also Ruzmaikin 1998;Rädler et al 1990;Moss 1999Moss , 2004Mason et al 2002;Fluri & Berdyugina 2005;Bassom et al 2005;Korhonen & Elstner 2005). Berdyugina et al (2006) noted that in most dynamo models the generated structures are expected to rotate quasirigidly, while the migration of active longitudes could be caused by a stroboscopic effect. Nevertheless, the significantly varying rate of asymmetry and rotation from one cycle to another, as well as the significant hemispheric differences provide observational constraints that need to be taken into account in dynamo modeling.…”

Magnetic activity of F stars observed byKepler

“…This would mean that these active longitudes are related to internal changes, such as the dynamo mechanisms occurring inside the star. In particular this has been studied theoretically with dynamo models (Berdyugina et al 2006;Weber et al 2013). …”