http://arxiv.org/abs/1704.09009
The magnetic activity of the Sun becomes stronger and weaker over roughly an 11 year cycle, modulating the radiation and charged particle environment experienced by the Earth as “space weather”. Decades of observations from the Mount Wilson Observatory revealed that other stars also show regular activity cycles in their Ca II H+K line emission, and identified two different relationships between the length of the cycle and the rotation rate of the star. Recent observations at higher cadence have allowed the discovery of shorter cycles with periods between 1-3 yr. Some of these shorter cycles coexist with longer cycle periods, suggesting that two underlying dynamos can operate simultaneously. We combine these new observations with previous data, and we show that the longer and shorter cycle periods agree remarkably well with those expected from an earlier analysis based on the mean activity level and the rotation period. The relative turbulent length scales associated with the two branches of cyclic behavior suggests that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, while a dynamo operating in deeper layers may dominate in less active stars. However, several examples of equally prominent long and short cycles have been found at all levels of activity. Deviations from the expected cycle periods show no dependence on the depth of the surface convection zone or on the metallicity. At the lowest activity levels, there may be a fundamental shift in the character and prevalence of cycles due to a reduction in magnetic braking. For some stars that exhibit longer cycles, we predict the periods of shorter cycles that might be detected with future high cadence observations.
A. Brandenburg, S. Mathur and T. Metcalfe
Mon, 1 May 17
17/46
Comments: 11 pages, 8 figures, 3 tables, submitted to ApJ
You must be logged in to post a comment.