Tachocline Alfvén waves manifested in stellar activity

Abstract

Context. The short-time (< 700 days) periodicities of both the stellar and solar activity that controls space weather are usually are discussed as manifestations of Rossby modes in tachoclines. Various interpretations of this phenomenon that have been proposed, in particular for the Sun, can be verified by considering the broad population of nonsolar-type stars. Aims. We look for surface stellar activity features, drifting in longitude, and compare their drift rates with those predicted for different low-frequency waves in stellar photospheres and tachoclines. Methods. Analogously to the Hovmöller diagrams in meteorology, we constructed a dynamic diagram of stellar activity pattern (DDSAP) to visualize the rotational variability of the stellar radiation flux as a function of rotation phase and time. We used the high-precision light curves of the fast-rotating main sequence stars, with rotation periods of 0.5 to 4 days, from the Kepler mission database. Results. We found quasi-periodic drifting lanes (DLs) of various durations and intensities in the DDSAPs for 108 stars. In the course of analysis, we carried out a correction of the stellar rotation periods by nullifying the drifts of the longest-lasing DLs that are presumably related to the long-lived starspot complexes co-rotating with the star. We discovered a clear elongated cluster of the absolute values of the DLs’ drift rates versus the stellar effective temperatures. This cluster cannot be attributed to any accidental contaminations of the light curves or manifestation of waves in the stellar photospheres, because of their extremely short timescales. An approximate equality of the absolute values of positive and negative drift rates of the considered DLs makes it impossible to interpret them in terms of Kelvin and/or magneto-Rossby waves in the stellar tachoclines. It is only global kink-type Alfvénic oscillations of the tachocline as a whole that allow us to interpret the estimated drift rates forming the above-mentioned cluster, as well as the related activity periodicities and turnover times in the convective zones. The corresponding magnetic field strength appears to be about 50 kG, which is approximately in the middle of the range of assumptions discussed in the literature. Conclusions. Alfvén waves are an important, albeit commonly ignored factor in stellar interiors. Apparently, the global tachocline’s Alfvén waves ought to play a role in triggering emergence of the magnetic flux tubes. Their manifestation in stellar activity opens up a unique way of probing the magnetic field strength in tachoclines of individual stars. Since the investigations of the tachocline waves performed thus far have been based on the shallow-fluid approximation, and also assuming a rigid fixed bottom of the tachocline layer, the global kink-type Alvénic disturbances of the whole tachocline layer have not been considered. The reported observational detection of signatures of such waves, manifested in specific longitude drifts of the stelar surface activity pattern, calls for a more detailed theoretical study.