# Evolution of coexisting long and short period stellar activity cycles

## TL;DR

This paper investigates the coexistence and characteristics of long and short stellar activity cycles across different stars, revealing insights into the underlying dynamo mechanisms and their relation to stellar activity levels.

## Contribution

It combines new high-cadence observations with previous data to analyze multiple stellar activity cycles and their relation to stellar properties, supporting the existence of multiple dynamo processes.

## Key findings

- Long and short cycle periods align with theoretical expectations based on activity level and rotation.
- Shorter cycles coexist with longer ones, indicating multiple dynamo processes.
- Cycle periods show no dependence on convection zone depth or metallicity.

## Abstract

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 have 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 suggest that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, whereas 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 of stars younger than 2.3 Gyr. Deviations from the expected cycle periods show no dependence on the depth of the convection zone or on the metallicity. For some stars that exhibit longer cycles, we compute the periods of shorter cycles that might be detected with future high-cadence observations.