Time evolution of magnetic activity cycles in young suns : The curious case of kappa Ceti

Context. A detailed investigation of the magnetic properties of young Sun-like stars can provide valuable information on our Sun's magnetic past and its impact on the early Earth. Aims. We determine the properties of the moderately rotating young Sun-like star kappa Ceti's magnetic and activity cycles using 50 yr of chromospheric activity data and six epochs of spectropolarimetric observations. Methods. The chromospheric activity was determined by measuring the flux in the Call H and K lines. A generalised Lomb-Scargle periodogram and a wavelet decomposition were used on the chromospheric activity data to establish the associated periodicities. The vector magnetic field of the star was reconstructed using the technique of Zeeman Doppler imaging on the spectropolarimetric observations. Results. Our period analysis algorithms detect a 3.1 yr chromospheric cycle in addition to the star's well-known similar to 6 yr cycle period. Although the two cycle periods have an approximate 1:2 ratio, they exhibit an unusual temporal evolution. Additionally, the spectropolarimetric data analysis shows polarity reversals of the star's large-scale magnetic field, suggesting a similar to 10 yr magnetic or Hale cycle. Conclusions. The unusual evolution of the star's chromospheric cycles and their lack of a direct correlation with the magnetic cycle establishes kappa Ceti as a curious young Sun. Such complex evolution of magnetic activity could be synonymous with moderately active young Suns, which is an evolutionary path that our own Sun could have taken.