A change in the relationship between chromospheric activity and the large-scale magnetic field for G stars on the main sequence

The internal magnetic fields of cool stars are known to power activity in stellar chromospheres, but there is still much to discover about the relationship between the large-scale magnetic field and chromospheric activity. We have revisited this relationship by surveying the chromospheric activity (logR'_HK) and surface-averaged longitudinal magnetic field strength (B_l) for 954 mid-F to mid-M dwarf stars using observations from 3 the PolarBase spectropolarimetric database. We computed the mean logR'_HK, mean B_l and their variability amplitudes from time-series observations, and found that for F, K and M stars, the mean logR'_HK declines fairly smoothly with the mean logB_l and also the amplitude of logR'_HK variability. Meanwhile, for G stars on the main sequence, chromospheric activity appears to drop from logR'_{HK ~} -4.4 to -4.8, with minimal change in the logB_l and the amplitude of logR'_HK variability. At a similar logR'_HK level, we also see a switch from mixed magnetic field geometries to dominantly poloidal magnetic field geometries, as shown by published large-scale magnetic field maps derived using Zeeman Doppler Imaging. These results support a change in the magnetic surface properties of G stars that occurs on the main sequence.