Left panel: photospheric magnetic activity index, S_ph (in ppm), as a function of the rotational period, P_rot (in days), of the 18 seismic solar analogs observed with the Kepler satellite. The mean activity level of the Sun calculated from the VIRGO/SPM observations is represented for a rotation of 25 days with its astronomical symbol, and its mean activity levels at minimum and maximum of the 11-yr cycle are represented by the horizontal dashed lines. Right panel: same as the left panel but as a function of the seismic age (in Gyr). The position of the Sun is also indicated for an age of 4.567 Gyr. The size of the symbols is inversely proportional to the rotation period, P_rot. In both panels, each star is referred by the same number as given in Tables 2 and 3.