Panel a: correlation between the total magnetic field strength and X-ray-to-bolometric luminosity ratio log L_X/L_bol. Individual targets are identified with Latin letters. The dashed line shows the best fitting power-law relation ⟨B⟩∝(L_X/L_bol)^0.37. The horizontal bar at the top illustrates typical variation of log L_X/L_bol over stellar activity cycles. Panel b: correlation between the total magnetic field strength and the Ca HK emission measure log . The dashed line corresponds to the best fitting relation .