Planetary magnetic fields and expected SPMI powers. Panel a shows the variation of the transmitted Poynting flux for HD 189733b on a [B_⋆, B_p] grid. The dashed contour lines show transmission efficiencies of (from the top) 8.0%, 8.1%, 8.2%, 8.3% and 8.4% with lighter shades representing higher values. Panels b and c illustrate the dependence of the required planetary magnetic field strengths to account for the observed power emitted from certain exoplanetary systems suspected of exhibiting SPMIs. Panel b presents a lower limit for the magnetic field strength, assuming that all the energy reaching the chromosphere is emitted in the Ca-II K band. In contrast, panel c offers a more realistic scenario, where the fields are calculated considering the constraint that 0.2% of the energy reaching the chromosphere is emitted in the Ca-II K band.