Critical magnetic flux density required for electrons, B_e, to be magnetically coupled to an external magnetic field on HAT-P-7b. The black dashed line indicates B = 10 G, which is a typical magnetic field strength for giant gas planets, the red dashed lines are values for the Earth magnetic field strength. If B > B_e then ω_ce ≫ ν_ne is fulfilled and the atmosphere is magnetised. All line colours are as in Fig. 2.