•Opacity results from electrons gyrating in coronal magnetic fields at fB = 2.8 106 B Hz. •In the non-flare (non-relativistic) corona this produces narrow resonances. •Opacity µ   n   B/(¶B/¶l)   (T/mc2 sin2q)s-1      where s = 1, 2, 3, … is the harmonic •Because T/mc2 is 1/3000, opacity drops by 3 orders of magnitude from one layer to the next