Gyroresonance layers
				•  Gyroresonance opacity is the only mechanism capable of making the corona optically thick at frequencies above 4 GHz • Emission comes from a surface of constant magnetic field in the corona •  Coronal temperatures indicate the presence of magnetic field strengths at appropriate strengths: microwaves are sensitive to fields in the range 200– 3000 G. •  High levels of circular polarization also indicate presence of strong magnetic fields and can be used to measure temperature gradients