Context : The Sun is an active source of radio emission ranging from long duration radio bursts associated with solar flares and coronal mass ejections to more complex , short duration radio bursts such as solar S bursts , radio spikes and fibre bursts .
While plasma emission is thought to be the dominant emission mechanism for most radio bursts , the electron-cyclotron maser ( ECM ) mechanism may be responsible for more complex , short-duration bursts as well as fine structures associated with long-duration bursts .

Aims : We investigate the conditions for ECM in the solar corona by considering the ratio of the electron plasma frequency \omega _ { p } to the electron-cyclotron frequency \Omega _ { e } .
The ECM is theoretically possible when \omega _ { p } / \Omega _ { e } ~ { } < ~ { } 1.

Methods : Two-dimensional electron density , magnetic field , plasma frequency , and electron cyclotron frequency maps of the off-limb corona were created using observations from SDO/AIA and SOHO/LASCO , together with potential field extrapolations of the magnetic field .
These maps were then used to calculate \omega _ { p } / \Omega _ { e } and Alfvén velocity maps of the off-limb corona .

Results : We found that the condition for ECM emission ( \omega _ { p } / \Omega _ { e } < 1 ) is possible at heights < 1.07 ~ { } R _ { \sun } in an active region near the limb ; that is , where magnetic field strengths are > 40 G and electron densities are > 3 \times 10 ^ { 8 } cm ^ { -3 } .
In addition , we found comparatively high Alfvén velocities ( > 0.02 c or > 6000 km s ^ { -1 } ) at heights < 1.07 ~ { } R _ { \sun } within the active region .

Conclusions : This demonstrates that the condition for ECM emission is satisfied within areas of the corona containing large magnetic fields , such as the core of a large active region .
Therefore , ECM could be a possible emission mechanism for high-frequency radio and microwave bursts .