Conditions for electron-cyclotron maser emission in the solar corona

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 ω_p to the electron-cyclotron frequency Ω_e. The ECM is theoretically possible when ω_p/ Ω_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 ω_p/Ω_e and Alfvén velocity maps of the off-limb corona.

Results. We found that the condition for ECM emission (ω_p/ Ω_e< 1) is possible at heights <1.07 R_⊙ in an active region near the limb; that is, where magnetic field strengths are >40 G and electron densities are >3 × 10⁸ cm^-3. In addition, we found comparatively high Alfvén velocities (>0.02c or >6000 km s^-1) at heights <1.07 R_⊙ 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.