The theory of the origin of zebra structure in solar radio emission

The strengths and weaknesses of different theories of the origin of the fine structure in dynamic spectra of solar radio emission observed in the form of quasi-equidistant bands of enhanced and reduced emission (zebra-structure) are discussed. Most papers interpreting the zebra-structure rest on the use of the plasma mechanism of radio-emission generation which involves the excitation of plasma (electrostatic) waves with their subsequent transformation into electromagnetic radiation. Plasma waves may be excited due to kinetic instability at hybrid resonance frequencies (in regions of the double plasma resonance in a distributed source). In some papers, a cause of appearance of emission and absorption bands is considered to be whistlers. An alternative theory suggests that there is a compact source with trapped plasma waves in the corona that easily provides the discrete spectrum. One more interpretation relies on special effects which may occur during radio wave propagation in the corona: alternate bands of strong and weaker emission are assumed to be a result of radio wave interference on some periodic structure in the corona. All mechanisms proposed in the literature are analysed with respect to the possibility of explaining as many observable fine-structure properties as possible within the framework of the source model with real physical parameters. Arguments are presented for the fact that the mechanism based on the action of the double plasma resonance in the inhomogeneous coronal loop can best explain all observable zebra-structure properties in decimetric and metric wavebands.