Polarization Characteristics of Type IV Bursts II. Physical Interpretation

Abstract

Abstract We attempt to investigate possible mechanisms applicable to type IV bursts, by which polarization characteristics may be consistently accounted for. A plausible model, corresponding to microwave type IV bursts (IV-μ), may be an inhomogenious one in which the emitting sources are distributed vertically over the lower corona corresponding to the frequencies of observation. The observed features may be accounted for if the effects of resonance absorption due to ambient thermal electrons and also the self-absorption due to radiating electrons are taken into account. For this purpose, it is necessary to postulate electrons of density about 102 ~ 103 cm–s, and with energies of about 105 ev. One mechanism applicable to the polarized parts of type IV bursts, which appear in the decimeter and meter range, may be the gyro-resonance radiation due to electrons of comparatively lower energies. Some consideration is given to the possibility of negative absorption, which is necessary to account for the observed high intensity. The condition for the strong polarization in the ordinary sense severely restricts the number of radiating electrons. An alternative possible mechanism is the radiation from coherent plasma waves excited by an electron beam. It is required to postulate sufficiently strong magnetic field and comparatively lower energy electrons of density about 107 cm–3 at a height of 0.3R⊙ above the photosphere. The difference in observed characteristics between the unpolarized and polarized parts of IV bursts may be attributed to a change with time in the configuration of the magnetic field in the source. The coupling between the ordinary and extraordinary modes of waves at the gyro-resonance frequencies is discussed as a possible source of the linear component of type IV bursts. The efficiency for production of linear component is greater for the wave propagating nearly perpendicularly to the magnetic lines of force.