The Variable Solar Radio Spectrum

Abstract

This paper reviews recent experimental and theoretical work concerning the slowly-varying component of solar radio emission, microwave bursts, and the four main spectral types of bursts observed in the meter and decimeter bands. Observational techniques are briefly summarized.The slowly-varying component is believed to be thermal in origin and to come from overdense regions above plages and sunspots. The physical characteristics of this component are tabulated and recent theories of emission processes are discussed. Microwave bursts, as observed by both fixed-frequency and sweep-frequency receivers, are categorized and emission mechanisms for each category are considered. The characteristics and possible origins of Type-I (noise storm) bursts, which originate high in the corona above active sunspots, are reviewed.Type-II (slow-drift) bursts are believed to be generated by disturbances propagating outward through the corona, sometimes non-radially, at velocities of about 1000–1500 km/sec, in regions where the electron density is about 10 times that given by the Baumbach-Allen distribution. Measurements at meter wavelengths of the brightness distribution across the Type-II disturbances indicate that the radiation originates in a bright central emitting core, surrounded by an extended halo of scattered radiation. Observations with a swept-frequency interferometer show that the radiation generated in the second harmonic comes from directions corresponding to much lower heights in the solar atmosphere than the fundamental. It has been suggested that radiation in the second harmonic is preferentially back-scattered from the point of generation; the second harmonic is then observed after reflection at much lower coronal heights.Type-Ill (fast-drift) bursts, generated by an exciter moving at approximately 0.5 c, are also believed to be generated in density condensations and to have a core and halo brightness distribution similar to that of Type-II bursts. Examination of the burst profiles gives information on the dispersion of the exciter as it moves outward through the corona.The complex properties of Type-IV (continuum) bursts, as observed throughout the complete radio spectrum (intensities, polarization, diameters, spectral characteristics, motions), are summarized and discussed in terms of existing models based on synchrotron and Cerenkov plasma wave emission processes.Finally, the relation of the radio bursts to accompanying X-ray and high-energy nucleon emission is briefly examined in terms of existing theories of electromagnetic emission and particle acceleration in the solar atmosphere.KeywordsRadio EmissionRadio BurstRadio SpectrumSolar RadioCoronal StreamerThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.