The Solar Corona in Active Regions and the Thermal Origin of the Slowly Varying Component of Solar Radio Radiation.

Using as a basis previously published K-corona observations made at Climax, Colorado, we derive a model for the enhancement of electron density in the corona above an active region. The electron densities in the quiet corona during the sunspot maximum 1957-1958 were found to be about twice those reported by van de Hulst for a maximum corona, while densities along the axis of the active region were found to be about twice those in the quiet corona at the same height. The several models advanced in the past to explain the slowly varying component of solar radio radiation are criticized mainly on the basis that they require coronal temperatures of from 6 to 10 X 106 K, while there is no evidence for the existence of such temperatures. The electron densities determined in the first section are combined with models of the chromosphere and of a plage to determine the appearance of the active region at radio wave lengths. Detailed calculations of the appearance of the active region at various radio frequencies were carried out and include the effect of refraction but neglect the effects of magnetic fields. Profiles of the solar disk at various radio frequencies for three coronal models with the active region at various positions on the solar disk are presented. These profiles are then used to predict such characteristics of the slowly varying component as its spectrum, apparent size, "height," and directivity. The comparison of these predictions with the observed properties leads to the conclusion that the slowly varying component is due solely to the enhancement of densities in the corona and in the chromospheric plage. There is no evidence that the temperature of the corona is modified from its normal value of about 2 X 106 K. A brief examination of the thermal radiation from an idealized loop prominence shows that temperatures of 3-4 X 105 K are sufficient to explain some of the features of the radio event called "gradual rise and fall."