OPTICAL-THICKNESS OF THE 2-4.5 GHZ SOLAR PLASMA EMISSION

For radio emission at the frequency corresponding to the second harmon ic of the local plasma frequency, the optical thickness in the solar a tmosphere is calculated. Three types of models are assumed: the model with radio emission from the narrow transition region,and models with radio emission from a cool and dense plasma filament embedded in hotte r plasma at the transition region and in the corona. The optical thick ness is computed by integration of the collisional (Free - free) absor ption along a radio-ray path radial in the solar atmosphere. In all mo dels considered the optical thickness can be sufficiently low for appr opriate parameters. For example, in the narrow (< 100 km) transition r egion where the density scale height is much less than that of the pre ssure one, the optical thickness can be lower than 1. Furthermore, the optical thickness can be decreased if the radio emission is generated in the cool and dense plasma filament surrounded by hotter and thinne r plasma. But the models differ in density scale heights and thus in d istances between plasma emission levels. This difference is essential for the interpretation of high frequency type III radio bursts.