HARMONIC STRUCTURE OF DECAMETRIC TYPE-III EMISSION AND RADIO-WAVE GROUP DELAY IN THE SOLAR CORONA

From solar observations with the UTR-2 decameter-wave radio telescope antenna, carried out continuously at fixed frequencies from 25 to 10 M Hz during type III and IIIb - III noise storms in 1979-1984, new data have been obtained on the frequency drift rates DELTAf/DELTAtf of nume rous type IIIF, IIIFb, and IIIH(b) decameter radio bursts in several n arrow bands DELTAf. The data confirm that the burst drift rates are pr actically independent on the phase of the 11-year solar activity cycle Measurements of the drift time intervals DELTAt(f) for bursts driftin g over narrow bands 25-20 and 12,5-10 MHz, separated by an octave, pro ve the reliability of the radio burst harmonic classification in which the emission components related with the fundamental (F) and second-h armonic (H) of local plasma frequency (in a source region of the solar corona) are strictly distinguished. In calculations for simple harmon ically homogeneous type III decametric bursts as well as complex ones, the power-law approximation is acceptable for the drift-rate frequenc y dependence, the power index being the same in the cases of F- and H- bursts. The burst frequency drift deceleration due to a group delay ef fect of radio waves, escaping from type III generation site, appears t o be relatively insignificant, because the source moves outwards throu gh the corona at a speed which is much less than the velocity of light .