ON THE RADIO-WAVE GROUP DELAY IN THE SOLAR CORONA FOR THE CASE OF DECAMETER TYPE-III BURSTS

The excess time delay of the radio waves during their propagation thro ugh inhomogeneous plasma is due to two main reasons: (1) the electroma gnetic wave group velocity differs from its vacuum value; (2) the ray path is lengthened by refraction. The group delay of type IIIb-III bur st emission is estimated by analyzing observations made with the UTR-2 array (Abranin et al. 1984). Owing to the harmonic origin of the type IIIb-III events it is possible to probe one and the same coronal regi on at two largely different frequencies. The group delay effect has to be more significant for the fundamental emission (type IIIb burst) th an for the second harmonic (type III burst). For the observed type III b-III pairs this difference is found smaller than the uncertainties in the data. The group delay time for the fundamental emission in the fr equency range 12.5-6.25 MHz does not exceed 1 s and the differential g roup delay is less than 0.3-0.5 s with a degree of confidence of appro ximately 70%. Radio wave propagation through the coronal plasma is sim ulated numerically choosing the scale height of the coronal density wh ich fits the mean frequency drift rate of the observed type IIIb-III b ursts. The calculated group delay times are-found considerably larger than the observed ones if the mean velocity v(s) of the burst source i s assumed equal to 0.3c. It is possible to reconcile the experimental and calculated group delay times choosing v(s) less-than-or-equal-to 0 .1c. Another possibility is to assume that the fundamental type III em ission is produced by the coalescence of plasma (Langmuir) waves with low frequency waves instead of plasma wave Rayleigh scattering by ther mal ions.