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 through inhomogeneous plasma is due to two main reasons: (1) the electromagnetic wave group velocity differs from its vacuum value; (2) the ray path is lengthened by refraction. The group delay of type IIIb-III burst 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 region at two largely different frequencies. The group delay effect has to be more significant for the fundamental emission (type IIIb burst) than for the second harmonic (type III burst). For the observed type IIIb-III pairs this difference is found smaller than the uncertainties in the data. The group delay time for the fundamental emission in the frequency range 12.5-6.25 MHz does not exceed 1 s and the differential group delay is less than 0.3-0.5 s with a degree of confidence of approximately 70%. Radio wave propagation through the coronal plasma is simulated numerically choosing the scale height of the coronal density which fits the mean frequency drift rate of the observed type IIIb-III bursts. The calculated group delay times are found considerably larger than the observed ones if the mean velocity v_s of the burst source is assumed equal to 0.3 c. It is possible to reconcile the experimental and calculated group delay times choosing v_s less than or equal to 0.1 c. Another possibility is to assume that the fundamental type III emission is produced by the coalescence of plasma (Langmuir) waves with low frequency waves instead of plasma wave Rayleigh scattering by thermal ions.