The purpose of this note is to examine the suggestion of Fokker (1970) to explain why the second branch of U-bursts is ill-developed and to present an alternate possible explanation which was suggested by an examination of the trajectories derived by him.
Let us assume along with Fokker (1970) that the source is a stream of particles which is capable of radiating plasma waves by the spontaneous Cerenkov effect and further amplifying them by the induced Cerenkov effect (Smith, 1970a). To determine the nature of the source more precisely, i.e. electron, proton or neutral stream, an analysis similar to Smith (1970b), taking into account the peculiarities of U-bursts (see below), would be required. Let us also assume, as did Fokker (1970) in Section 4.6 of his paper, that most of the emission from U-bursts is near the second harmonic of the plasma frequency. Although this suggestion has appeared repeatedly in the literature (Zheleznyakov, 1964;Kundu, 1965), the only explanation which has been offered is that fundamental emission is much more directive than second harmonic emission. However, in the case of Type III bursts, when scattering of radiation by coronal inhomogeneities (Fokker, 1965) is taken into account, fundamental emission becomes much less directive (Smith, 1970a) and a more complete analysis of this question, taking scattering into account, would be very desirable. One of the strongest indications that most of the emission from Type IIi bursts is near the fundamental of the plasma frequency is that, although their distribution in longitude is almost uniform (Swarup et al., 1960;Fokker, 1970), strong Type III bursts come mostly from the center of the disk (Swarup et al., 1960). Thus, to decide the question of the dominance of second harmonic emission from the observational point of view, it would be very desirable to see not only the distribution in longitude of all U-bursts (Fokker, 1970), but also the distribution in longitude of strong U-bursts.
We now examine the self-consistency of Fokker's suggestion to explain why the second branches of U-bursts are less developed than the first branches. He has proposed that this is due to a difference in the efficiencies of emission for streams travelling towards higher and lower densities; namely, he suggests that streams travelling opposite to the direction of grad n e (n e -electron density) should emit more efficiently. However, exactly the opposite result, namely that streams travelling in the direction * Postdoctoral Fellow on the U.S.-U.S.S.R. Cultural Exchange ** Present address: High Altitude Observatory, Boulder, Colo., U.S.A.