Center-to-limb variations indicate that a type III burst is likely to be emitted in a radial direction while a type IIIb emission is more perpendicular to the stream.
Present theories of type III bursts by stream-plasma interaction do not emphasize the center to limb variations of the characteristics of these bursts. Actually the occurrence probability of type III bursts does not reveal any strong center to limb effect, and these variations are assumed to be small. This is certainly true for the type II! bursts associated with -flares. But we shall show below that, on low frequencies, in particular at decameter wavelengths where the type III bursts are mostly observed in storms, several characteristics of these storms change with the longitude of the source. One of these effects has been found by who showed that the average frequency drift rate of storm type III bursts is larger when the source is near the center of the disc than when it is close to the limb, but this effect can be explained by purely geometrical considerations. On the contrary, the effects that we shall describe refer to the physics of the radio emission by stream-plasma interaction.
We shall first recall that: (a) the decameter storms are observed at the same time as type I storms at higher frequencies.
(b) they are formed by a great variety of fast bursts, with a majority of type III and type IIIb bursts.
(c) inside the decameter range, the type III bursts observed during a storm do not differ significantly from the flare associated type III bursts, except in their average intensity. Then the theory of the two emissions is likely the same, the difference being only in the origin of the streams of electrons.
(d) the type IIIb bursts (de la NoE and Boischot, 1972) drift in frequency at a rate similar to that of type III bursts, and they are certainly due to a streamplasma interaction, too.
(e) it is not yet clear whether type IIIb and type III bursts are emitted by the same stream of electrons or by two distinct streams of similar electrons.
Then any difference in the directivity of type IIIb and type III bursts will give information on the different mechanisms by which a stream-plasma interaction can give an emission of radio waves.
1. Center to Limb Variation of the Decameter Storm Characteristics
Even though several kinds of bursts are observed during a decameter storm, there is generally one kind, either type III or type IIIb bursts, which clearly dominates.