RELATIVISTIC TYPE-III SOLAR RADIO-BURSTS

Observations with the ARTEMIS multichannel radiospectrograph reveal a new class of type III solar radio bursts generated by the relativistic head of electron beams. Compared to normal type III bursts, these bur sts, which we call type IIId's drift faster towards lower frequencies, have a shorter duration at fixed frequency and start at higher freque ncies. An even more dramatic difference is that they often disappear s uddenly before reaching the 100 MHz level. Sometimes they occur in fro nt of normal type III's, forming characteristic pairs which cannot be fundamental-harmonic pairs because of the large difference in drift ra tes between the two components, giving a highly variable frequency rat io. The apparent speeds of type IIId sources, deduced from the frequen cy drift rates, are often superluminous, especially at the center of t he solar disc where beams come towards the observer. This is due to th e radiation time of flight effect similar to those found in extragalac tic radiojets. Correcting for this effect, we find speeds close to the speed of light. The shortest characteristic drift times (0.0 +/- 0.1 s) are inconsistent with the differential group delay of fundamental r adiation which must be 0.3 s or more; we conclude that type IIId's are emitted on the harmonic mode (2 x plasma frequency). Fundamental-harm onic pairs occur mainly at f less than or similar to 100 MHz. Type III d - normal type III pairs, which occur at frequencies f greater than o r similar to 100 MHz, turn out to be of a different nature. We interpr et them as due to the generation of plasma waves by the usual bump-on- the-tail instability in two zones along the electron beam, both radiat ing on the harmonic mode. The first zone, radiating type IIId's, is du e to the relativistic electrons concentrated at the head of beams whos e energy spectra are somewhat harder than average; when electrons have lost enough energy through this process to go back to subrelativistic speeds, velocity dispersion comes into effect and electrons start spr eading longitudinally; then the first zone stops radiating. The second zone of plasma waves, radiating normal type III's, is the ''classical '' one, and corresponds to the bulk of the beam, made up of subrelativ istic electrons. Depending on the distance to the acceleration source, on the spectral index of the electron energy spectra and other factor s, we may or may not see the radiation from either one of the two pote ntial radiation zones.