INTERPLANETARY TYPE-III RADIO-BURSTS THAT APPROACH THE PLASMA FREQUENCY - ULYSSES OBSERVATIONS

We study a set of solar type III radio bursts where the emission is visible from the high-frequency limit of the radio astronomy receiver of the Ulysses Unified Radio and Plasma wave (URAP) experiment down to low frequencies, sometimes near the plasma frequency, and where Langmuir wave spikes are recorded by the radio and/or plasma receivers. For 14 of the 16 type III bursts studied, the emission at the onset of the burst was at the fundamental of the plasma frequency as deduced from the progression of start times at decreasing frequency and the time of commencement of the Langmuir waves. For about 12 of these cases, the spectrum of radio flux density at the time of the plasma waves is continuous just to the plasma frequency or the flux density decreases sharply within a few kHz of the plasma frequency. In two cases there is a good association of a type III burst with plasma wav es but the fundamental radiation cuts off at about 1/2 and 1/3 of the plasma frequency. While not studied in detail in this paper, we note that there are frequently cutoffs of radio emission well above the plasma frequency, but in the absence of closely associated plasma waves there is no means of knowing whether the radiation is at the fundamental or at the harmonic. In two cases the evidence indicates that the radiation near burst onset changed from fundamental near the Sun to the harmonic, starting at 0.3 AU in one case and 0.9 AU in the other case. This and previous studies suggest that such events are rare: almost always when a Langmuir wave association exists, the type III radiation at burst onset is at the fundamental. Harmonic radiation typically becomes dominant at later times, near or after the time of maximum flux density. The speeds of electrons that produced the type III bursts ranged from 0.04 c to 0.13 c. The brightness temperatures of the radiation at the lowest frequency measurable at the times of Langmuir wave spikes ranged from T-B approximate to 10(10) K to 10(12) K. These results pose questions regarding the emission of radio emission by Langmuir waves. When Langmuir waves are observed, why is it only sometimes that radio radiation is emitted by them at the fundamental? Put another way, why is there often a gap or a cutoff in the radiation at a frequency well above the plasma frequency? In the few cases where the radiation at times of Langmuir wave spikes is at the harmonic, why is there no fundamental?