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 vi sible from the high-frequency limit of the radio astronomy receiver of the Ulysses Unified Radio and Plasma wave (URAP) experiment down to l ow frequencies, sometimes near the plasma frequency, and where Langmui r wave spikes are recorded by the radio and/or plasma receivers. For 1 4 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 t he progression of start times at decreasing frequency and the time of commencement of the Langmuir waves. For about 12 of these cases, the s pectrum of radio flux density at the time of the plasma waves is conti nuous just to the plasma frequency or the flux density decreases sharp ly within a few kHz of the plasma frequency. In two cases there is a g ood association of a type III burst with plasma waves but the fundamen tal radiation cuts off at about 1/2 and 1/3 of the plasma frequency. W hile not studied in detail in this paper, we note that there are frequ ently cutoffs of radio emission well above the plasma frequency, but i n 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 on set 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 st udies 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 late r 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 0.1 3 c. The brightness temperatures of the radiation at the lowest freque ncy measurable at the times of Langmuir wave spikes ranged from T-B ap proximate to 10(10) K to 10(12) K. These results pose questions regard ing the emission of radio emission by Langmuir waves. When Langmuir wa ves are observed, why is it only sometimes that radio radiation is emi tted 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?