THE LOW RADIO-FREQUENCY LIMIT OF SOLAR-TYPE-III BURSTS - ULYSSES OBSERVATIONS IN AND OUT OF THE ECLIPTIC

We measure the low frequency limit f(lo) of 1028 type III bursts and compare it with the observed plasma, frequency at the spacecraft f(p). We consider three periods: when Ulysses was in the ecliptic plane near the Sun, far from the Sun, and out of the ecliptic; the average plasma frequency was 20, 10, and 5 kHz respectively. The major results are:
1) During the entire period of Ulysses flight November 1990 to March 1995, 9 kHz is the lowest frequency at which type III electromagnetic radiation was ever seen, even though f(p) was often 3-4 kHz.
2) The distribution of cutoffs is independent of ecliptic latitude, and of f(p) at the spacecraft (excepting of course that f(lo) greater than or equal to f(p)) and is broadly spread between 17 and 300 kHz.
3) When f(p) is about 20, 10 and 5 kill;, only 15, 5 and 0% respectively of about 20, 10 and 5 kill;, only 15, 5 and 0% respectively of type III burst radiation approaches within a factor of two of f(p).

We develop three hypotheses to explain these results:
1) that the low-frequency cutoff of type III bursts is intrinsic to the radiation mechanism,
2) that it is due to directivity of the radiation, and
3) that it is due to propagation conditions between the burst source and the spacecraft. The evidence for intrinsic cutoffs is strong, but propagation effects play a role in some cases.