Kilometre-wave radio observations of solar type III bursts by Ulysses compared with decametre-wave observations from the Earth

Observations of solar type III bursts, made at kilometric frequencies by the radio receiver of the Ulysses Unified Radio and Plasma Wave (URAP) investigation, are compared with simultaneous observations, made at decametric frequencies at the Nançay Radio Astronomy Station of the ParisMeudonObservatory, during the period following the spacecraft encounter with Jupiter until the south solar polar pass. Of the events suitable for study, 57 can be identified in both frequency bands having delay times consistent with prediction based upon geometry and upon the frequency drift between the lowest frequency observed on the ground (25MHz) and the highest frequency observed by URAP (940 kHz). The good agreement between calculated and measured delay times suggests that the large delay anomalies, reported by Steinberg et al. (1984) sometimes to be in excess of 8 minutes in duration, may be confined to frequencies below the 500 kHz limit studied by these authors and not detectable at 940 kHz with the time resolution of the URAP receiver obtained in this study. There are cases when bursts recorded at Ulysses are not seen at Nançay. This may just be due to the source at 25MHz being beyond the solar limb and so obscured from the Earth. There are other cases, however, when bursts recorded at the Earth are not seen at Ulysses. Such cases do not appear to be correlated with either the EarthSun-Ulysses (ESU) angle orwith the heliographic latitude of the spacecraft. As the source region at 940 kHz is large and at a considerable distance from the Sun, some part of it will usually be visible from the spacecraft no matter what the relative positions of the Sun, the source and Ulysses may be; this suggests either that there may sometimes be a low frequency cutoff, inherent to the burst itself somewhere between 940 kHz and25MHz, or else that the emission has somehow been occulted. Representative examples are presented and discussed.