Characteristics of Jupiter's Decametric Radio Source Measured with Arc-Second Resolution

Long-baseline measurements of Jupiter's decametric radiation at 34 1llz were made with an intensity interferometer between Boulder, Colorado, and Clark Lake, California (120000 X), and with a phase interferometer between Boulder and the Arecibo Ionospheric Observatory (487000 N). Theory and observations show that intensity and phase interferometers work equally well for the type of radiation observed. On time scales of less than 0.1 second, the radiation received at the two ends of the baselines was highly correlated, giving an upper limit to the size of an incoherent source of 0'.' 1, or 400 km, at Jupiter. The observations also imply an upper limit to a possible coherent source of 1", or 4000 km; this limit results from measuring the time history of the radiation at the two stations, i.e., the radiation pattern of the coherent source. The stability of the fringe rate shows that the source position is stable to within 700 km on time scales of at least 10 seconds, and apparent position shifts caused by interplanetary scintillations are less than 0"2. We find that the observations are most simply interpreted in terms of a small source near the feet of b-connected magnetic field lines which emits into a thin conical sheet nearly perpendicular to the field lines.