Striated spectral activity in Jovian and Saturnian radio emission

Abstract

Examination of high time resolution frequency‐time spectrograms of radio emission measured near the Voyager 1 and 2 encounters with Jupiter reveals occasional striation patterns within the normally diffuse hectometric radiation. The patterns are characterized by distinctive banded structures of enhanced intensity meandering in frequency over time scales of minutes to tens of minutes. This banded form of striated spectral activity (SSA) has an occurrence probability of the order of 5% during the three weeks before and after Jupiter encounters. Plots of single 6‐s frequency sweeps often exhibit a slow rise in intensity followed by a sharp drop‐off in each band as frequency decreases. Banded SSA is often preceded or followed by chaotic SSA in which banding of the emission becomes discontinuous or unrecognizable, although the intensity modulation is still evident. Although SSA normally occurs in the frequency range of roughly 0.2–1.0 MHz, similar but longer‐lasting patterns have been found occasionally in decametric emission above 10 MHz. Analogous modulation has also been observed in the Saturnian radio emission, suggesting that SSA may be a common feature intrinsic to the radio emission at both planets. Observed properties suggest a number of constraints on the possible source mechanism which make theoretical interpretations difficult.