Microstructure of Jovian decametric S bursts

We have developed a method for displaying the spectral structure of Jupiter 's decametric radio S bursts on timescales down to a few microseconds, 2 or ders of magnitude finer than has been achieved elsewhere. By employing an e xtremely sensitive antenna (640 dipoles, at 26 MHz) and selecting only rela tively weak S bursts that possess the simplest possible spectral;shape, we identify frequently occurring structural features that we associate with lo calized emission centers. On timescales having better than about 30 mu s re solution we find that the S burst baseband oscillation land therefore the R F oscillation) is modulated to form distinct pulses, which we refer to as s ubpulses. Still finer time resolution reveals that within individual subpul ses the baseband land RF) oscillation often displays segments in which the usually drifting phase term abruptly becomes essentially constant and, afte r remaining so for 10 to 100 mu s, abruptly resumes its random drift. It is these abruptly starting and ending segments of phase coherence that we att ribute to isolated powerful centers of cyclotron maser wave amplification, which happen for brief intervals to be the only ones that are active. We be lieve that the more usual phase-incoherent condition (i.e., one in which th e instantaneous frequency drifts randomly within the emission band) is due to the fact that the resultant radiation is the sum of two or more componen ts from neighboring wave amplification centers emitting at slightly differe nt frequencies, with independently varying intensities. Possible models for the production of subpulses and phase coherent intervals are discussed.