FINE-STRUCTURE IN PLASMA-WAVES AND RADIATION NEAR THE PLASMA FREQUENCY IN EARTHS FORESHOCK

Novel observations are presented of intrinsic fine structure in the fr equency spectrum of electromagnetic (EM) radiation and plasma waves ne ar the electron plasma frequency fp during a period of unusually high interplanetary magnetic field strength. Measured using the wideband re ceiver on the ISEE 1 spacecraft, fine-structured emissions are observe d both in the solar wind and the foreshock. The fine structure is show n to correspond to emissions spaced above fp near half harmonics of th e electron cyclotron frequency f(ce), i.e., near f(p) + nf(ce)/2. Thes e appear to be the first space physics observations of emissions space d by f(ce)/2. Indirect but strong arguments are used to discriminate b etween EM and electrostatic (ES) signals, to identify whether ISEE 1 i s in the solar wind or the foreshock, and to determine the relative fr equencies of the emissions and the local f(p). The data are consistent with generation of the ES and EM emissions in the foreshock, with sub sequent propagation of the EM emissions into the solar wind. It remain s possible that some emissions currently identified as ES have signifi cant EM character. The ES and EM emissions often merge into one anothe r with minimal changes in frequency, arguing that their source regions and generation mechanisms are related and imposing significant constr aints on theories. The f(ce)/2 ES and EM fine structures observed may be intrinsic to the emission mechanisms or to superposition of two ser ies of signals with f(ce) spacing that differ in starting frequency by f(ce)/2. Present theories for nonlinear wave coupling processes, cycl otron maser emission, and other linear instability processes are all u nable to explain multiple EM and/or ES components spaced by N f(ce)/2 above f(p) for f(p)/f(ce) much greater than 1 and typical foreshock be am parameters. Suitable avenues for further theoretical research are i dentified. Empirically, the observed fine structures appear very simil ar to those in split band and multiple-lane type II solar radio bursts ; interpretation of both these type II fine structures in terms of f(c e)/2 splitting is suggested, thereby supporting and generalizing a sug gestion by Wild (1950). A possible application to continuum radiation is mentioned. The ubiquity of these fine structures in the Earth's fp radiation and foreshock waves remains unknown. Only the ISEE 1 wideban d receiver has sufficient frequency resolution (less than or similar t o 100 Hz) to perform a dedicated-search. Further study of the ubiquity of these fine structures, of how reliably the splitting corresponds t o f(ce)/2, and of the other interpretations above is necessary.