Fine structure in plasma waves and radiation near the plasma frequency in Earth's foreshock

Abstract

Novel observations are presented of intrinsic fine structure in the frequency spectrum of electromagnetic (EM) radiation and plasma waves near the electron plasma frequency fp during a period of unusually high interplanetary magnetic field strength. Measured using the wideband receiver on the ISEE 1 spacecraft, fine‐structured emissions are observed both in the solar wind and the foreshock. The fine structure is shown to correspond to emissions spaced above fp near half harmonics of the electron cyclotron frequency fce, i.e., near fp + nfce/2. These appear to be the first space physics observations of emissions spaced by fce/2. Indirect but strong arguments are used to discriminate between EM and electrostatic (ES) signals, to identify whether ISEE 1 is in the solar wind or the foreshock, and to determine the relative frequencies of the emissions and the local fp. The data are consistent with generation of the ES and EM emissions in the foreshock, with subsequent propagation of the EM emissions into the solar wind. It remains possible that some emissions currently identified as ES have significant EM character. The ES and EM emissions often merge into one another with minimal changes in frequency, arguing that their source regions and generation mechanisms are related and imposing significant constraints on theories. The fce/2 ES and EM fine structures observed may be intrinsic to the emission mechanisms or to superposition of two series of signals with fce spacing that differ in starting frequency by fce/2. Present theories for nonlinear wave coupling processes, cyclotron maser emission, and other linear instability processes are all unable to explain multiple EM and/or ES components spaced by ∼fce/2 above fp for fp/fce ≫ 1 and typical foreshock beam parameters. Suitable avenues for further theoretical research are identified. Empirically, the observed fine structures appear very similar to those in split band and multiple‐lane type II solar radio bursts; interpretation of both these type II fine structures in terms of fce/2 splitting is suggested, thereby supporting and generalizing a suggestion 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 wideband receiver has sufficient frequency resolution (≲ 100 Hz) to perform a dedicated search. Further study of the ubiquity of these fine structures, of how reliably the splitting corresponds to fce/2 and of the other interpretations above is necessary.