FORESHOCK LANGMUIR-WAVES FOR UNUSUALLY CONSTANT SOLAR-WIND CONDITIONS- DATA AND IMPLICATIONS FOR FORESHOCK STRUCTURE

Plasma wave data are compared with ISEE 1's position in the electron f oreshock for an interval with unusually constant (but otherwise typica l) solar wind magnetic field and plasma characteristics. For this peri od, temporal variations in the wave characteristics can be confidently separated from sweeping of the spatially varying foreshock back and f orth across the spacecraft. The spacecraft's location, particularly th e coordinate D-f downstream from the foreshock boundary (often termed DIFF), is calculated by using three shock models and the observed sola r wind magnetometer and plasma data. Scatterplots of the wave field ve rsus D-f are used to constrain viable shock models, to investigate the observed scatter in the wave fields at constant D-f, and to test the theoretical predictions of linear instability theory. The scatterplots confirm the abrupt onset of the foreshock waves near the upstream bou ndary, the narrow width in D-f of the region with high fields, and the relatively slow falloff of the fields at large D-f, as seen in earlie r studies, but with much smaller statistical scatter. The plots also s how an offset of the high-field region from the foreshock boundary. It is shown that an adaptive, time-varying shock model with no free para meters, determined by the observed solar wind data and published shock crossings, is viable but that two alternative models are not. Foresho ck wave studies can therefore remotely constrain the bow shock's locat ion. The observed scatter in wave field at constant D-f is shown to be real and to correspond to real temporal variations, not to unresolved changes in D-f. By comparing the wave data with a linear instability theory based on a published model for the electron beam it is found th at the theory can account qualitatively and semiquantitatively for the abrupt onset of the waves near D-f = 0, for the narrow width and offs et of the high-field region, and for the decrease in wave intensity wi th increasing D-f. Quantitative differences between observations and t heory remain, including large overprediction of the wave fields and th e slower than predicted falloff at large D-f of the wave fields. These differences, as well as the unresolved issue of the electron beam spe ed in the high-field region of the foreshock, are discussed. The intri nsic temporal variability of the wave fields, as well as their overpre diction based on homogeneous plasma theory, are indicative of stochast ic growth physics, which causes wave growth to be random and varying i n sign, rather than secular.