Ih. Cairns et al., FORESHOCK LANGMUIR-WAVES FOR UNUSUALLY CONSTANT SOLAR-WIND CONDITIONS- DATA AND IMPLICATIONS FOR FORESHOCK STRUCTURE, J GEO R-S P, 102(A11), 1997, pp. 24249-24264
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.