With increasing penetration into the electron foreshock the characteristics of the electrostatic waves driven by streaming electrons change continuously from the familiar intense waves near the electron plasma frequency fp to weak bursts of broadband waves initially significantly above fp and then well below fp. Growth well below fp has been demonstrated theoretically for slow, cold electron beams, and the broadband waves below fp in the foreshock have been interpreted in terms of the very cold or sharp ''cutoff'' feature of a cutoff distribution for small cutoff speeds. However, an approximate theoretical criterion indicates that the electron beams studied hitherto are unstable to reactive rather than kinetic growth, thereby favoring very narrow-band growth contrary to the observed broadband growth. In this paper we determine conditions for kinetic growth well above and below fp for both cold and warm beams over a wide range of beam densities and speeds. We verify that kinetic growth below fp is possible for cold, slow beams and for warm, dense beams (over wide range of beam velocities).

We than argue that the observed broadband waves below fp are naturally explained in terms of slow, dilute electron beams with cold features but that an explanation involving warm, dense beams is inconsistent with the available data. Broadband growth above fp is explained in terms of faster, warmer beams. A unified qualitative theory for the narrow-band and broad-band waves is proposed involving (1) cutoff distributions, (2) the spatial variations in the cutoff velocity throughout the foreshock, (3) initial reactive growth with modification of the particle distribution and subsequent kinetic growth for cutoff velocities appropriate for the waves near and above fp, (4) initial kinetic growth and preservation of the cutoff feature for cutoff velocities appropriate to growth below fp, and (5) limitation of kinetic growth by inhomogeneous quasilinear relaxation. This theory can account qualitatively for the spatial variations in the intensity, center frequency, and bandwidth of the electron plasma waves in the foreshock. However, futher work is required to understand the preservation of the cold features required for growth below fp and the bursty nature of the broadband waves. ¿ American Geophysical Union 1988