On the beam speed and wavenumber of intense electron plasma waves near the foreshock edge

Using high time resolution particle and wave data from the Wind spacecraft, we examine several crossings of the electron foreshock-solar wind boundary. We show that the most intense electron plasma waves, observed near the foreshock boundary, often occur coincident with a flux of electrons with energies between 1 keV and 27 keV. This corresponds to electron beam speeds of 9v_th ≤ v_b ≤ 50v_th, rather than v_b ≈ 5v_th, as is inferred from reduced distribution functions obtained by other instruments. Assuming Landau coupling, the resonant index of refraction is then 3 ≤ N₀ ≤ 15, which implies that the waves are susceptible to strong scattering by ambient density fluctuations. The most intense electric fields are not well correlated with beam speed, and the distribution of electric field occurrence is broadly aligned with the interplanetary magnetic field direction. We compare the estimated maturity and bandwidth of the instability with those expected of the electrostatic decay instability and nonlinear Landau damping and find that our observations show a narrower bandwidth than expected. We suggest that the observations are consistent with scattering of electrostatic Langmuir waves to small wavenumber. The observed transverse polarization may be explained by the electromagnetic nature of the small-wavenumber z-mode, or as evidence of incident, reflected, and transmitted electrostatic components. The generation of electromagnetic emission at f_pe and 2f_pe is discussed in the context of the observations.