Particle data from ISEE 1 sampled in the earth's magnetotail show the presence of energetic ion beams in the boundary layer of the plasma sheet. A theory of instabilities driven by the beams is developed and compared with wave data sampled simultaneous to the particle data. We conclude that the ion beams generate broadband electrostatic bursts of noise. The electrostatic noise correlates well with the occurrence of the beams, and the spectrum is consistent with that predicted from a negative energy beam instability under magnetotail conditions. The theory predicts that a spectrum of growing waves can be driven for frequencies from 0.001 ωpe up to ωpe, the electron plasma frequency, with a spectral peak typically near 0.01 ωpe or lower, in agreement with the wave data. Furthermore, as one moves away from the source region perpendicular to the magnetic field, the high-frequency components of the observed wave spectra are predicted to disappear gradually, leaving the low-frequency part of the spectrum, also as is observed. Evidence is given for significant pitch angle scattering of the beams by the broadband electrostatic noise leading to more isotropic ion distributions.