During the active injection of an electron beam, a broad spectrum of waves is generated. In this paper examples of spectra from the recent Echo 7 experiment are presented. These results show that the characteristics of the emission can change substantially with altitude. Two-dimensional (three velocity) relativistic electromagnetic particle simulations are used to investigate the changes in the plasma conditions required to account for the observed spectral variations. It is shown that many of these variations can be accounted for by assuming that the ratio of the electron plasma frequency &ohgr;pe to cyclotron frequency &OHgr;e is less than unity at the lower altitudes of about 200 km and near or above unity at apogee of about 300 km. In the former case, whistlers with a cutoff at &ohgr;pe, lower hybrid and plasma waves are driven by the parallel beam energy while electromagnetic fundamental z mode and second harmonic x mode and electrostatic upper hybrid waves are driven by the perpendicular beam energy through the maser instability.

E¿B drifts driven by perpendicular electric fields associated with the beam-plasma interaction can also be important in generating maser emission, particularly for field-aligned injection where there is no intrinsic perpendicular beam energy. The power in the electrostatic waves is a few percent of the beam energy and that in the electromagnetic waves a few tenths of a percent. In the latter case, where &ohgr;pe/&OHgr;e increases above unity, emission in the fundamental z mode and second harmonic x mode become suppressed. The dominant emissions below &OHgr;e are strong whistlers and lower hybrid waves while above &OHgr;e the dominant waves are electrostatic Bernstein and upper hybrid waves, with frequencies near harmonics close to the spacecraft and near half harmonics downstream from the spacecraft for &ohgr;pe/&OHgr;e≂2. This difference in emission with distance is due to the local depletion of plasma near the spacecraft. Weak second harmonic plasma radiation is also identified in the simulations, but this component is probably below the noise threshold of the Echo 7 receiver. ¿ Copyright 1990 by the American Geophysical Union