Abstract

Energetic electron beams are frequently used as active probes of space plasmas. Often the assumed test particle nature of these electrons is violated when the electron beam stimulates plasma wave emissions. Such complex phenomena have been observed on rockets and satellites and are being modeled in laboratory plasmas. The large vacuum chamber at NASA Johnson Space Center in Houston, Texas has been used for modeling F-region type ionospheric plasmas. A VLF receiver has been flown into an auroral plasma and the spectra from this flight will be compared to VLF spectra obtained in the NASA/JSC laboratory chamber. The electron beam is believed to have produced beam plasma discharge (BPD) on the rocket similar to that seen in the lab. At times during the rocket flight the electron beam was operated at 4 kilovolts and the electron current modulated at 3 kilohertz from 0 to 80 milli-amps. This resulted in the beam pulsing in and out of BPD and a variety of propagating wave modes.

The laboratory VLF electric field spectra during BPD show a characteristic peak at a few kilohertz with amplitudes over 100 mV/m. This peak broadens and moves to higher frequencies as the current is increased at a fixed electron voltage. Other features of BPD in the lab as seen in the VLF spectra include appearance of the spectral peak prior to optical BPD threshold, differences between E͂ and B͂ spectra below the peak and oscillation in and out of BPD even under a steady state electron gun current on time scales of 100 ms.