Optical observations of the beam-plasma discharge phenomenon

Abstract

Spectroscopic observations of optical emissions from the beam-plasma discharge (BPD) phenomenon were made with NASA's vacuum chamber facility, at the Johnson Space Center, configured to simulate the physical conditions of magnetospheric electron beam injection into the ionospheric/upper-atmospheric environment. Nonlinear N 2 and N + 2 optical emission growth rates (with respect to incremental electron beam current values) were observed from the chamber gas during transition to the BPD state. For electron-beam currents ( I) near the BPD transition value ( I c ), the band emissions from the chamber gas produced by relatively low energy (⩽ 50 eV) electrons interacting with N 2 were anomalously more intense than those requiring higher energy (> 100 eV) electrons to excite them. For I ⪢ I c , the optical emissions increased linearly with I (as was the case for I < I c ) and their ratios decreased significantly from the peak values attained when I ≈ I c ). These observations suggest that during BPD some of the energy of the primary electron beam is efficiently transferred, via wave-particle interactions, to local electrons produced through ionization of the chamber gas; the resulting suprathermal electrons provide an additional source of excitation for the relatively low energy states ( A, B and C) of N 2. Such nonlinear excitation of upper atmospheric gas may occur in certain auroral events wherein the current due to the precipitating electrons approaches a value close to I c . It may explain the unusual red enhancement in the spectral distribution of optical radiation from type-B red-lower-border auroras, and the formation of the auroral thin layer.