OPTICAL OBSERVATIONS OF THE BEAM-PLASMA-DISCHARGE PHENOMENON

Spectroscopic observations of optical emissions from the beam-plasma d ischarge (BPD) phenomenon were made with NASA's vacuum chamber facilit y, at the Johnson Space Center, configured to simulate the physical co nditions of magnetospheric electron beam injection into the ionospheri c/upper-atmospheric environment. Nonlinear N2 and N2+ optical emission growth rates (with respect to incremental electron beam current value s) were observed from the chamber gas during transition to the BPD sta te. For electron-beam currents (I) near the BPD transition value (I(c) ), the band emissions from the chamber gas produced by relatively low energy (less-than-or-equal-to 50 eV) electrons interacting with N2 wer e anomalously more intense than those requiring higher energy (> 100 e V) electrons to excite them. For I much greater than I(c), the optical emissions increased linearly with I (as was the case for I < I(c)) an d their ratios decreased significantly from the peak values attained w hen I almost-equal-to I(c). These observations suggest that during BPD some of the energy of the primary electron beam is efficiently transf erred, via wave-particle interactions, to local electrons produced thr ough ionization of the chamber gas; the resulting suprathermal electro ns provide an additional source of excitation for the relatively low e nergy states (A, B and C) of N2. Such nonlinear excitation of upper at mospheric gas may occur in certain auroral events wherein the current due to the precipitating electrons approaches a value close to I(c). I t may explain the unusual red enhancement in the spectral distribution of optical radiation from type-B red-lower-border auroras, and the fo rmation of the auroral thin layer.