Measurement and theory of the spatial distribution of excitation by high‐current electron beams injected into the ionosphere

Spatial distributions of energy deposition rates in EXCEDE Spectral's 3 keV, 7 A electron beam injection at E region altitudes derived from onboard photographic images of the air fluorescence are presented and compared with a time‐dependent computational model of the collective beam‐plasma interaction. The volume excitation rates decrease with a scale length of order 1 m near the rocket, are ∼2 orders of magnitude greater than predicted from single particle transport calculations, and at fixed distances from the accelerator vary by less than a factor 3 over an ambient density range of at least 40. The data are consistent with a discharge model in which the driving beam plasma instability is an absolute hydrodynamic instability with energy deposition controlled by trapping and axial dimension by ambipolar expansion of the plasma.