Observations and modeling of neutral gas releases from the APEX satellite

Results from experiments of neutral xenon gas releases in the ionosphere ar e presented. The releases were made from the scientific satellite APEX, whi ch had various plasma and field diagnostics. An enhancement of HF wave acti vity over a broad band (0.1-10 MHz) is shown to occur during releases made in the sunlight at high pitch angles to the magnetic field. No changes of p lasma density were detected, but electron temperature measurements indicate electron energy enhancements during the releases. Theoretical calculations based on the conditions of the APEX releases show that charge-exchange col lisions and elastic scattering of the injected Xe neutrals by the ambient p lasma can lead to an ion beam instability that may account for the observed wave and electron energy enhancements. The interaction is akin to the crit ical ionization velocity effect. We calculate that because of the low injec tion density, this turbulence-fueled ionization should have a low yield (<1 %), which may explain the lack of detectable plasma density enhancement.