Observations and modeling of neutral gas releases from the APEX satellite

Results from experiments of neutral xenon gas releases in the ionosphere are presented. The releases were made from the scientific satellite APEX, which had various plasma and field diagnostics. An enhancement of HF wave activity 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 plasma 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 collisions and elastic scattering of the injected Xe neutrals by the ambient plasma can lead to an ion beam instability that may account for the observed wave and electron energy enhancements. The interaction is akin to the critical ionization velocity effect. We calculate that because of the low injection density, this turbulence-fueled ionization should have a low yield (<1%), which may explain the lack of detectable plasma density enhancement.