Early Time Evolution of Turbulence in the Space Environment by Neutral Beam Injection

The Space Measurement of A Rocket‐released Turbulence mission will demonstrate the production and evolution of turbulence in the space environment via a cascade of plasma physics processes. A sounding rocket will inject neutral barium atoms into the upper ionosphere at high speed across the magnetic field. The barium atoms photoionize, and the magnetic field confines them, leading to an ion ring distribution in velocity space that is unstable to electrostatic lower hybrid waves. Nonlinear induced scattering of lower hybrid waves produces electromagnetic magnetosonic and whistler waves that rapidly escape the source region and propagate into the radiation belts. In this paper, we present models and simulations of early time parts of this cascade until the lower hybrid waves are generated. We study the neutral atom injection, expansion, and ionization analytically and computationally. In a realistic scenario, the velocity distribution function is found to be nongyrotropic instead of a perfect ring in the plane perpendicular to the magnetic field. Using these results, we examine a representative region of the barium cloud and perform particle‐in‐cell simulations of the excitation of the lower hybrid waves. Of particular interest is the energy extracted from the kinetic energy of the barium ions, as this will ultimately affect the amplitude of the electromagnetic waves in the radiation belts. We find that for velocity distribution functions that deviate from an ideal ring distribution, the energy extracted from the fast barium ions increases.