Electromagnetic instabilities produced by neutral-particle ionization in interplanetary space

This article examines the consequence of the ionization (e.g., photoionization) of a small population of neutral atoms (hydrogen or helium) in interplanetary space. It is found that, even if the density of the newly ionized particles is only a very small fraction of that of the solar wind, these particles can efficiently excite electromagnetic waves by means of a new collective instability. The instability is driven by an anisotropy in kinetic energy of the newly ionized particles. The typical linear growth rate is γ ≃ (ωin/2½) (υ0⊥/c) where ωin is the plasma frequency of the newly ionized ions, and υ0⊥ is the characteristic speed of the newly ionized ions perpendicular to the ambient magnetic field in the frame of the solar wind.