Computer simulations of cometary‐ion/ion instabilities and wave growth

Abstract

The growth of electromagnetic fluctuations driven by newborn cometary ions is studied by means of hybrid computer simulations of homogeneous plasmas. The simulations are one‐dimensional with the direction of spatial variation in the direction of the ambient magnetic field B0 Newborn ions are injected into the simulations at a constant rate, and are given a velocity relative to the solar wind which makes a nonzero angle α with respect to B0. For relatively strong free energy injection rates corresponding to oxygen ion injection at relatively small values of α, the fluctuating field energy density exhibits exponential temporal growth of the ion/ion right‐hand resonant instability. At the end of this growth regime there is a peak value of the fluctuating magnetic field energy density; at 0°≤α≲45° this quantity is approximately proportional to cos8α. For relatively weak free energy injection rates corresponding to oxygen ion injection at relatively large α and proton injection at all α values, there are two distinct regimes. At early times the fluctuating field energy density exhibits linear temporal growth, whereas at later times the fluctuating field energy approaches a constant value in an asymptotic regime. The exponential growth regime corresponds to fluctuating magnetic field energy density spectra with clear peaks at the oxygen ion cyclotron resonance wave number in agreement with observations. In contrast, spectra of the linear temporal growth regime exhibit less distinct peaks at shorter wavelengths which then show a transfer of wave energy to longer wavelengths (an apparent “inverse cascade”) as the asymptotic regime is approached. This absence of a clear spectral peak at the proton cyclotron resonance frequency is similar to cometary observations. The simulations and observations share two additional properties: a linear correlation between cometary proton energy density and proton resonant magnetic fluctuations, as well as a shift of magnetic fluctuation energy of the oxygen‐ion/proton modes to shorter wavelengths and weaker amplitudes as α increases toward 90°.