Computer simulations of two‐pickup‐ion instabilities in a cometary environment

Abstract

The velocity‐space evolution of newborn cometary ions is studied by means of one‐dimensional electromagnetic hybrid computer simulations of homogeneous plasmas. Newborn ions are injected into the simulations at a constant rate with velocity relative to the solar wind which is parallel to the ambient magnetic field. Both protons and oxygen ions are injected and both self‐consistently contribute to the growth of low‐frequency electromagnetic fluctuations. At weak injection rates, corresponding to relatively large distances from the cometary nucleus, the most important growing modes are two ion‐ion right‐hand resonant instabilities. The proton‐proton right‐hand mode grows more rapidly, with fluctuating magnetic field energy densities that exhibit linear temporal growth and that pitch angle scatter the protons to thin shelllike velocity distributions. The oxygen‐proton right‐hand mode has a smaller growth rate, but corresponds to a much larger free energy injection rate, so that its fluctuating magnetic field energy densities eventually exhibit exponential temporal growth to a peak amplitude, phase bunching, and magnetically trapping the oxygen ions. At times well after peak amplitude, the longer wavelength oxygen‐proton fluctuations stochastically accelerate some of the injected protons. An exponential energy distribution of accelerated ions is obtained such that the effective temperature of the high energy protons is larger than that of the heavier energetic ions. These simulations yield a scaling of the peak fluctuating field amplitudes as a function of injected beam properties and local injection rates. There is good agreement between this scaling and observations of magnetic field amplitudes at comet Giacobini‐Zinner.