Revisiting stability of pickup ion rings

Abstract:

Do we really understand ring distribution stability in weakly turbulent plasmas? The first measurement of the interstellar magnetic fluctuation spectrum by Voyager 1 was consistent with interstellar Kolmogorov turbulence spectrum, ruling out any significant local wave sources, such as an instability. We show that both linear dispersion analysis and kinetic plasma computer simulations should be approached with much caution when drawing conclusions about the wave generation and resulting particle scattering in pitch angle, especially for very tenuous rings. Using hybrid-kinetic and PIC models we carefully examine the nonlinear phase of pickup ion (PUI) scattering on self-generated fluctuations to reveal a critical dependence on the number of pseudo-particles present in the simulation. It is shown that narrow rings modeled with very high number of particles (1 million per cell) could become stable in the nonlinear phase. The second part of this presentation deals with more realistic pickup rings derived from the distribution of primary energetic neutral atoms (ENAs) originating in the solar wind and the inner heliosheath. The width of these distributions allow them to remain stable on the timescales of at least a few months. These results lend additional support to the secondary PUI theory of the origin of the IBEX ribbon.