Coupled System for Pickup-Ion-Driven Turbulence in the Outer Solar Wind

Interstellar neutral atoms slowly drifting into the heliosphere become ionized in the supersonic solar wind, where they temporarily form unstable ring-beam distributions about the magnetic field. These newly ionized particles scatter towards a more stable isotropy, self-consistently generating quasi-parallel-propagating ion cyclotron (IC) waves. These waves then drive the turbulent cascade in the outer solar wind. Evidence of this driven cascade is found in the anomalous heating of the core solar wind protons, most visible beyond 40 AU, and the slow decay of the turbulent fluctuations. Attempts to quantify this driving found that the single quasilinear interaction with IC waves alone, resulting in so-called bispherical distributions, provided far too much energy transfer from the particles to the waves/turbulence, predicting proton temperatures in excess of those observed. An additional wave-particle process was invoked, serving to moderate the scattering to the bispherical distribution, but the specific scattering process has not been rigorously studied. We point out that the turbulent cascade itself, modeled as a critically-balanced spectrum of oblique kinetic Alfvén waves (KAWs), can provide the moderating interaction on the newly-ionized pickup ions through their cyclotron-resonant energization of these particles. Since the degree of IC scattering determines the level of turbulent driving, this coupled system can be self-regulating. That is, changes in the background conditions, such as in the ionization rate, will lead to compensating changes in the driven turbulent cascade to modify the level of IC scattering of newly-ionized pickup ions and thus the level of energy transfer back to the turbulent driving. We will model this combined interaction and investigate the possibility of such a self-regulating coupled system. This KAW interaction may also explain the secondary pickup ion heating which has been observed by the SWAP instrument on the New Horizons spacecraft.