Turbulence and Ion Acceleration in the Outer Heliosphere

Abstract

The transport parameters of suprathermal and energetic particles in the heliosphere are intimately linked to the properties of the plasma turbulence in the supersonic solar wind flow and in the subsonic heliosheath plasma flow beyond the termination shock. Based on observations with the magnetometers on board the Helios, ACE, Ulysses, and Voyager spacecraft, and on theoretical calculations, the quantitative evolution of transport parameters of suprathermal particles over heliocentric distances is estimated. From these transport parameters, the stochastic acceleration efficiencies and spatial pressure profiles of suprathermal ions in the solar wind termination region are derived. The scattering mean free path in the heliosheath plasma also yields the injection threshold speed and characteristic time-scales for first-order Fermi acceleration of ions at the termination shock. The theoretical results are compared to observations of suprathermal ions, i.e. the termination shock energetic particles (TSPs) and the Anomalous Cosmic Rays (ACRs), with the Voyager spacecraft in the outer heliosphere, and with data on energetic neutral atoms (ENAs) detected with the CELIAS/HSTOF sensor onboard SOHO and with the ASPERA-3 sensor onboard Mars Express.