Acceleration Site of Anomalous Cosmic Rays: New Results from Voyager 2

Abstract:

We have analyzed data collected from the Voyager 2 spacecraft during calibration manuevers for the magnetic field instrument while the spacecraft was located in the inner heliosheath of the heliosphere. We present anisotropy results for anomalous cosmic ray protons in the energy range ~0.5-35 MeV for all such manuevers that occurred in the heliosheath. We find that the radial component of the anisotropy vector, when converted to a radial solar wind speed via the Compton-Getting effect, generally agrees with similar Compton-Getting-method determinations from the Low-Energy Charged Particle experiment using 28-43 keV proton data. These values, however, often differ significantly from the radial solar wind speeds measured directly from the Plasma Science instrument. The results further suggest that the radial solar wind speed derived from the Compton-Getting method in the heliosheath may underestimate the speed as the spacecraft nears the heliopause. This raises a question about the existence of a stagnation region at the nose of the heliosphere, which was inferred from Voyager 1 observations involving the Compton-Getting method. The radial speed disagreements are not well-understood. The tangential and normal components of the anisotropy vector appear to provide information about the diffusive flow of anomalous cosmic rays. We find tangential components of the diffusive flow that are in reasonable agreement, both in direction and magnitude, with a 2D, steady-state, numerical model of the acceleration and transport of anomalous cosmic rays, involving diffusive shock acceleration along a non-spherical solar wind termination shock. The results suggest that there exists a diffusive flow of these particles from the flank or tail of the heliosphere towards the nose, which gives support to models of anomalous cosmic rays in which they are accelerated back along the flank or tail of the solar wind termination shock, rather than further out in the heliosheath.