University of Maryland
Acceleration of low-energy Ions in the quiet-time Solar Wind and at the Termination Shock
The long-anticipated crossing of the Termination Shock (TS) by Voyager 1 in December 2004 marked a milestone in our exploration of the heliosphere. At the location of the crossing, the shock proved to be only a modest accelerator of low-energy (40 - ~2000 keV/nuc) ions whose intensity increased abruptly, but just by a factor of about ten, with no evidence of the expected acceleration of Anomalous Cosmic Rays at higher energies. Immediately upstream of the TS and, most remarkably, in the entire heliosheath traversed by Voyager 1 during the last twelve months, the spectral index of the power law distribution functions in velocity space was -5. Suprathermal tails are observed ubiquitously in the heliosphere, primarily by Ulysses and ACE. In the quiet solar wind, between 1 and 5.4 AU, the tails have a common spectral shape, a velocity distribution function that is a power law with the same spectral index of -5 that is observed upstream and downstream of the TS at 94 AU. It will be shown that in the quiet-time solar wind, power law spectra with the unique spectral index of -5 are expected if the tails are formed by stochastic acceleration due to random compressions and expansions generated in part by variations in the pressure of the suprathermal (several tens of keV/nucleon) particles, and a cascade in energy, analogous to turbulent (Kolmogorov) cascades. Furthermore, we will argue that the -5 spectral shapes of the low-energy ions observed at the Termination Shock, both upstream and downstream, require that the pressure of the accelerated particles is behaving like that of a simple ideal gas, without heat flux, and that the intensity increase across the Termination Shock can be determined by assuming that the pressure of the accelerated particles behaves according to the Rankine-Hugoniot relationship. The approach taken here is contrasted with diffusive shock acceleration.