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Observations made possible in the last decade with particle detectors and ion mass spectrometers capable of identifying major ion species in the energy range of several keV to several MeV, revealed the ubiquitous presence of suprathermal tails on the velocity distributions of solar wind and pickup ions. In the majority of cases these tail have a unique spectral shape, namely a power law with index –5 (in the solar wind frame) with a rollover at energies somewhere between ~0.1 and ~10 MeV, depending on ambient solar wind conditions. The universality of these powerlaw tails having the unique –5 spectral index begs for theoretical explanations. Several different mechanisms have been proposed, and there is a hot debate on the merits of each of these in accounting for the observations.

Both Voyager spacecraft have now crossed the termination shock (TS) and in neither crossing was there any evidence for the acceleration of Anomalous Cosmic Rays (ACRs), previously expected to be accelerated at the TS. This and numerous other energetic particle observations in the heliosphere cast some doubt on the universality of diffusive shock acceleration, and several alternative mechanisms for producing ACRs have been suggested.

On the observational side, suprathermal particle spectra from primarily Ulysses (SWICS), ACE (SWICS and ULEIS) and Voyagers (LECP) have been reported, in most cases for protons. Observations and analysis of tails measured by STEREO, Cassini, and Wind, and of heavy ions as well as electrons, have been few. On the theoretical side, established theories and models have difficulties in explaining the common and unique suprathermal tail spectra as well as the acceleration of ACRs. A nonconventional theory that can explain these unique tails as well as ACR acceleration is not generally accepted or understood.

The formation of an international team of European and U.S. scientists working on these problems is thus very timely. Our goals are: (a) to establish uniform data selection criteria; (b) using these selection criteria, to focus on analysis of suprathermal ion and electron data measured by most of the heliospheric spacecraft, thus documenting more completely how often and under what conditions the –5 power-law tails occur; and (c) to resolve outstanding theoretical issues of models that explain the observed suprathermal tails and ACR acceleration.