Ubiquitous Suprathermal Tails on the Solar Wind and Pickup Ion Distributions

Abstract

Using time‐of flight mass spectrometers on Ulysses and ACE we measure the velocity distributions of particles in the poorly explored suprathermal range that extends from about twice to fifty times the solar wind speed. We find that H+, He++ and He+ are always present in this energy (or speed) range at variable intensity levels that depend on solar wind conditions. Contrary to most common expectations, even during most quiet times, in the absence of shocks and other interplanetary disturbances, suprathermal power‐law tails on solar wind distributions are observed that extend to the highest energies measured. Suprathermal tails are observed in distributions of solar wind and interstellar pickup ions, not just for protons but also in distributions of all heavy ions that can be measured. Tails are seen in the quiet and disturbed slow wind, as well as in the super‐quiet fast wind from polar coronal holes. Instead of diminishing rapidly with heliocentric distance, tails are found to persist to at least 5.4 AU and become harder. We suggest that pre‐accelerated particles contained in these tails may well be the population that is injected for further acceleration by shocks. It remains unknown what processes produce these tails and whether these tails persist to very large distances.