Injection and acceleration of interstellar helium at interplanetary shocks

Although, substantial progress has been made in the understanding of particle injection and acceleration at shocks, the processes involved are not fully understood. Meanwhile numerical simulations have become powerful enough to include different species, such as heavy ions and electrons, and observations are now more sensitive to resolve individual ion species and charge states even during less intense solar energetic particle events. As a consequence, the combination of theory, simulations, and observations provides the tools to study the physical processes at interplanetary discontinuities in detail. In this talk we will report on recent results using ACE/SEPICA and SOHO/CELIAS/STOF to observe interstellar pickup He⁺ as well as coronal and/or solar wind He²⁺ at interplanetary discontinuities. Apparently He⁺ is injected and accelerated with very high efficiency compared with He²⁺. Therefore, this pair, He⁺ and He²⁺, represents an ideal probe for the injection efficiency, because it comes from two well-defined sources with different velocity distributions. We will discuss the most recent findings as well as their implications on shock structure and acceleration in comparison with theory and numerical simulations.