Spectral Properties of Heavy Ions Accelerated by Interplanetary Shocks Near 1 AU

Energetic ions above ~0.05 MeV nucleon^-1 are accelerated routinely at interplanetary (IP) shocks driven by fast coronal mass ejections (CMEs). Theoretically, these observations have been interpreted in terms of diffusive shock acceleration of ions originating from either the bulk solar wind or a suprathermal tail consisting predominantly of solar wind ions. Recently, however, composition measurements from ACE have provided new insights into the question of the origin and injection of the source population for such IP shocks. In particular, observations from the Ultra-Low Energy Isotope Spectrometer on board the Advanced Composition Explorer have unexpectedly shown large enrichments of the rare isotope ³He over the solar wind value in a number of CME-driven IP shock events. The acceleration of tracer ions like ³He in such events with enhancements several times above the solar wind values is clear evidence that solar wind is not the exclusive source of material that is accelerated at interplanetary shocks. Indeed, a detailed survey of the heavy ion composition measurements has shown that solar wind ions may not even be the dominant source of material in such IP shocks. In this talk, we will present spectral properties of heavy ions in the 0.05-10.0 MeV nucleon^-1 energy range accelerated in over 70 IP shock events, and compare the spectral indices with key predictions of shock acceleration theory. We will discuss these new results in terms of seed populations and currently known rigidity-dependent shock acceleration theories.