Authors: Klecker, B.
Reference: In 'Solar Eruptions and Energetic Particles'; Eds: N. Gopalswamy, J. Torsti. AGU Monograph, Vol 165, 147-156, 2006.
Over the last $\sim$ 10 years, advanced instrumentation onboard several spacecraft (e.g. WIND, SAMPEX, SOHO, and ACE), extended our ability to explore energy spectra, elemental, isotopic, and ionic charge composition of solar energetic particle (SEP) events in a wide energy range from $\sim$ 10 keV/nuc to $\sim$ 100 MeV/nuc. Due to the much improved sensitivity of the instrumentation, spectral and compositional measurements are now available for a large range of particle intensities, i.e. not only for large SEP events generally associated with coronal mass ejections (CMEs) and coronal / interplanetary shocks, but also for small, flare associated events. Originally, these two types of events were classified as gradual and impulsive, based on the duration of the associated soft X-ray emission, but this two-class paradigm of SEP events was also useful to distinguish differences in the acceleration processes involved. However, in the last few years, the two-class paradigm has been challenged by several observations, as, for example, significant enrichment of $^3$He and heavy ions and high Fe charge states of $\sim$ 20 at high energies, found in interplanetary shock related events. In this paper the recent observations on ion composition will be reviewed and their implication for our understanding of the different acceleration scenarios will be discussed.