Multi-Spacecraft Observations of Energetic Heavy Ions Accelerated by Interplanetary Shocks Near Earth

Energetic ion intensity enhancements that occur in association with the arrival of interplanetary (IP) shocks driven by coronal mass ejections (CMEs) near Earth orbit are known as Energetic Storm Particle or ESP events. Such ESP events are believed to occur due to diffusive shock acceleration of solar wind ions. Although shock acceleration theory has been well studied and widely accepted, many key properties of the accelerated particles such as ion composition, peak intensity, and spectral index are at odds with theoretical expectations. During solar cycle 23, the Wind spacecraft and the Advanced Composition Explorer (ACE) have observed nearly ∼200 common IP shocks near 1 AU. A sizable number (30-40%) of these shocks were also accompanied by increases in the intensities of ions with 100's of keV to a few MeV in energy. These simultaneous observations offer us a unique opportunity to investigate various properties of energetic particles during individual IP shock events. We will examine the differences and similarities in the spatial and temporal evolution of the intensities, spectra, and composition of heavy ions from 4He through Fe during several shock events observed by two sophisticated mass spectrometers, the SupraThermal-through-Energetic Particle Telescope (STEP) on board Wind and the Ultra-Low Energy Isotope Spectrometer (ULEIS) on board ACE. We will also characterize our results in terms of the locally measured IP shock properties. Finally, we will discuss the main challenges that hinder current theoretical and experimental investigations and highlight new opportunities that will enable us to quantitatively model and predict properties of the IP shock accelerated populations.