Suprathermal Tails in the Solar Wind and at Interplanetary Discontinuities

Ion mass spectrometers on several spacecraft have identified suprathermal tails on the velocity distributions of solar wind and pickup ions as a common feature. These tail distributions typically show a power law with a rollover at higher energies. The power law behavior of these suprathermal tails has been studied during so-called quiet times of the solar wind (no significant wave activity) as well as during active periods of the solar cycle. Several studies reported different results for this power law index, ranging from -9 to -1. In this study we used data from the PLASTIC and IMPACT instruments on STEREO spacecraft A in 2008 during a pronounced solar minimum. We investigate the temporal and spatial evolution of these tails at interplanetary structures such as shocks, stream interfaces, magnetic clouds, Alfvénic turbulence, and quiet times. We focus on He⁺ data to investigate ion acceleration, the formation of suprathermal tails and their association with Alfvénic turbulence. The results of this study show that in shocks and stream interfaces, the spectral index varies dramatically in space and time. Both pre-existing energetic ion populations as well as elevated Alfvénic turbulence impact the spectral slope. Quiet times, which may represent thermalized plasma, tend to show spectral slopes which are closer to -5.