Particle acceleration and turbulence transport in heliospheric plasmas

Plasma turbulence at various length scales affects practically all mechanisms proposed to be responsible for particle acceleration in the heliosphere. In this paper, we concentrate on providing a synthesis of some recent efforts to understand particle acceleration in the solar corona and inner heliosphere. Acceleration at coronal and interplanetary shock waves driven by coronal mass ejections (CMEs) is the most viable mechanism for producing large gradual solar energetic particle (SEP) events, whereas particle acceleration in impulsive flares is assumed to be responsible for the generation of smaller impulsive SEP events. Impulsive events show enhanced abundances of ³He and heavy ions over the gradual SEP events. Gradual events often show charge states consistent with acceleration of ions in a dilute plasma at 1-2 MK temperature, while impulsive events have higher charge states. The division of SEP events to gradual and impulsive has been challenged by the discovery of events, which show intensity-vs.-time profiles typical for gradual events but, especially at the highest energies (above 10 MeV/nucl), abundances and charge states more typical of impulsive events. Although a direct flare component cannot be ruled out, we find that particle acceleration at quasi-perpendicular shocks in the low corona also offer a plausible explanation for the hybrid events. By carefully modeling shock acceleration and coronal turbulence and its modification by the accelerated particles, a consistent picture of gradual events thus emerges from the shock acceleration hypothesis.