Authors: Steven R. Cranmer (CU Boulder)

In the parts of the solar corona and solar wind that experience the fewest particle-particle collisions, the various constituents (i.e., protons, electrons, and heavy ions) are not in thermal equilibrium with one another. The particles exhibit a range of different flow speeds, temperatures, and velocity distribution anisotropies, and these differences can be used to probe the kinetic processes responsible for heating the plasma. In this scene-setting talk I will review what we know observationally about such collisionless processes in the corona and heliosphere, and also provide my perspective on what we still need to understand theoretically. To form a complete picture, it is important to combine in-situ measurements with remote-sensing data (particularly results from ultraviolet coronagraph spectroscopy in low-density regions such as coronal holes). Traditionally, these observations seemed to point toward the ubiquity of ion cyclotron resonance as a primary energization mechanism for protons and other ions. However, models of MHD turbulent cascade do not appear to easily generate these kinds of high-frequency waves. Thus, over the last few decades we have explored theoretical models that include stochastic violation of magnetic moment conservation, wave-particle instabilities, and the elusive helicity barrier. Can we hope to make sense of this hodge-podge of nonlinear physics and identify a chain of events that uniquely explains the observational data? Attend this session to be part of the adventure!