Dissipation and Excitation: The Role of Kinetic-Scale Waves and Instabilities in the Evolution of the Solar Wind

One of the grand challenges of heliophysics is quantifying the distribution and flow of energy in the solar wind as it is transported from the Sun’s surface through the solar system and cascaded from immense flows to small fluctuations and particle heating. This task is necessary for characterizing the energy budget in the heliosphere, but also serves as a natural laboratory for studying analogous astrophysical systems beyond the reach of in situ observations. There are a number of competing mechanisms that have been proposed to govern the transport and dissipation of energy that are expected to dominate under different conditions, including the excitation and dissipation of kinetic-scale waves. Such waves are observed in the inner heliosphere, coincident with non-equilibrium velocity distribution structures whose presence is enabled by the weakly collisional plasma state dictated by the system’s low density and high temperature. More detailed measurements of the structure of the velocity distributions, enabled by recent in situ missions including MMS, Parker Solar Probe, and Solar Orbiter, allow for more sophisticated studies of energy transfer mechanisms, including both the damping and excitation of waves. This talk will review these recent studies and consider how future multipoint, multiscale missions such as HelioSwarm will continue to improve our understanding of the thermodynamic role of ion-scale waves and fluctuations in the solar wind.