Comparing Parker Solar Probe observations with theory and simulations of reflection-driven Alfven turbulence

Abstract:

Alfven Waves (AW's) and AW turbulence play an important role in several models for the heating of the solar corona and the acceleration of solar wind. In the heating scenario, the role of AW's is to transport energy from convective motions on the photosphere into the solar atmosphere, corona, and solar wind, where as the role of turbulence is to transfer the energy of these AW's to smaller scales where kinetic mechanisms can efficiently heat the ambient plasma. When density fluctuations are neglected, AW turbulence can only arise from the nonlinear interaction between counter-propagating AW's, in which case a source of sunward-propagating AW's are needed for this scenario to work. A possible source of these sunward-propagating fluctuations is the non-WKB reflection of AW's due to the inhomogeneities of the background plasma, in which case the turbulence is called reflection-driven. In this work we present a direct comparison between Parker Solar Probe observations of many turbulent properties of the solar wind, such as turbulent amplitudes and correlation lengths, with predictions from phenomenological models and high-resolution numerical simulations of reflection-driven AW turbulence. The observations, spanning heliocentric distances from 0.16 AU to 0.8 AU, are found broadly consistent with predictions from theory and simulations, suggesting that reflection-driven AW's may play an important role in the radial evolution of solar wind turbulence.