Authors: Niranjana Shankarappa (University of Arizona), Kristopher Klein (University of Arizona)

The heating of protons and electrons as the result of the dissipation of turbulence plays a crucial role in the thermodynamics of the solar wind and other analogous plasma environments. Quantifying the bifurcation of energy between these populations is necessary for fully characterizing these systems. Using Parker Solar Probe (PSP) observations, we seek to characterize how the relative heating rates vary as a function of radial distance and plasma conditions, enabling us to better constrain the thermodynamics of this never before sampled region of our heliosphere. For this study, we focus on Landau damping, one plausible mechanism for damping of the turbulent cascade. Observations from the first several encounters of PSP find that plasma beta, the ratio of thermal to magnetic pressures, is not significantly smaller than unity, implying that Landau damping may be relevant for these intervals. We apply a particular theoretical model, developed in Howes et al 2008 JGR, to determine proton and electron heating rates due to low-frequency Alfvenic turbulence where dissipation is mediated through Landau damping as a function of observable plasma parameters. The model considers a steady-state cascade of wavevector anisotropic turbulent fluctuations from the inertial through dissipation ranges, connecting the MHD and kinetic descriptions. Using magnetic field and thermal plasma observations from the first two PSP perihelion, we distinguish high-frequency circularly polarized waves from the low-frequency turbulence and apply the cascade model to spectra constructed from the latter. We find that the model accurately describes the observed power spectrum for approximately 50 percent of intervals in encounters 1 and 2, bolstering the relevance of Landau damping in the young solar wind. We verify the strong dependency of the ratio of proton-to-electron heating rates on plasma beta and the consistency of the assumption of a critically balanced cascade. We estimate high magnitudes for the Kolmogorov constant, which is inversely proportional to the non-linear energy cascade rate. We verify that our estimates of heating rates are comparable to other empirical estimates and determine fits for these estimates as a function of plasma beta and heliocentric distance.