Authors: Waverly Gorman (University of Arizona), Kristopher Klein (University of Arizona), Jimmy Juno (Princeton Plasma Physics Laboratory), Jason TenBarge (Princeton University)

Heliospheric plasma is governed by complex kinetic processes that mediate energy transfer and dissipation. Capturing these effects in simulations, without incurring significant computational cost, requires innovative models that preserve essential physics while remaining computationally feasible. The Parallel Kinetic Perpendicular Moment (PKPM https://arxiv.org/abs/2505.02116) model, implemented within the Gkyell simulation framework, offers a promising reduced approach that retains full kinetic physics along the mean magnetic field while employing a fluid closure in the perpendicular direction, improving computational efficiency. To discern what physics is included in this reduced model, we simulated kinetic Alfvén waves with varying perpendicular wavenumbers and plasma betas, comparing the PKPM result to the full hot plasma dispersion relation from PLUME. We find that at low to order unity and lower beta, the lowest order PKPM System of Equations– which includes the zeroth Fourier harmonic and the first two Laguerre coefficients in vperp– agrees well with the full kinetic solutions. For larger betas, at low parallel wavenumbers (large scales), the lowest order PKPM model deviates near the proton gyroscale. The expected dip in Alfvén wave frequency, which produces the high-beta spectral gap where Alfvén waves become non-propagating, occurs due to increased Transit Time Damping compared to the dominance of Landau damping at lower beta. By decomposing the damping mechanisms in PLUME, we show that the PKPM dispersion matches the Landau-only contribution, suggesting that the current lowest order PKPM does not capture Transit Time Damping. Preliminary results indicate that including higher-order Fourier harmonics in the PKPM model may resolve this discrepancy.