Simulations of Decaying Kinetic Alfv\'en Wave Turbulence: Intermittent and Coherent Structures

We simulate decaying kinetic Alfv\'en wave turbulence in a strong guide field, appropriate for modeling interstellar turbulence at scales $\leq 10\rho_s$. Ion flow decouples from the system at these scales, while electron density ($n_e$) fluctuations equipartition with the magnetic field. Stable circularly symmetric structures form in $J$, $\mathrm{B}$ and $n_e$ fields after a few Alfv\'en times; nonlinear magnetic shear prevents turbulence from mixing the structures into the background and allow the structures to persist for many Alfv\'en times. $J$ filaments are large in amplitude and spatially localized, and their associated $\mathrm{B}$ and $n_e$ structures are less localized, consistent with the Biot-Savart law and KAW equipartitioning. Ensemble-averaged pdfs indicate $n_e$ and $\nabla n_e$ deviate strongly from Gaussian statistics following the onset of structure formation. The non-Gaussian $\nabla n_e$ statistics are especially of interest as a possible explanation of $\tau \propto D^4$ scaling of pulsar signal widths $\tau$ with distance-to-source $D$.---Work supported by NSF.