Disruption of Alfvénic turbulence by magnetic reconnection in a collisionless plasma

Abstract: We calculate the disruption scale $\lambda_{\rm D}$ at which sheet-like structures in dynamically aligned Alfv\'enic turbulence are destroyed by the onset of magnetic reconnection in a low-$\beta$ collisionless plasma. The scaling of $\lambda_{\rm D}$ depends on the order of the statistics being considered, with more intense structures being disrupted at larger scales. The disruption scale for the structures that dominate the energy spectrum is $\lambda_{\rm D}\sim L_\perp^{1/9}(d_e\rho_s)^{4/9}$, where $d_e$ is the electron inertial scale, $\rho_s$ is the ion sound scale, and $L_\perp$ is the outer scale of the turbulence. When $\beta_e$ and $\rho_s/L_\perp$ are sufficiently small, the scale $\lambda_{\rm D}$ is larger than $\rho_s$ and there is a break in the energy spectrum at $\lambda_{\rm D}$, rather than at $\rho_s$. We propose that the fluctuations produced by the disruption are circularised flux ropes, which may have already been observed in the solar wind. We predict the relationship between the amplitude and radius of these structures and quantify the importance of the disruption process to the cascade in terms of the filling fraction of undisrupted structures and the fractional reduction of the energy contained in them at the ion sound scale $\rho_s$. Both of these fractions depend strongly on $\beta_e$, with the disrupted structures becoming more important at lower $\beta_e$. Finally, we predict that the energy spectrum between $\lambda_{\rm D}$ and $\rho_s$ is steeper than $k_\perp^{-3}$, when this range exists. Such a steep "transition range" is sometimes observed in short intervals of solar-wind turbulence. The onset of collisionless magnetic reconnection may therefore significantly affect the nature of plasma turbulence around the ion gyroscale.