Title: 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 Alfvénic 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.