Disruption of Alfv\'enic turbulence by magnetic reconnection in a collisionless plasma

TL;DR

This paper investigates how magnetic reconnection disrupts Alfvénic turbulence in collisionless plasmas, affecting the energy spectrum and creating flux ropes, with implications for solar wind observations.

Contribution

It introduces a scaling for the disruption scale in collisionless turbulence and links it to observable structures and spectral features in plasma turbulence.

Findings

Disruption scale depends on turbulence statistics and plasma parameters.

Disrupted structures are likely circular flux ropes observed in the solar wind.

Energy spectrum steepens between the disruption scale and ion sound scale.

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.