Magnetic Reconnection May Control the Ion-Scale Spectral Break of Solar Wind Turbulence

TL;DR

This study investigates how magnetic reconnection influences the ion-scale spectral break in solar wind turbulence, finding that reconnection likely contributes to steepening the spectral slope especially at low electron beta values.

Contribution

It provides a statistical analysis linking the spectral break scale to reconnection-related disruption scales, highlighting reconnection's role in turbulence dissipation at ion scales.

Findings

Steepest spectral indices occur at low electron beta (0.1-1).

Break scale correlates well with the disruption scale $\\lambda_D$.

Reconnection may cause steep spectral slopes steeper than -3.

Abstract

The power spectral density of magnetic fluctuations in the solar wind exhibits several power-law-like frequency ranges with a well defined break between approximately 0.1 and 1 Hz in the spacecraft frame. The exact dependence of this break scale on solar wind parameters has been extensively studied but is not yet fully understood. Recent studies have suggested that reconnection may induce a break in the spectrum at a "disruption scale" $\lambda_D$, which may be larger than the fundamental ion kinetic scales, producing an unusually steep spectrum just below the break. We present a statistical investigation of the dependence of the break scale on the proton gyroradius $\rho_i$, ion inertial length $d_i$, ion sound radius $\rho_s$, proton-cyclotron resonance scale $\rho_c$ and disruption scale $\lambda_D$ as a function of $\beta_{\perp i}$. We find that the steepest spectral indices of the dissipation range occur when $\beta_e$ is in the range of 0.1-1 and the break scale is only slightly larger than the ion sound scale (a situation occurring 41% of the time at 1 AU), in qualitative agreement with the reconnection model. In this range the break scale shows remarkably good correlation with $\lambda_D$. Our findings suggest that, at least at low $\beta_e$, reconnection may play an important role in the development of the dissipation range turbulent cascade and causes unusually steep (steeper than -3) spectral indices.