Electron Transport in Coronal Loops: The Influence of the Exponential Separation of Magnetic Field Lines

Observations by the TRACE spacecraft have shown that coronal emission in the extreme ultraviolet is characterized by filamentary structures within coronal loops, with transverse sizes close to the instrumental resolution. Starting from the observed filament widths and using the concepts of braided magnetic fields, an estimate of the turbulence level in the coronal loops can be obtained. Magnetic turbulence in the presence of a background magnetic field can be strongly anisotropic, and such anisotropy influences the separation of magnetic field lines, as well as the magnetic field line diffusion coefficient. Careful computations of the magnetic field line diffusion coefficient D m and of the rate of exponential separation of magnetic field lines h , also allowing for the possibility of anisotropic magnetic turbulence, enable computation of the effective perpendicular diffusion coefficient for electrons. When compared with observations this yields magnetic turbulence levels on the order of δ B / B 0 = 0.05–0.7, which are larger than previous estimates. These values of the magnetic fluctuation level support the idea that magnetic turbulence can contribute to coronal heating by means of MHD turbulence dissipation. It is also found that field line transport is not governed by the quasilinear regime, but by a nonlinear regime which includes an intermediate and the percolation regimes.