Dynamic effects of a magnetic field on diffusion in a chaotic flow

In this Letter we examine the effect of different strength magnetic fields on the two-dimensional version of the 'circularly-polarized' Galloway-Proctor flow. We show that the average stretching in the region, which is measured by the effect on the spatial average of the finite-time Lyapunov exponents, is decreased unless the field is such that the magnetic energy everywhere is Rm times smaller than the equipartition value. Further, we show that the greatest reduction in the average stretching in the region does not occur for the strongest field considered in this work, namely an equipartition strength field. Instead, the reduction of the average Lyapunov exponent is found to be greatest for a field with a strength that is less than equipartition.