http://arxiv.org/abs/1711.01290
The motions of small-scale magnetic flux elements in the solar photosphere can provide some measure of the Lagrangian properties of the convective flow over the depth which the elements sample. Measurements of these motions have thus been critical in estimating the turbulent diffusion coefficient in flux-transport dynamo models and in determining the Alfven wave excitation spectrum for coronal heating models. We examine the motions of magnetic elements in a 24 hour long Hinode/NFI magnetogram sequence with 90 second cadence, and study both the scaling of their mean squared displacement and the shape of their displacement probability distribution as a function of time. We find, in agreement with previous work, that the mean squared displacement scales super-diffusively with a slope of about 1.48. This is true in other studies even for temporal increments as small as 5 seconds for which ballistic scaling would be expected. Using high cadence MURaM simulations, we show that the observed super-diffusive scaling at short temporal increments is an artifact of interpreting changes in elements’ barycenter position as their true motion, as these barycenter positions are subject to random flux evolution. We also find that for long temporal increments, beyond granular lifetimes, the observed displacement distribution deviates from that expected for a diffusive process, changing from Rayleigh to Gaussian. The evolution of the distribution agrees well with an analytic model that accounts for advection by the supergranular flows in addition to the granular motions. These results complicate the interpretation of magnetic element motions on both short and long time scales and suggest that their use in turbulent diffusion or wave excitation models may be problematic. We propose an alternative indirect method of using passive tracers that is likely more robust, in the measured photospheric flow field.
P. Agrawal, M. Rast, M. Gosic, et. al.
Tue, 7 Nov 17
77/118
Comments: submitted to ApJ
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