http://arxiv.org/abs/1402.5934
We study structure and dynamics of the turbulent phostospheric magnetic field in the active region NOAA 11158 by characterizing the spatial and temporal scaling properties of the line-of-sight (LOS) component. Using high-resolution high-cadence LOS magnetrograms from SDO/HMI, we measured power-law exponents $\alpha$ and $\beta$ describing wavenumber- ($k$) and frequency-domain ($f$) Fourier power spectra, respectively, and investigated their evolution during the passage of the active region through the field of view of HMI. The (flaring) active region NOAA 11158 displays an average one-dimensional spatial power spectral density that follows approximately the power law $k^{-3}$; a spectrum characteristic of a passive scalar field in a turbulent two-dimensional medium. In addition, we found that values of $\alpha$ capture systematic changes in the configuration of the LOS photospheric magnetic field during flaring activity in the corona. Position-dependent values of the temporal scaling exponent $\beta$ showed that, on average, the core of the active region scales with $\beta >$ 3 surrounded by a diffusive region with an approximately $f^{-2}$-type spectrum. Our results indicate that only a small fraction (6%) of the LOS photospheric magnetic flux displays $\beta\approx\alpha$, implying that the Taylor’s frozen-in-flow turbulence hypothesis is typically not valid for the magnetic flux convected by turbulent photospheric flows. In consequence, both spatial and temporal variations of plasma and magnetic field must be included in a complete description of the turbulent evolution of the active region.
J. Guerra, A. Pulkkinen, V. Uritsky, et. al.
Tue, 25 Feb 14
2/59
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