Spatio-Temporal Scaling of Turbulent Photospheric Line-of-Sight Magnetic Field in Active Region NOAA 11158
We studied the structure and dynamics of the turbulent photospheric magnetic field in active region NOAA 11158 by characterizing spatial and temporal scaling properties of the line-of-sight (LOS) component. Using high-resolution high-cadence LOS magnetograms from SDO/HMI, we measured the power-law exponents α and β that describe Fourier power spectra in wavenumber (k) and frequency (f) domains, and we investigated their evolution during the passage of the active region through the field of view of HMI. The flaring active region NOAA 11158 produces a one-dimensional spatial power spectral density that approximately follows a k −2 power law – a spectrum that suggests parallel MHD fluctuations in an anisotropic turbulent medium. In addition, we found that the values of α capture systematically change in the configuration of the LOS photospheric magnetic field during flaring activity in the corona. Position-dependent values of the temporal scaling exponent β showed that, on an average, the core of the active region scales with β>3 surrounded by a diffusive region with an approximately f −2-type spectrum. Our results indicate that only about 1 – 3 % of the studied LOS photospheric magnetic flux displays β≈α, implying that Taylor’s hypothesis of frozen-in-flow turbulence is typically invalid for this scalar field in the presence of turbulent photospheric flows. In consequence, both spatial and temporal variations of the plasma and magnetic field must be included in a complete description of the turbulent evolution of active regions.
KeywordsActive regions Flares, relation to magnetic field Magnetic fields, photosphere Photospheric turbulence
We thank the SDO/HMI and SDO/AIA teams for the data used in this study. This work was done under CEPHEUS cooperative agreement between The Catholic University of America and NASA Goddard Space Flight Center. We thank Karin Muglach for useful discussions.
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