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Register a new userTurbulent Diffusion in the Photosphere as Derived from Photospheric Bright Point Motion
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Added by
Valentyna Abramenko
Publication date
2011
fields
Physics
and research's language is
English
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On the basis of observations of solar granulation obtained with the New Solar Telescope (NST) of Big Bear Solar Observatory, we explored proper motion of bright points (BPs) in a quiet sun area, a coronal hole, and an active region plage. We automatically detected and traced bright points (BPs) and derived their mean-squared displacements as a function of time (starting from the appearance of each BP) for all available time intervals. In all three magnetic environments, we found the presence of a super-diffusion regime, which is the most pronounced inside the time interval of 10-300 seconds. Super-diffusion, measured via the spectral index, $gamma$, which is the slope of the mean-squared displacement spectrum, increases from the plage area ($gamma=1.48$) to the quiet sun area ($gamma=1.53$) to the coronal hole ($gamma=1.67$). We also found that the coefficient of turbulent diffusion changes in direct proportion to both temporal and spatial scales. For the minimum spatial scale (22 km) and minimum time scale (10 sec), it is 22 and 19 km$^{2}$ s$^{-1}$ for the coronal hole and the quiet sun area, respectively, whereas for the plage area it is about 12 km$^{2}$ s$^{-1}$ for the minimum time scale of 15 seconds. We applied our BP tracking code to 3D MHD model data of solar convection (Stein et al. 2007) and found the super-diffusion with $gamma=1.45$. An expression for the turbulent diffusion coefficient as a function of scales and $gamma$ is obtained.
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To explore the magnetic flux dispersion in the undisturbed solar photosphere, magnetograms acquired by Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) were utilized. Two areas, a coronal hole area (CH) and an area of super-granulation pattern, SG, were analyzed. We explored the displacement and separation spectra and the behavior of the turbulent diffusion coefficient, $K$. The displacement and separation spectra are very similar to each other. Small magnetic elements (of size 3-100 squared pixels and the detection threshold of 20 Mx sm$^{-2}$) in both CH and SG areas disperse in the same way and they are more mobile than the large elements (of size 20-400 squared pixels and the detection threshold of 130 Mx sm$^{-2}$). The regime of super-diffusivity is found for small elements ($gamma approx 1.3 $ and $K$ growing from $sim$100 to $sim$ 300 km$^2$ s$^{-1}$). Large elements in the CH area are scanty and show super-diffusion with $gamma approx 1.2$ and $K$ = (62-96) km$^2$ s$^{-1}$ on rather narrow range of 500-2200 km. Large elements in the SG area demonstrate two ranges of linearity and two diffusivity regimes: sub-diffusivity on scales (900-2500) km with $gamma=0.88$ and $K$ decreasing from $sim$130 to $sim$100 km$^2$ s$^{-1}$, and super-diffusivity on scales (2500-4800) km with $gamma approx 1.3$ and $K$ growing from $sim$140 to $sim$200 km$^2$ s$^{-1}$. Comparison of our results with the previously published shows that there is a tendency of saturation of the diffusion coefficient on large scales, i.e., the turbulent regime of super-diffusivity is gradually replaced by normal diffusion.
One of the most important features in the solar atmosphere is magnetic network and its rela- tionship with the transition region (TR), and coronal brightness. It is important to understand how energy is transported into the corona and how it travels along the magnetic-field lines be- tween deep photosphere and chromosphere through the TR and corona. An excellent proxy for transportation is the Interface Region Imaging Spectrograph (IRIS) raster scans and imaging observations in near-ultraviolet (NUV) and far-ultraviolet (FUV) emission channels with high time-spatial resolutions. In this study, we focus on the quiet Sun as observed with IRIS. The data with high signal to noise ratio in Si IV, C II and Mg II k lines and with strong emission intensities show a high correlation in TR bright network points. The results of the IRIS intensity maps and dopplergrams are compared with those of Atmo- spheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) instruments onboard the Solar Dynamical Observatory (SDO). The average network intensity profiles show a strong correlation with AIA coronal channels. Furthermore, we applied simultaneous observations of magnetic network from HMI and found a strong relationship between the network bright points in all levels of the solar atmosphere. These features in network elements exhibited high doppler velocity regions and large mag- netic signatures. A dominative fraction of corona bright points emission, accompanied by the magnetic origins in photosphere, suggest that magnetic-field concentrations in the network rosettes could help couple between inner and outer solar atmosphere.
Convective flows are known as the prime means of transporting magnetic fields on the solar surface. Thus, small magnetic structures are good tracers of the turbulent flows. We study the migration and dispersal of magnetic bright features (MBFs) in intergranular areas observed at high spatial resolution with Sunrise/IMaX. We describe the flux dispersal of individual MBFs as a diffusion process whose parameters are computed for various areas in the quiet Sun and the vicinity of active regions from seeing-free data. We find that magnetic concentrations are best described as random walkers close to network areas (diffusion index, gamma=1.0), travelers with constant speeds over a supergranule (gamma=1.9-2.0), and decelerating movers in the vicinity of flux emergence and/or within active regions (gamma=1.4-1.5). The three types of regions host MBFs with mean diffusion coefficients of 130 km^2/s, 80-90 km^2/s, and 25-70 km^2/s, respectively. The MBFs in these three types of regions are found to display a distinct kinematic behavior at a confidence level in excess of 95%.
By use of the high-resolution spectral data and the broadband imaging obtained with the Goode Solar Telescope at the Big Bear Solar Observatory on 2013 June 6, the spectra of three typical photospheric bright points (PBPs) have been analyzed. Based on the H$alpha$ and Ca II 8542 AA line profiles, as well as the TiO continuum emission, for the first time, the non-LTE semi-empirical atmospheric models for the PBPs are computed. The attractive characteristic is the temperature enhancement in the lower photosphere. The temperature enhancement is about 200 -- 500 K at the same column mass density as in the atmospheric model of the quiet-Sun. The total excess radiative energy of a typical PBP is estimated to be 1$times$10$^{27}$ - 2$times$10$^{27}$ ergs, which can be regarded as the lower limit energy of the PBPs. The radiation flux in the visible continuum for the PBPs is about 5.5$times$10$^{10}$ ergs cm$^{-2}$ s$^{-1}$. Our result also indicates that the temperature in the atmosphere above PBPs is close to that of a plage. It gives a clear evidence that PBPs may contribute significantly to the heating of the plage atmosphere. Using our semi-empirical atmospheric models, we estimate self-consistently the average magnetic flux density $B$ in the PBPs. It is shown that the maximum value is about one kilo-Gauss, and it decreases towards both higher and lower layers, reminding us of the structure of a flux tube between photospheric granules.
The sub-arcsec bright points (BP) associated with the small scale magnetic fields in the lower solar atmosphere are advected by the evolution of the photospheric granules. We measure various quantities related to the horizontal motions of the BPs observed in two wavelengths, including the velocity auto-correlation function. A 1 hr time sequence of wideband H$alpha$ observations conducted at the textit{Swedish 1-m Solar Telescope} (textit{SST}), and a 4 hr textit{Hinode} textit{G}-band time sequence observed with the Solar Optical telescope are used in this work. We follow 97 textit{SST} and 212 textit{Hinode} BPs with 3800 and 1950 individual velocity measurements respectively. For its high cadence of 5 s as compared to 30 s for textit{Hinode} data, we emphasize more on the results from textit{SST} data. The BP positional uncertainty achieved by textit{SST} is as low as 3 km. The position errors contribute 0.75 km$^2$ s$^{-2}$ to the variance of the observed velocities. The textit{raw} and textit{corrected} velocity measurements in both directions, i.e., $(v_x,v_y)$, have Gaussian distributions with standard deviations of $(1.32,1.22)$ and $(1.00, 0.86)$ km s$^{-1}$ respectively. The BP motions have correlation times of about $22 - 30$ s. We construct the power spectrum of the horizontal motions as a function of frequency, a quantity that is useful and relevant to the studies of generation of Alfven waves. Photospheric turbulent diffusion at time scales less than 200 s is found to satisfy a power law with an index of 1.59.
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