Article

# Detection of Vortex Tubes in Solar Granulation from Observations with SUNRISE

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(Impact Factor: 5.34). 10/2010; 723(2):L180. DOI: 10.1088/2041-8205/723/2/L180
Source: arXiv

ABSTRACT

We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface.

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Available from: Wolfgang Schmidt
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• "High-resolution observations of solar granulation from the balloon-borne solar observatory SUNRISE (Barthol et al., 2011) reveal sub-structures such as granular lanes which typically consist of a bright and a dark edge, that travel from the visible boundary of granules into the granule itself (Steiner et al., 2010). By comparing the observations with numerical simulations, Steiner et al. (2010) interpret these structures to be signatures of vortex tubes which are a fundamental structure element of turbulence and are important for transferring energy from large to small scales. On the other hand, granules are also considered good tracers of the large-scale flows (Stangalini, 2014), particularly that of supergranular flows (Hart, 1956). "
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ABSTRACT: We employ different shapes of apodizing windows in the local correlation tracking (LCT) routine to retrieve horizontal velocities using numerical simulations of convection. LCT was applied on a time sequence of temperature maps generated by the Nirvana code with four different apodizing windows, namely--Gaussian, Lorentzian, trapezoidal and triangular, with varying widths. In terms of correlations (between the LCT-retrieved and simulated flow field), the triangular and the trapezoidal perform the best and worst, respectively. On segregating the intrinsic velocities in the simulations on the basis of their magnitudes, we find that for all windows, a significantly higher correlation is obtained for the intermediate and high-velocity bins and only modest or weak values in the low-velocity bins. The differences between the LCT-retrieved and simulated flow fields were determined spatially which show large residuals at or close to the boundary of granules. The extent to which the horizontal flow vectors retrieved by LCT compare with the simulated values, depends entirely on the width of the central peak of the apodizing window for a given $\sigma$. Even though LCT suffers from a lack of spatial content as seen in simulations, its simplicity and speed could serve as a viable first-order tool to probe horizontal flows--one that is ideal for large data sets.
Full-text · Article · Feb 2015 · Solar Physics
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##### Article: Sunrise: Instrument, mission, data, and first results
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ABSTRACT: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. After a brief description of instruments and data, first qualitative results are presented. In contrast to earlier observations, we clearly see granulation at 214 nm. Images in Ca II H display narrow, short-lived dark intergranular lanes between the bright edges of granules. The very small-scale, mixed-polarity internetwork fields are found to be highly dynamic. A significant increase in detectable magnetic flux is found after phase-diversity-related reconstruction of polarization maps, indicating that the polarities are mixed right down to the spatial resolution limit, and probably beyond. Comment: accepted by ApJL
Full-text · Article · Aug 2010 · The Astrophysical Journal Letters
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##### Article: SUNRISE/IMaX observations of convectively driven vortex flows in the Sun
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ABSTRACT: We characterize the observational properties of the convectively driven vortex flows recently discovered on the quiet Sun, using magnetograms, Dopplergrams and images obtained with the 1-m balloon-borne Sunrise telescope. By visual inspection of time series, we find some 3.1e-3 vortices/(Mm^2 min), which is a factor of 1.7 larger than previous estimates. The mean duration of the individual events turns out to be 7.9 min, with a standard deviation of 3.2 min. In addition, we find several events appearing at the same locations along the duration of the time series (31.6 min). Such recurrent vortices show up in the proper motion flow field map averaged over the time series. The typical vertical vorticities are <= 6e-3 1/sec, which corresponds to a period of rotation of some 35 min. The vortices show a preferred counterclockwise sense of rotation, which we conjecture may have to do with the preferred vorticity impinged by the solar differential rotation. Comment: To appear in ApJL. 5 Figs, 4 pages. The two animations associated with the work can be downloaded from http://www.iac.es/proyecto/solarhr/imaxvortex.html References updated in V2
Full-text · Article · Sep 2010 · The Astrophysical Journal Letters