Dark Intergranular Lanes Research Articles

Towards the Identification and Classification of Solar Granulation Structures Using Semantic Segmentation

Solar granulation is the visible signature of convective cells at the solar surface. The granulation cellular pattern observed in the continuum intensity images is characterised by diverse structures e.g., bright individual granules of hot rising gas or dark intergranular lanes. Recently, the access to new instrumentation capabilities has given us the possibility to obtain high-resolution images, which have revealed the overwhelming complexity of granulation (e.g., exploding granules and granular lanes). In that sense, any research focused on understanding solar small-scale phenomena on the solar surface is sustained on the effective identification and localization of the different resolved structures. In this work, we present the initial results of a proposed classification model of solar granulation structures based on neural semantic segmentation. We inspect the ability of the U-net architecture, a convolutional neural network initially proposed for biomedical image segmentation, to be applied to the dense segmentation of solar granulation. We use continuum intensity maps of the IMaX instrument onboard the Sunrise I balloon-borne solar observatory and their corresponding segmented maps as a training set. The training data have been labeled using the multiple-level technique (MLT) and also by hand. We performed several tests of the performance and precision of this approach in order to evaluate the versatility of the U-net architecture. We found an appealing potential of the U-net architecture to identify cellular patterns in solar granulation images reaching an average accuracy above 80% in the initial training experiments.

Granular-scale Magnetic Flux Emergence and its Associated Features in an Emerging Active Region

Abstract Using the high-resolution photosphere and chromosphere observations made by the 1 m New Vacuum Solar Telescope, we studied the granular-scale magnetic flux emergence occurring in emerging active region NOAA 12579. Supplementary observations are also provided by the spacecraft Solar Dynamics Observatory. The studied granular-scale flux emergence took place at two different locations. One is completely embedded in the unipolar region of the following sunspots (Case 1), while another is located at the central part in the active region (Case 2). We find that both cases initially emerge from a dark patch like a wide intergranular lane, but showing the different subsequent features. In Case 1, the emerging granule grows in an elongated feature and reaches its maximum size of almost of 5″ × 3″, with an elongated speed of about 2–3 km s−1. An eruption (i.e., surge) with bright footpoints is observed after the emerging granule reaches its maximum scale. There is a time delay of more than 10 minutes between the appearance of the abnormal granule and the Hα surge. Furthermore, its footpoints are clearly rooted at the intergranular lane. We propose that the eruptive surge could be triggered by the reconnection between the emerging magnetic flux and the preexisting ambient field, leading to the localized heating and bidirectional flows. In Case 2, the granular cell emerging is simultaneously associated with bright points with opposite magnetic polarity, showing the separating motion between them and a bunch of newly formed arch filament systems. We infer that the bright points are due to the strong-field magnetic concentration in the dark intergranular lanes rather than the instantaneous Ellerman bombs.

Structure of photosphere under high resolution: granules, faculae, micropores, intergranular lanes

The fundamental small-scale structures such as granules, faculae, micropores that are observed in the solar photosphere under high resolution are discussed. As a separate constituent of the fine structure, a continuous net of dark intergranular lanes is considered. The results due to image processing of micropores and facular knots obtained on modern adaptive optics telescopes are presented. For intergranular lanes and micropores, a steady-state magnetic diffusion model is proposed, in which the horizontal-vertical plasma flows converging to a intergranular lane (and to the body of a micropore) compensate for the dissipative spreading of the magnetic flux at a given scale. A theoretical estimate of the characteristic scales of these structures in the photosphere is obtained as 20–30 km for the thickness of dark intergranular lanes (and the diameter of the thinnest magnetic tube in the solar photosphere), 200–400 km for the diameter of micropores. A model of a facular knot with the darkening core on the axis, which physically represents a micropore, stabilizing the entire magnetic configuration over a time interval of up to 1 day, is briefly described.

Brightness of Solar Magnetic Elements As a Function of Magnetic Flux at High Spatial Resolution

Abstract We investigate the relationship between the photospheric magnetic field of small-scale magnetic elements in the quiet-Sun (QS) at disk center and the brightness at 214, 300, 313, 388, 397, and 525.02 nm. To this end, we analyzed spectropolarimetric and imaging time series acquired simultaneously by the Imaging Magnetograph eXperiment magnetograph and the SuFI filter imager on board the balloon-borne observatory during its first science flight in 2009, with high spatial and temporal resolution. We find a clear dependence of the contrast in the near ultraviolet and the visible on the line-of-sight component of the magnetic field, B LOS, which is best described by a logarithmic model. This function effectively represents the relationship between the Ca ii H-line emission and B LOS and works better than the power-law fit adopted by previous studies. This, along with the high contrast reached at these wavelengths, will help with determining the contribution of small-scale elements in the QS to the irradiance changes for wavelengths below 388 nm. At all wavelengths, including the continuum at 525.40 nm, the intensity contrast does not decrease with increasing B LOS. This result also strongly supports the fact that has resolved small strong magnetic field elements in the internetwork, resulting in constant contrasts for large magnetic fields in our continuum contrast at 525.40 nm versus the B LOS scatterplot, unlike the turnover obtained in previous observational studies. This turnover is due to the intermixing of the bright magnetic features with the dark intergranular lanes surrounding them.

Height-dependent Velocity Structure of Photospheric Convection in Granules and Intergranular Lanes with Hinode/SOT

Abstract The solar photosphere is the visible surface of the Sun, where many bright granules, surrounded by narrow dark intergranular lanes, are observed everywhere. The granular pattern is a manifestation of convective motion at the photospheric level, but its velocity structure in the height direction is poorly understood observationally. Applying bisector analysis to a photospheric spectral line recorded by the Hinode Solar Optical Telescope, we derived the velocity structure of the convective motion in granular regions and intergranular lanes separately. The amplitude of motion of the convective material decreases from 0.65 to 0.40 km s−1 as the material rises in granules, whereas the amplitude of motion increases from 0.30 to 0.50 km s−1 as it descends in intergranular lanes. These values are significantly larger than those obtained in previous studies using bisector analysis. The acceleration of descending materials with depth is not predicted from the convectively stable condition in a stratified atmosphere. Such convective instability can be developed more efficiently by radiative cooling and/or a gas pressure gradient, which can control the dynamical behavior of convective material in intergranular lanes. Our analysis demonstrated that bisector analysis is a useful method for investigating the long-term dynamic behavior of convective material when a large number of pixels is available. In addition, one example is the temporal evolution of granular fragmentation, in which downflowing material develops gradually from a higher layer downward.

Identifying and Tracking Solar Photospheric Bright Points Based on Three-dimensional Segmentation Technology

Photospheric bright points (PBPs) are tiny short-lived brightening phenomena that can be seen within dark inter-granular lanes. In this paper, we develop a new method to identify and track a PBP in a 3D space-time cube. Different from the common-adopted strategy of “identifying-before-tracking”, this algorithm is based on the strategy of “identifying-by-tracking”. This method can identify a PBP when its intensity is still rather weak, and can avoid the discontinuous evolution of PBP caused by the algorithm of Laplacian morphological dilation (LMD) to recognize completely the evolutionary process of a PBP. The statistics on a group of G-band data observed by Hinode/SOT (Solar Optical Telescope) in quiet regions indicate that the average lifetime of isolated PBP is as long as 3minutes, and the longest lifetime is up to 27minutes, which means that the lifetime of PBPs is longer than that obtained by the previous LMD algorithm. Furthermore, it is also found that the mean intensity contrast of PBPs is 1.02 times of the mean photospheric intensity, which is weaker than that calculated by the LMD algorithm, and that the intensity of PBP exhibits a periodical oscillation of 2∼3minute during the whole lifetime.

AUTOMATIC DETECTION AND EXTRACTION ALGORITHM OF INTER-GRANULAR BRIGHT POINTS

Inter-granular Bright Points (igBPs) are small-scale objects in the Solar photosphere which can be seen within dark inter-granular lanes. We present a new algorithm to automatically detect and extract igBPs. Laplacian and Morphological Dilation (LMD) technique is employed by the algorithm. It involves three basic processing steps: (1) obtaining candidate "seed" regions by Laplacian; (2) determining the boundary and size of igBPs by morphological dilation; (3) discarding brighter granules by a probability criterion. For validating our algorithm, we used the observed samples of the Dutch Open Telescope (DOT), collected on April 12, 2007. They contain 180 high-resolution images, and each has a <TEX>$85{\times}68\;arcsec^2$</TEX> field of view (FOV). Two important results are obtained: first, the identified rate of igBPs reaches 95% and is higher than previous results; second, the diameter distribution is <TEX>$220{\pm}25km$</TEX>, which is fully consistent with previously published data. We conclude that the presented algorithm can detect and extract igBPs automatically and effectively.

Magnetic and velocity fields of a solar pore

Context. Solar pores are intermediate-size magnetic flux features that emerge at the surface of the Sun. The absence of a filamentary penumbra indicates that there is a relatively simple magnetic structure with a prevailing vertical magnetic field. Aims. Relations between the magnetic field components, line-of-sight velocities, and horizontal motions in and around a large pore (Deff = 8. �� 5) are analysed to provide observational constraints on theoretical models and numerical simulations. Methods. Spectropolarimetric observations in Fe I 617.3 nm of the pore NOAA 11005 with the IBIS spectrometer attached to the Dunn Solar Telescope are inverted into series of maps of thermal, magnetic, and velocity parameters using the SIR code. Horizontal velocities are obtained from series of white-light images by means of local correlation tracking. Results. The magnetic field B extends from the visible pore border of more than 3. �� 5 and has a radial structure in a form of spines that are co-spatial with dark intergranular lanes. The horizontal component Bhor is more extended than the vertical component Bz. The temperature linearly decreases with increasing Bz, by about −300 K kG −1 in the photosphere and −800 K kG −1 in the umbra. The temperature contrast of granulation increases with increasing magnetic field strength and is then suppressed for Bz > 1200 G. Granular upflows dominate in regions with Bz 1000 G. Horizontal motions of granules start to be damped for Bz > 500 G and recurrently exploding granules appear only in magnetic fields comparable to or weaker than the equipartition field strength 400 G.

Vortices in simulations of solar surface convection

We report on the occurrence of small-scale vortices in simulations of the convective solar surface. Using an eigenanalysis of the velocity gradient tensor, we find the subset of high vorticity regions in which the plasma is swirling. The swirling regions form an unsteady, tangled network of filaments in the turbulent downflow lanes. Near-surface vertical vortices are underdense and cause a local depression of the optical surface. They are potentially observable as bright points in the dark intergranular lanes. Vortex features typically exist for a few minutes, during which they are moved and twisted by the motion of the ambient plasma. The bigger vortices found in the simulations are possibly, but not necessarily, related to observations of granular-scale spiraling pathlines in "cork animations" or feature tracking.

SUNRISE: INSTRUMENT, MISSION, DATA, AND FIRST RESULTS

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.

HIGHEST RESOLUTION OBSERVATIONS OF THE QUIETEST SUN

Highest resolution observations made with the new 1.6 m aperture solar telescope in Big Bear Solar Observatory during this time of historic inactivity on the Sun reveal new insights into the small-scale dynamics of the Sun's photosphere. The telescope's unprecedented resolution enabled us to observe that the smallest scale photospheric magnetic field seems to come in isolated points in the dark intergranular lanes, rather than the predicted continuous sheets confined to the lanes, and the unexpected longevity of the bright points implies a deeper anchoring than predicted. Further, we demonstrated for the first time that the photospheric plasma motion and magnetic fields are in equipartition over a wide dynamic range, and both cascade energy to ever-smaller scales according to classical Kolmogorov turbulence theory. Finally, we discovered tiny jet-like features originating in the dark lanes that surround the ubiquitous granules that characterize the solar surface.

The Contrast of Magnetic Flux Concentrations at Near‐Infrared and Visible Wavelengths

To reconcile discrepancies in the observed contrast of magnetic flux concentrations, we compute synthetic filtergrams from solar magnetoconvection models in four different passbands: two continuum bands, at 1626 and 575 nm, the G band, and the line wing of the Ca II K line. Magnetic flux concentrations are selected by their G-band brightness. In the near-infrared the selected flux concentrations appear inconspicuous with intensities that are on average below that of the synthetic average quiet Sun, while their contrast in the visible passband is mostly positive. When we account for limited telescope resolution and imperfect seeing, the contrasts of bright points in the visible become increasingly negative as a result of mixing with the dark intergranular lanes in which they are embedded. The correlation in bright-point intensity between different passbands becomes increasingly poor from the G band to the visible and the near-infrared. This is caused in part by the varying amount of spectral lines in each of the bands. Further, we find that the largest magnetic field concentrations, representative of micropores or magnetic knots, are dark in all four passbands. The brightenings in the Ca II K line wing do not coincide with magnetic flux concentrations on the spatial scale of the simulation. Finally, we find that the rms intensity contrast in all four passbands depends on the amount of magnetic field present in the simulation, with higher contrast for lower average magnetic field strength.

Simulations of Solar Granulation. I. General Properties

Numerical simulations provide information on solar convection not available by direct observation. We present results of simulations of near surface solar convection with realistic physics: an equation of state including ionization and three-dimensional, LTE radiative transfer using a four-bin opacity distribution function. Solar convection is driven by radiative cooling in the surface thermal boundary layer, producing the familiar granulation pattern. In the interior of granules, warm plasma ascends with ≈ 10% ionized hydrogen. As it approaches and passes through the optical surface, the plasma cools, recombines, and loses entropy. It then turns over and converges into the dark intergranular lanes and further into the vertices between granulation cells. These vertices feed turbulent downdrafts below the solar surface, which are the sites of buoyancy work that drives the convection. Only a tiny fraction of the fluid ascending at depth reaches the surface to cool, lose entropy, and form the cores of these downdrafts. Granules evolve by pushing out against and being pushed in by their neighboring granules, and by being split by overlying fluid that cools and is pulled down by gravity. Convective energy transport properties that are closely related to integral constraints such as conservation of energy and mass are exceedingly robust. Other properties, which are less tightly constrained and/or involve higher order moments or derivatives, are found to depend more sensitively on the numerical resolution. At the highest numerical resolution, excellent agreement between simulated convection properties and observations is found. In interpreting observations it is crucial to remember that surfaces of constant optical depth are corrugated. The surface of unit optical depth in the continuum is higher above granules and lower in the intergranular lanes, while the surface of optical depth unity in a spectral line is corrugated in ways that are influenced by both thermal and Doppler effects.

The Effect of Photospheric Granulation on the Determination of the Lithium Abundance in Solar‐type Stars

I investigate non-local thermodynamic equilibrium (non-LTE) formation of the 670.6 nm Li I resonance doublet in the presence of convective surface inhomogeneities in solar-type stars. This doublet is widely used for lithium abundance determination in stars. It has been suggested that the presence of hot and cool elements in a stellar atmosphere due to convective heat transport might lead to an underestimate of lithium abundance by as much as a factor of 10 when the equivalent width of the doublet is analyzed in terms of a one-dimensional plane-parallel model atmosphere. To explore this possibility, I solved the two-dimensional non-LTE radiative transfer equations for a sufficiently large lithium model atom in a hydrodynamic simulation snapshot of the solar granulation. This was done for different values of the lithium abundance ranging from ALi = 0.0 to 3.3. In all cases the effects of the inhomogeneities in the atmosphere on lithium line strength are small, never amounting to more than 0.1 dex in the derived abundance. This occurs mainly for three reasons. First, because of the exponential decrease of density with height in the gravitationally stratified stellar atmosphere, radiation escapes mostly vertically with little horizontal exchange. Some lateral transfer does occur at the boundaries between hot and cold elements, but the effect of this exchange on the spatially averaged line strength cancels out. It leads to a smoothing over the surface rather than to a diminishing overall strength of the doublet. Second, the sharp drop in temperature over hot upwelling material, in contrast to the much shallower gradient over the dark intergranular lanes, causes the 670.6 nm doublet to be deeper and narrower in the former and broader in the latter. Consequently, the contrast of equivalent line width between profiles emerging over hot upflows and cold downflows is small. Finally, because of its small abundance the opacity scale in lithium ionizing continua is mostly set by H- bound-free processes. Optical depth unity at the photoionization edges, therefore, follows the contours of electron temperature, moderating contrast in the ionizing radiation field.

Local Properties of the Sun‘s Seismic Events

We made simultaneous high resolution observations of the solar granular field and the Sun‘s seismic events. We find that these events occur in the dark intergranular lanes. The events are preceded by the darkening of an already dark lane. On the leading edge of each event, there is a second more abrupt and precipitous darkening lasting a minute or two.

Solar oscillations in strong and weak fraunhofer lines over a quiet region

We have analysed a 35-min-long time sequence of spectra in the Caii H line, Nai D1 and D2 lines, and in a large number of strong and weak Fei lines taken over a quiet region at the center of the solar disk. The time series of these spectra have been observed simultaneously in these lines under high spatial, spectral, and temporal resolution at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory. We have derived the line profiles and their central intensity values at the sites of the chromospheric bright points, which are visible in the H line for easy identification. We have done a power spectrum analysis for all the lines, using their central intensity values to determine the period of oscillations. It is shown that the 3 Fei lines, present ∼ 23 A away from the core of the H line representing the pure photospheric lines, Nai D1 and D2 lines, 6 Fei lines at the wings of H line, and Can H line exhibit 5-min, 4.05-min, 3.96-min, and 3.2-min periodicity in their intensity oscillations, respectively. Since all these lines form at different heights in the solar atmosphere from low photosphere to middle chromosphere and show different periodicities in their intensity oscillations, these studies may give an idea about the spatial and temporal relation between the photospheric and chromospheric intensities. Therefore these studies will help to better understand the physical mechanisms of solar oscillations. It is clearly seen that the period of intensity oscillations decreases outward from the low photosphere to the middle chromosphere. Since we have studied a single feature at a time on the Sun (i.e., bright points seen in the H line) in all these spectral lines simultaneously, this may explain about the footpoints of the bright points, the origin of 3-min oscillations, and the relation to other oscillations pertaining to these locations on the Sun. We have concluded that 80% of the bright points are associated with dark elements in the true continuum, and they may seem to have a relationship with the dark intergranular lanes of the photosphere, after carefully examining the brightness (bright threads) extending from the core to the far wings of the H line at the locations of a large number of bright points, using their time sequence of spectra.

On the relation between magnetic field structures and granulation

view Abstract Citations (76) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Relation between Magnetic Field Structures and Granulation Title, A. M. ; Tarbell, T. D. ; Topka, K. P. Abstract Narrow-band observations of a solar emerging-flux region, obtained at 525.02 nm with resolution 0.5 arcsec using the Sacramento Peak vacuum-tower telescope and the Spacelab 2 tunable filter on December 10, 1979, are reported. The data are presented in histograms and characterized in detail. Magnetic-flux concentrations of 600-1000 G are found to be located in areas of suppressed downflow near holes or pores in the granulation pattern, while 100-500-G concentrations occur in areas with stronger downdrafts, in cell-like structures of diameter 2-5 arcsec, the fields being aligned with dark intergranular lanes. It is inferred that granulation concentrates the magnetic field in the downflow lanes (as found in numerical simulations by Nordlund, 1983), and that the magnetic structures are more diffuse and longer-lasting than the bright spots; thus bright-point maps are contained in, rather than equivalent to, magnetic-field maps. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165339 Bibcode: 1987ApJ...317..892T Keywords: Magnetic Field Configurations; Solar Granulation; Solar Magnetic Field; Field Strength; Image Resolution; Iron; Line Spectra; Magnetic Flux; Astrophysics; SUN: GRANULATION; SUN: MAGNETIC FIELDS full text sources ADS |

Time-Dependent Magneto-Convection with Hexagonal Planform

High-resolution observations indicate that very strong, small scale magnetic fields in the solar photosphere are concentrated into ropes which emerge through it. The scale of these ropes is only a few hundred kilometres across (Stenflo 1976) and their strength is estimated to vary between 1,000 and 2,000 G (Harvey 1977). These features are closely related to photospheric granular convection. The flux is observed as X-ray bright spots and sheet-like crinkles in the dark intergranular lanes, and is buffeted and shifted about by the granules (Dunn and Zirker 1973). Further, the crinkles can be resolved into separate features which outline small micropores as though a flux sheet at the end of a convection cell has separated into several isolated tubes (Galloway and Weiss 1981).

A morphological study of the solar granulation

Some properties of the photospheric granulation near the centre of the quiet sun have been investigated on the basis of two high-definition Stratoscope photographs. We obtain (1) a number density of 54 granules per 10″ × 10″, (2) a total number of granules on the whole surface of 6.3 × 106, (3) a mean cell diameter of 1″.5 or 1100 km and a mean granule diameter of 1″.2 or 850 km, the difference of 250 km being ascribed to the mean width of the dark intergranular lanes, (4) frequency distributions of cell diameters and granule diameters (Figures 1 and 2), and (5) an isophotal map in relative intensity (Figure 3). In general, larger granules are brighter than smaller granules (Figure 4). Cross-section profiles are shown for some granules and intergranular lanes (Figures 5 and 6). These quantities have not yet been corrected for the finite resolution of the telescope.