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Register a new userBright points in the inter-network quiet Sun
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Authors
J. Sanchez Almeida
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High resolution G-band images of the interior of a supergranulation cell show ubiquitous Bright Points (some 0.3 BPs per Mm^2). They are located in intergranular lanes and often form chains of elongated blobs whose smallest dimension is at the resolution limit (135 km on the Sun). Most of them live for a few minutes, having peak intensities from 0.8 to 1.8 times the mean photospheric intensity. These BPs are probably tracing intense magnetic concentrations, whose existence has been inferred in spectro-polarimetric measurements. Our finding provides a new convenient tool for the study of the inter-network magnetism, so far restricted to the interpretation weak polarimetric signals.
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We present a visual determination of the number of bright points (BPs) existing in the quiet Sun, which are structures though to trace intense kG magnetic concentrations. The measurement is based on a 0.1 arcsec angular resolution G-band movie obtained with the Swedish Solar Telescope at the solar disk center. We find 0.97 BPs/Mm^2, which is a factor three larger than any previous estimate. It corresponds to 1.2 BPs per solar granule. Depending on the details of the segmentation, the BPs cover between 0.9% and 2.2% of the solar surface. Assuming their field strength to be 1.5 kG, the detected BPs contribute to the solar magnetic flux with an unsigned flux density between 13 G and 33 G. If network and inter-network regions are counted separately, they contain 2.2 BPs/Mm^2 and 0.85 BPs/Mm^2, respectively.
We study the motions of G band bright points (GBPs) in the quiet Sun to obtain the characteristics of different motion types. A high resolution image sequence taken with the Hinode/Solar Optical Telescope (SOT) is used, and GBPs are automatically tracked by segmenting 3D evolutional structures in a space time cube. After putting the GBPs that do not move during their lifetimes aside, the non stationary GBPs are categorized into three types based on an index of motion type. Most GBPs that move in straight or nearly straight lines are categorized into a straight motion type, a few moving in rotary paths into a rotary motion, and the others fall into a motion type we called erratic. The mean horizontal velocity is 2.18 km/s, 1.63 km/s and 1.33 km/s for straight, erratic and rotary motion type, respectively. We find that a GBP drifts at a higher and constant velocity during its whole life if it moves in a straight line. However, it has a lower and variational velocity if it moves in a rotary path. The diffusive process is ballistic, super and sub diffusion for straight, erratic and rotary motion type, respectively. The corresponding diffusion index and coefficients are 2.13 and 850 km2/s, 1.82 and 331 km2/s, 0.73 and 13 km2/s. In terms of direction of motion, it is homogeneous and isotropical, and usually persists between neighbouring frames, no matter what motion type a GBP belongs to.
CONTEXT: The quiet Sun magnetic fields produce ubiquitous bright points (BPs) that cover a significant fraction of the solar surface. Their contribution to the total solar irradiance (TSI) is so-far unknown. AIMS: To measure the center-to-limb variation (CLV) of the fraction of solar surface covered by quiet Sun magnetic bright points. The fraction is referred to as fraction of covered surface, or FCS. METHODS: Counting of the area covered by BPs in G-band images obtained at various heliocentric angles with the 1-m Swedish Solar Telescope on La Palma. Through restoration, the images are close to the diffraction limit of the instrument (~0.1 arcsec). RESULTS: The FCS is largest at disk center (~1 %), and then drops down to become 0.2 % at mu= 0.3 (with mu the cosine of the heliocentric angle. The relationship has large scatter, which we evaluate comparing different subfields within our FOVs. We work out a toy-model to describe the observed CLV, which considers the BPs to be depressions in the mean solar photosphere characterized by a depth, a width, and a spread of inclinations. Although the model is poorly constrained by observations, it shows the BPs to be shallow structures (depth < width) with a large range of inclinations. We also estimate how different parts of the solar disk may contribute to TSI variations, finding that 90 % is contributed by BPs having mu > 0.5, and half of it is due to BPs with mu > 0.8.
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points. These points can be found directly in a potential field extrapolation or their density can be estimated from Fourier spectrum of the magnetogram. The spectral estimate, which assumes that the extrapolated field is random, homogeneous and has Gaussian statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHOs MDI. The majority of null points occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null point, with altitude greater than 1.5 Mm, above every 322 square Mm patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minimum shows that the null point density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete elements with mean flux 1.0e19 Mx distributed randomly with density n=0.007 per square Mm.
We detected 2.8 bright points (BPs) per Mm$^2$ in the Quiet Sun (QS) with the New Solar Telescope (NST) at Big Bear Solar Observatory; using the TiO 705.68 nm spectral line, at an angular resolution ~ 0.1 to obtain 30 min data sequence. Some BPs formed knots that were stable in time and influenced the properties of the granulation pattern around them. The observed granulation pattern within ~ 3 of knots presents smaller granules than those observed in a normal granulation pattern; i.e., around the knots a suppressed convection is detected. Observed BPs covered ~ 5% of the solar surface and were not homogeneously distributed. BPs had an average size of 0.22, they were detectable for 4.28 min in average, and had an averaged contrast of 0.1% in the deep red TiO spectral line.
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