Burkart Bovelet

Bio: Burkart Bovelet is an academic researcher from University of Göttingen. The author has contributed to research in topics: Brightness & Magnetic flux. The author has an hindex of 4, co-authored 5 publications receiving 181 citations.

A New Algorithm for Pattern Recognition and its Application to Granulation and Limb Faculae

Abstract: We have developed a new pattern-recognition algorithm based on multiple intensity clips which assures an optimal adaptation to the solar structure under study. The shapes found at higher clip levels are repeatedly extended to lower levels, thus filling more and more of the observed intensity contours. Additionally, at each intensity threshold new shapes, exceeding the level, are integrated. The number and height of the levels can be optimized making this `multiple level tracking' algorithm (MLT) superior to commonly used Fourier-based recognition techniques (FBR). The capability of MLT is demonstrated by application to the intensity structure of solar granulation near the disk center, both speckle reconstructed and not. Comparisons with Doppler maps prove its reliability. The granular pattern recognized by MLT differs essentially from that obtained with FBR. Elongated `snake-like' granules do not occur with MLT and, consequently, the perimeter-area distribution exhibits only a marginal `second branch' of higher fractal dimension, which dramatically diminishes the better the MLT pattern matches the granular structure. The final distribution obtained with optimized parameters has a single fractal dimension near 1.1, making the question of a `critical size', a `second branch', and the often discussed dimension of 4/3; highly questionable. This result is equally obtained from application of MLT to the corresponding Doppler velocity map of granular up-flows. In contrast, the pattern of down-flows contains some elongated `snake-like' structures with higher fractal dimension. We also use the new algorithm to recognize speckle-reconstructed limb faculae, which MLT separates from their granular surroundings, and compare both, granules and faculae, using large statistical samples. The facular grains near cosθ=57° exhibit a slightly different ellipticity than the (geometrically foreshortened) adjacent granules. However, small facular grains are more round than small granules and larger grains are more similar to granules.

Brightness and size of small-scale solar magnetic flux concentrations

Abstract: The new 1 m Swedish Solar Telescope SST on La Palma allows to observe inter-granular G -band bright points (igBP) in solar active regions at an unprecedented spatial resolution. The igBP are reasonably assumed to be small-scale magnetic flux-concentrations. A sample of more than 1500 igBP shows tight relations of diameter and brightness in the G -band and in the continuum; it covers a diameter range of 100 km to 300 km, with a most frequent value near 160 km. Features larger than 300 km formerly reported, evidently result from insufficient spatial resolution; that upper diameter limit is close to the typical width of inter-granular lanes, and suggests a “gap” to small pores. The lack of igBP with sizes below 130 km is discussed not to arise from the finite spatial resolution of the 1 m telescope.

The quiet Sun's magnetic flux estimated from CaIIH bright inter-granular G-band structures

Abstract: We determine the number density and area contribution of small-scale inter-granular calcium-II bright G-band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations. In a 149" x 117" G-band image of the disk center at the activity minimum, 7593 small inter-granular structures ['IGS']were segmented with the `multiple-level tracking' pattern recognition algorithm ['MLT_4']. The scatter-plot of the continuum versus the G-band brightness shows the known magnetic and non-magnetic branches. These branches are largely disentangled by applying an intrinsic Ca-II excess criterion. The thus obtained 2995 structures contain 1152 G-band bright points ['BP'] and 1843 G-band faint points ['FP']. They show a tendency of increasing size with decreasing G-band excess, as expected from the `hot wall' picture. Their Ca-H and G-band brightness are slightly related, resembling the known relation of Ca-II and magnetic field strength. The magnetic flux density of each individual BP and FP is estimated from their G-band brightness according to MHD-model calculations. The entity of BP and FP covers the total field-of-view ['FOV'] with a number density of 0.32/Mm^2 and a total area contribution of 2.0%. Their individual calibrations yield a mean flux density of 20 Mx/cm^2 in the entire FOV and 13 Mx/cm^2 for inter-network regions.

The quiet Sun's magnetic flux estimated from Ca II H bright inter-granular G-band structures

Abstract: Aims. We determine the number density and area contribution of small-scale inter-granular Ca II bright G -band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations. Methods. In a $149\arcsec\,\times\,117\arcsec$ G -band image of the disk center at the activity minimum, 7593 small inter-granular structures were segmented with the “multiple-level tracking” pattern recognition algorithm. The scatterplot of the continuum versus the G -band brightness shows the known magnetic and non-magnetic branches. These branches are largely disentangled by applying an intrinsic Ca II H excess criterion. The thus obtained 2995 structures contain 1152 G -band bright points (BP) and 1843 G -band faint points (FP). They show a tendency toward increasing size with decreasing G -band excess, as expected from the “hot wall” picture. Their Ca II H and G -band brightness are slightly related, resembling the known relation of Ca II and magnetic field strength. The magnetic flux density of each individual BP and FP is estimated from their G -band brightness according to MHD model calculations. Results. The entity of BP and FP covers the total FOV with a number density of 0.32 / Mm 2 and a total area contribution of 2.0%. Their individual calibrations yield a mean flux density of 20 Mx/cm 2 in the entire FOV and 13 Mx/cm 2 for inter-network regions.

25 Years of Self-Organized Criticality: Solar and Astrophysics

Abstract: Shortly after the seminal paper “Self-Organized Criticality: An explanation of 1/f noise” by Bak et al. (1987), the idea has been applied to solar physics, in “Avalanches and the Distribution of Solar Flares” by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.

25 Years of Self-Organized Criticality: Solar and Astrophysics

Abstract: Shortly after the seminal paper {\sl "Self-Organized Criticality: An explanation of 1/f noise"} by Bak, Tang, and Wiesenfeld (1987), the idea has been applied to solar physics, in {\sl "Avalanches and the Distribution of Solar Flares"} by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.

Statistics of Active Region Complexity: A Large-Scale Fractal Dimension Survey

Abstract: A quantification of the magnetic complexity of active regions using a fractal dimension measure is presented. This fully automated approach uses full-disk MDI magnetograms of active regions from a large data set (2742 days of the SOHO mission, 9342 active region images) to compare the calculated fractal dimension of each region to both its Mount Wilson classification and flare rate. Each Mount Wilson class exhibits a similar fractal dimension frequency distribution, possibly suggesting a self-similar nature of all active regions. Solar flare productivity exhibits an increase in both the frequency and GOES X-ray magnitude of flares from regions with higher fractal dimension. Specifically, a lower threshold fractal dimension of 1.2 and 1.25 exists as a necessary, but not sufficient, requirement for an active region to produce M- and X-class flares, respectively, within 24 hr of the observation.

Transverse Component of the Magnetic Field in the Solar Photosphere Observed by SUNRISE

Abstract: We present the first observations of the transverse component of a photospheric magnetic field acquired by the imaging magnetograph SUNRISE/IMaX. Using an automated detection method, we obtain statistical properties of 4536 features with significant linear polarization signal. We obtain a rate of occurrence of 7 × 10–4 s–1 arcsec–2, which is 1-2 orders of magnitude larger than the values reported by previous studies. We show that these features have no characteristic size or lifetime. They appear preferentially at granule boundaries with most of them being caught in downflow lanes at some point. Only a small percentage are entirely and constantly embedded in upflows (16%) or downflows (8%).

Image Processing Techniques and Feature Recognition in Solar Physics

Abstract: This review presents a comprehensive and systematic overview of image-processing techniques that are used in automated feature-detection algorithms applied to solar data: i) image pre-processing procedures, ii) automated detection of spatial features, iii) automated detection and tracking of temporal features (events), and iv) post-processing tasks, such as visualization of solar imagery, cataloguing, statistics, theoretical modeling, prediction, and forecasting. For each aspect the most recent developments and science results are highlighted. We conclude with an outlook on future trends.