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Statistical Properties of Flaring and Sub-Flaring Activity in the Solar Atmosphere  

Manolis K. Georgoulis   Submitted: 2003-12-17 17:05

We review the observational, theoretical, and numerical efforts to interpret the statistical behavior of solar flares and sub-flares, and their role to coronal heating. The observed X-ray flares cannot heat the corona. The role of smaller-scale events is still inconclusive, although very recent observational evidence may shed light to the controversy. Statistical and classical modeling reproduces the statistics of large flares. Part of these models also predict a statistical behavior for the small-scale events consistent with that of Parker's nanoflares if these events indeed heat the solar corona.

Authors: Manolis K. Georgoulis
Projects:

Publication Status: Invited Review, 6th Hellenic Astronomical Conference, Athens, Greece, 15-17 September 2003
Last Modified: 2004-02-11 14:58
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On the Force-Freeness of Photospheric Magnetic Fields in Solar Active Regions: The Current Density Method  

Manolis K. Georgoulis   Submitted: 2002-06-25 13:46

We introduce and apply a method to investigate the force-free character of the photospheric magnetic fields in solar active regions. The vertical current density, inferred from vector magnetograms, is decomposed into a helical (assumed field-aligned) component, and a component perpendicular to the magnetic field. Contrary to the classical method of calculating the integrated Lorentz forces, the current density method does not require balanced magnetic configurations, i.e. zero net flux, and it provides spatial information, i.e. maps of force-free vs. non-force-free areas. A revised force-free condition and a new expression for the twist parameter are also introduced. The new twist parameter is more accurate than the conventional one in that it discards contributions from currents that are non-field-aligned by definition. The current density method is applied on high-resolution (~0.5 arcsec) photospheric vector magnetograms of the emerging flux region NOAA 8844, acquired by the Flare Genesis Experiment. We find that the photospheric magnetic fields in our active region are not force-free, but they tend to become force-free ~30 hr after our observations. We conclude that the degree of force-freeness is a function of the evolutionary stage of an active region, determined by the mean inclination of the magnetic field vector with respect to the photosphere. Non-force-free magnetic fields are due to the flux emergence process and the subsequent intense photospheric flows which, in turn, give rise to strong shear. Force-freeness is achieved by the completion of the flux emergence process. The non-force-free character of the magnetic fields is manifested by non-field-aligned, azimuthal, currents, that flow around flux tubes. These currents are stronger than field-aligned currents, in many cases. The azimuthal currents are associated with shear and they become detectable due to a low-lying magnetic canopy of inclined flux tubes, extending above the interspot area.

Authors: Georgoulis, M. K., Rust, D. M., and Bernasconi, P. N.
Projects:

Publication Status: ApJ (submitted)
Last Modified: 2002-06-25 13:46
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Statistics, Morphology, and Energetics of Ellerman Bombs  

Manolis K. Georgoulis   Submitted: 2002-04-08 16:42

We investigate the statistical properties of Ellerman bombs in the dynamic emerging flux region NOAA 8844, underneath an expanding arch filament system. High-resolution chromospheric Hα filtergrams (spatial resolution 0.8arcsec), as well as photospheric vector magnetograms (spatial resolution 0.5arcsec), and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. Hα observations reveal the first ``seeing-free'' data set on Ellerbombs and one of the largest samples of these events. We find that Ellerman bombs occur and recur in preferential locations in the low chromosphere, either above, or in the absence of, photospheric neutral magnetic lines. Ellerman bombs are associated with photospheric downflows and their loci follow the transverse mass flows on the photosphere. They are small-scale events, with typical size 1.8 imes 1.1;arcsec^2, but this size depends on the instrumental resolution. A large number of Ellerman bombs are probably undetected, due to limited spatial resolution. Ellerman bombs occur in clusters which exhibit fractal properties. The fractal dimension, with an average value sim 1.4, does not change significantly in the course of time. Typical parameters of Ellerman bombs are interrelated and obey power-law distribution functions, as in case of flaring and sub-flaring activity. We find that Ellerman bombs may occur on separatrix, or quasi-separatrix, layers, in the low chromosphere. A plausible triggering mechanism of Ellerman bombs is stochastic magnetic reconnection caused by the turbulent evolution of the low-lying magnetic fields and the continuous re-shaping of separatrix layers. The total energies of Ellerman bombs are estimated in the range (1027,1028);erg, the temperature enhancement in the radiating volume is sim 2 imes 103;K and the timescale of radiative cooling is short, of the order of a few seconds. The distribution function of the energies of Ellerman bombs exhibits a power-law shape with an index sim -2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions.

Authors: M. K. Georgoulis, D. M. Rust, P. N. Bernasconi, and B. Schmieder
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Publication Status: ApJ (in press)
Last Modified: 2002-04-08 16:42
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A Comparison Between Statistical Properties of Solar X-Ray Flares and Avalanche Predictions in Cellular Automata Statistical Flare Models  

Manolis K. Georgoulis   Submitted: 2001-01-15 13:38

We perform a tentative comparison between the statistical properties of cellular automata statistical flare models including a highly variable, non-linear external driver, and the respective properties of the WATCH flare data base, constructed during the maximum of solar cycle 21. The model is based on the concept of Self-Organized Criticality (SOC). The frequency distributions built on the measured X-ray flare parameters show the following characteristics: (1) The measured parameters (total counts, peak count rates and, to a lesser extent, total duration) are found to be correlated to each other. Overall distribution functions of the first two parameters are robust power laws extending over several decades. The total-duration distribution function is represented by either two power laws or a power law with an exponential roll-over. (2) By sub-grouping the peak count rate and the total counts as functions of duration and constructing frequency distributions on these sub-groups, it is found that the slope systematically decreases with increasing duration. (3) No correlation is found between the elapsed time interval between successive bursts arising from the same active region and the peak intensity of the flare. Despite the inherent weaknesses of the SOC models to simulate realistically a number of physical processes thought to be at work in solar active regions and in flares' energy release, we show that the model is able to reproduce the bulk of the above statistical properties. We thus underline two main conclusions: (i) A global, statistical approach for the study of rapid energy dissipation and magnetic field line annihilation in complex, magnetized plasmas may be of equal importance with the localized, small-scale Magnetohydrodynamic (MHD) simulations, and (ii) refined SOC models are needed to establish a more physical connection between the cellular automata evolution rules and the observations. Key words: Sun: activity - Sun: corona - Sun: flares - Sun: magnetic fields - Sun: X-rays

Authors: Georgoulis, M. K., Vilmer, N. & Crosby, N. B.
Projects:

Publication Status: A&A (in press)
Last Modified: 2001-01-15 13:38
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Abstracts by Author
Statistical Properties of Flaring and Sub-Flaring Activity in the Solar Atmosphere
On the Force-Freeness of Photospheric Magnetic Fields in Solar Active Regions: The Current Density Method
Statistics, Morphology, and Energetics of Ellerman Bombs
A Comparison Between Statistical Properties of Solar X-Ray Flares and Avalanche Predictions in Cellular Automata Statistical Flare Models

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