arXiv subjects

M. Georgoulis

Publications and source records attributed to M. Georgoulis.

4 recordsLinked to original sources

arXiv2017

Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. Part III: Twist Number Method

We study the writhe, twist and magnetic helicity of different magnetic flux ropes, based on models of the solar coronal magnetic field structure. These include an analytical force-free Titov--Démoulin equilibrium solution, non force-free magnetohydrodynamic simulations, and nonlinear force-free magnetic field models. The geometrical boundary of the magnetic flux rope is determined by the quasi-separatrix layer and the bottom surface, and the axis curve of the flux rope is determined by its overall orientation. The twist is computed by the Berger--Prior formula that is suitable for arbitrary geometry and both force-free and non-force-free models. The magnetic helicity is estimated by the twist multiplied by the square of the axial magnetic flux. We compare the obtained values with those derived by a finite volume helicity estimation method. We find that the magnetic helicity obtained with the twist method agrees with the helicity carried by the purely current-carrying part of the field within uncertainties for most test cases. It is also found that the current-carrying part of the model field is relatively significant at the very location of the magnetic flux rope. This qualitatively explains the agreement between the magnetic helicity computed by the twist method and the helicity contributed purely by the current-carrying magnetic field.

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arXiv2016

An observationally-driven kinetic approach to coronal heating

Coronal heating through the explosive release of magnetic energy remains an open problem in solar physics. Recent hydrodynamical models attempt an investigation by placing swarms of 'nanoflares' at random sites and times in modeled one-dimensional coronal loops. We investigate the problem in three dimensions, using extrapolated coronal magnetic fields of observed solar active regions. We apply a nonlinear force-free field extrapolation above an observed photospheric magnetogram of NOAA active region (AR) 11158. We then determine the locations, energy contents, and volumes of 'unstable' areas, namely areas prone to releasing magnetic energy due to locally accumulated electric current density. Statistical distributions of these volumes and their fractal dimension are inferred, investigating also their dependence on spatial resolution. Further adopting a simple resistivity model, we infer the properties of the fractally distributed electric fields in these volumes. Next, we monitor the evolution of 10^5 particles (electrons and ions) obeying an initial Maxwellian distribution with a temperature of 10 eV, by following their trajectories and energization when subjected to the resulting electric fields. For computational convenience, the length element of the magnetic-field extrapolation is 1 arcsec, much coarser than the particles collisional mean free path in the low corona. The presence of collisions traps the bulk of the plasma around the unstable volumes, or current sheets (UCS), with only a tail of the distribution gaining substantial energy. Assuming that the distance between UCS is similar to the collisional mean free path we find that the low active-region corona is heated to 100-200 eV, corresponding to temperatures exceeding 2 MK, within tens of seconds for electrons and thousands of seconds for ions. Fractally distributed, nanoflare-triggening fragmented UCS ...

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arXiv2009

The correlation of fractal structures in the photospheric and the coronal magnetic field

This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions.Our results show no correlation between the fractal dimensions of the observed 2d photospheric structures and the extrapolated unstable volumes in the corona, when nonlinear force-free extrapolation is used. This result is independent of efforts to (1) bring the photospheric magnetic fields closer to a nonlinear force-free equilibrium and (2) omit the lower part of the modeled magnetic field volume that is almost completely filled by unstable volumes. A significant correlation between the fractal dimensions of the photospheric and coronal magnetic features is only observed at the zero level (lower limit) of approximation of a current-free (potential) magnetic field extrapolation. We conclude that the complicated transition from photospheric non-force-free fields to coronal force-free ones hampers any direct correlation between the fractal dimensions of the 2d photospheric patterns and their 3d counterparts in the corona at the nonlinear force-free limit.

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arXiv2002

Statistical properties of the energy release in emerging and evolving active region

The formation and evolution of active regions is an inherently complex phenomenon. Magnetic fields generated at the base of the convection zone follow a chaotic evolution before reaching the solar surface. In this article, we use a 2-D probabilistic Cellular Automaton (CA) to model the statistical properties of the magnetic patterns formed on the solar surface and to estimate the magnetic energy released in the interaction of opposite polarities. We assume that newly emerged magnetic flux tubes stimulate the emergence of new magnetic flux in their neighborhood. The flux-tubes move randomly on the surface of the sun, and they cancel and release their magnetic energy when they collide with magnetic flux of opposite polarity, or diffuse into the ``empty'' photosphere. We assume that cancellation of magnetic flux in collisions causes "flares" and determine the released energy as the difference in the square of the magnetic field flux. The statistics of the simulated "flares" follow a power-law distribution in energy, with power-law index a=2.2 +/- 0.1. The size distribution function of the simulated active regions exhibits a power law behavior with index k = 1.93 +/- 0.08, and the fractal dimension of the magnetized areas on the simulated solar surface is close to D = 1.42 +/- 0.12. Both quantities, D and k, are inside the range of the observed values.

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