Synthetic extreme ultraviolet images for the validation of the magnetofrictional inner coronal simulation

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Title: Synthetic extreme ultraviolet images for the validation of the magnetofrictional inner coronal simulation
Author: Kalliokoski, Milla
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
Thesis level: master's thesis
Discipline: Physics
Abstract: Solar eruptions are key large-scale heliospheric structures that impact the terrestrial space weather, and may even harm technology both in space and on the ground. The processes triggering these eruptions are closely related to the magnetic field of the Sun and, in particular, to the complex evolution of the magnetic field in the solar corona. Therefore, comprehensive understanding of the coronal dynamics is required to be able to predict hazardous solar events. Since directly measuring the coronal magnetic field is difficult, the field evolution must be studied by alternative means, namely via data-driven modeling. One such model is the magnetofrictional method (MFM) which provides computationally efficient modeling, where photospheric magnetogram measurements are used to compute the time-dependent boundary condition. To evaluate the physical validity of the output of the MFM simulations, the modeled coronal evolution must be compared with actual solar observations. Observations at the extreme ultraviolet (EUV) and X-ray wavelengths show that the corona is populated by bright arcs of plasma. These structures – the coronal loops – are supported by closed magnetic field lines. Hence, coronal loops present a visual indication of the topology of the coronal magnetic field. Coronal loops exist especially in the so-called active regions which are the origin of solar eruptions. Observations of coronal loops and methods to numerically model them are presented in this thesis, in addition to discussing the formation, observation and modeling of coronal EUV emission. In this thesis, the performance of the MFM technique is assessed by investigating two inner coronal simulations of the same active region complex, employing different assumptions in the calculation of the boundary conditions. The simulations encompass a time period of four days on June 2012, when two Earth-directed coronal mass ejections (CMEs) are observed to erupt from the studied region. The model is compared with real coronal EUV observations via synthesizing mock coronal EUV images based on an ad hoc proxy for coronal EUV emission using a visualization toolkit created by the author. Realistic coronal emission cannot be directly computed from the MFM output, since the thermodynamic properties of the coronal plasma are not modeled. The degree of correspondence between synthetic and observed coronal EUV images is examined by visually considering the intensity structure and the connectivity of coronal loops in the images. Also the magnetic field topology in itself is investigated. While neither of the studied simulations produce eruptive loop structures associated with the observed CMEs, the other of the two simulations is found to qualitatively reproduce observed coronal loop structures and their evolution.

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