Evolution of coronal magnetic fields

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This work employs a numerical method to calculate the magnetic field above solar active regions using photospheric vector magnetic field measurements. The magnetic field significantly influences the structure of the outer solar atmosphere, and our approach is based on the force-free assumption, meaning coronal currents align with the magnetic field. We extrapolated the measured photospheric field vector into the chromosphere and corona to derive the 3D magnetic field structure in these layers. Both potential (current-free) and nonlinear force-free field models were utilized to calculate the coronal magnetic field, with the latter being the most sophisticated approximation. By analyzing the coronal magnetic field above two solar active regions, we demonstrated that nonlinear force-free field extrapolations are effective for investigating its evolution before and after solar eruptions, such as flares and coronal mass ejections. We calculated the magnetic energy content, free magnetic energy, and energy density distribution, revealing that magnetic energy builds up slowly before eruptions, with stored energy correlating to the magnitude of explosive events. Additionally, we localized the height and position in the solar atmosphere where excess energy was stored and released during eruptions, providing evidence for a coronal implosion scenario. As solar eruptions occur, some coronal magnetic energy transforms into kinetic energy o