Coronal dynamics driven by magnetic flux emergence

Abstract

The corona is the Sun’s outer atmosphere which is more than 100 times hotter than the solar surface. It can be brilliantly observed in the extreme-ultraviolent (EUV) and soft X-ray passbands. What heats the corona is one of the fundamental questions in solar and plasma physics. The answer must address the origin of the energy input into the corona and how the observed coronal features form and evolve as the consequence of the energy input. To understand the built-up of the corona during the formation of the active region through magnetic flux emergence in the photosphere, we use the output of a magnetic flux emergence simulation to drive a magnetohydrodynamics (MHD) simulation for the corona. The braiding of magnetic fieldlines in the photosphere induces currents in the corona. The Ohmic dissipation of the induced currents heats the coronal plasma to over 1 MK. The proper treatment on the energy balance, as in the real corona, allows the model to synthesise EUV emission directly comparable to observations. In the coronal model numerous bright coronal EUV loops form during the formation of a sunspot pair in the model photosphere. The coronal loops are rooted at the outer edge of the sunspots, where an enhanced upward Poynting flux is produced by the interaction of flows and magnetic field structures. The thermal dynamics and energetics of the plasma in individual magnetic fieldlines are consistent with the expectation of traditional one dimensional loop models with prescribed heat input. At each instance of time, EUV loops are along magnetic field lines. However, their temporal evolution can be radically different, because the EUV emission is governed by the convolution of the temperature and density of the coronal plasma. When the footpoints of emerging magnetic fieldlines consecutively move through a spot of enhanced energy input at the outer edge of the sunspot, an apparently static EUV structure is created by the plasma in the emerging magnetic fieldlines. This gives an essentially new view on the relation of EUV loops to magnetic fieldlines. Moreover, transverse oscillations of coronal loops triggered in the model can be clearly identified in synthetic observations. For observations of the Sun, the technique of coronal seismology is used to deduce the physical properties in an oscillating loop. We apply the same technique to our synthetic data to derive the average field strength in the loop and compare it to the actual value in the simulation. It is close to the average field strength that would give an identical total wave travel time through the coronal loop. This result can serve as a benchmark for coronal seismology. The results in this thesis shed new light on dynamics during the built-up of coronal loop structures in response to the emergence of magnetic flux in the photosphere. This model highlights the power of realistic three dimensional models to resemble features in the real corona. It also emphasises the essential necessity of treating the plasma and the magnetic field at the same time, in order to self-consistently model dynamics of the coronal plasma.