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Project

Wave properties in the low solar atmosphere

The solar atmosphere is in a dynamic state and ubiquitous waves can be observed in all layers. At the same time, above the photosphere, the temperature increases in the chromosphere and corona with increasing distance from the solar surface. It is yet not clear which non-thermal process supplies the upper layers with sufficient energy to maintain their temperature. This is termed the coronal heating problem. The ubiquitous waves in the solar atmosphere are a good candidate for transporting wave energy from the solar interior to the outer layers, where the energy can then be dissipated by numerous processes. Heating by waves, i.e. heating by fast plasma movements, is called AC heating. However, in order to understand how much of this wave energy reaches the corona and in what form, we need to study the wave propagation and transformation from the base of the photosphere.

The low layers of the solar atmosphere - the photosphere, the chromosphere, and the transition region - are highly stratified with density, pressure, and temperature changing by orders of magnitude, especially in the transition region. In addition, the influence of the magnetic field on plasma motions changes significantly, which promotes wave mode conversions. An acoustic cutoff region prevents the propagation of low frequency acoustic waves and the steep gradients of the transition region cause significant wave reflection. Most of these complex processes happen at too small scales to be sufficiently resolved by current observational possibilities. We thus rely on mathematical modeling and investigations through theoretical calculations and numerical simulations.

In this study we aim to shed light on the propagation and transformation of acoustic waves through and in the low solar atmosphere by the means of numerical simulations. Step by step, our model atmospheres increase in complexity. Waves are excited at the bottom of the domain, which corresponds to the base of the photosphere, via vertically polarized mono-periodic acoustic perturbations. The resulting waves and their energy fluxes are then studied as they propagate upward. The models that include cylindrical geometries are subject to tube (sausage and kink) mode excitation. Occurring tube modes are identified by comparing the simulation data to the eigenfunctions and energy content of a simplified theoretical model that locally resembles the simulated atmosphere.

We start our analysis in Chapter 3 with a high frequency linear driver and a tightly packed system of straight loops with constant radius, that extend into the low corona. Apart from plane waves, fast sausage surface and slow sausage body waves were excited. For loops inclined from the vertical and thus the driver polarization, we also found fast kink surface waves. In addition, acoustic wave flux was converted into magnetic wave flux as a function of loop inclination. In Chapter 4 we focused on a much smaller part of the solar atmosphere. In order to explain the strong damping obtained by observations, we simulated a solar pore located in the photosphere. The model pore has an increasing radius with increasing height and was driven with a realistic amplitude. The energy flux damping is highly dependent on the driver location and for simulations featuring a localized driver inside the pore, significant damping was achieved. The damping resulted from two geometrical effects: geometric spreading and lateral wave leakage. Finally, in Chapter 5 we simulated a full coronal loop that expands with increasing distance from the photosphere. This model is more realistic then the previous ones and was driven with a low frequency driver, which causes the presence of an acoustic cutoff region. The cutoff region and the steep transition region hinder acoustic waves from reaching the corona and only 2% of the initial driver energy reaches this layer. Throughout the model atmosphere, a mix of propagating and standing acoustic waves was formed. The standing waves cause an oscillation of the transition region height. We also found indications of sausage waves being excited. Due to the changing plasma parameters with height, different sausage modes are possible at different positions along the loop.

Date:25 Sep 2017 →  31 Aug 2021
Keywords:corona, plasma, AC heating, coronal loop, coronal waves
Disciplines:Atmospheric sciences, Physical geography and environmental geoscience, Atmospheric sciences, challenges and pollution, Astronomy and space sciences
Project type:PhD project