Interaction of Coronal Waves with Coronal Holes

The solar corona is the outermost layer of the solar atmosphere, and although we can now observe details of its structure thanks to sophisticated satellite missions, many important questions remain open. The most interesting open question that has been puzzling scientists for more than 100 years is why the solar corona is so hot. A large part of the scientific research in solar physics tries to explain why this occurs. The Sun exhibits different structures and phenomena indicative of solar activity, and those structures are the ones we want to study in order to understand the dynamics of the solar corona better in an attempt to answer important questions that affect our daily lives. Two of the most exiting phenomena that occur in the solar corona are coronal holes and coronal waves. Coronal holes are dark regions in the corona. The reason why they appear dark is because the density of particles in coronal holes is lower than the density of their surroundings. They are of special interest since they are the source of high-speed streams, which strongly influence the structure of interplanetary space and determine Space Weather, which has a strong effect on satellite communication and therefore affecting our daily lives. Coronal waves are propagating disturbances in the solar corona that may or may not interact with coronal holes. When a coronal wave interacts with a coronal hole, their interaction leads to different interesting physical phenomena, such as the so-called reflected or refracted waves. Studying these phenomena helps us understand the processes that govern dynamics in the solar corona, which allows us to perform improvements of Space Weather forecast. Due to certain limitations in the observations of the Sun, we need to perform computer simulations of the solar atmosphere in order to supplement the observational data and therefore get a more comprehensive picture of the dynamic processes. Computer simulations of the solar atmosphere are usually based on solving numerically the standard magnetohydrodynamic equations. Results of such simulations show mostly good agreement with observations. However, in order to get more accurate results and be able to describe the physics of the corona better, we use a more advanced theoretical approach including numerical magnetohydrodynamic simulations. The aim of our project is to develop a new theoretical model including a full three-dimensional approach for studying dynamic processes in the solar corona. For the first time, we will perform 2D and 3D simulations of coronal holes interacting with coronal waves including a complex coronal hole geometry. Overall, we expect new and important insights in coronal wave propagation, the properties of coronal holes as well as an improvement of Solar wind models and Space Weather forecast.

This project focused on the interaction of two of the many exciting phenomena in the solar corona: coronal holes and coronal waves. The solar corona is the outermost layer of the solar atmosphere, and although sophisticated satellite missions now allow us to observe its structure in great detail, many important questions remain open. These include, for example, why coronal waves reflected at coronal hole boundaries can be faster than the incoming waves, and how the shape of a coronal hole influences the interaction features. The results of the project show that at least four key factors are essential for understanding and explaining coronal wave-coronal hole interactions: (i) a realistic initial density profile of the incoming coronal wave that consists of both an enhanced and a depleted part, (ii) the incident angle of the coronal wave as it approaches the coronal hole, (iii) the density of the coronal hole, and (iv) the shape of the coronal hole. Using magnetohydrodynamic simulations, we were able to demonstrate that a specific combination of these four factors can explain interaction features that have been reported in observations but could not previously be fully and comprehensively explained. More specifically, we found that a combination of small coronal hole density, a concave coronal hole boundary, and a realistic density profile of the incoming coronal wave leads to a large reflected phase speed, a significantly enhanced reflected density amplitude compared to the incoming wave, and a depleted density region between the incoming and reflected waves. All of these features have been observed, but until now lacked a comprehensive explanation. Moreover, these findings allow us, in certain cases, to infer properties of coronal holes, such as their density. Therefore, by combining numerical simulations, analytical formulas, and observational data, the project shows that such interactions can be used as a diagnostic tool to estimate coronal hole density and other key parameters of coronal wave - coronal hole interaction that usually cannot be obtained directly from measurements. Overall, the results significantly improve our understanding of how disturbances propagate through the solar corona and provide new insights into coronal hole properties, which are particularly important for developing more accurate solar wind models and space-weather forecasts.