Numerical modelling of waves in the solar atmosphere

Abstract

The Sun is the closest star to Earth and the bringer of life for all of us. Remove the Sun and Earth is rendered a lifeless, icy rock floating in outer space. It stands to reason that a thorough understanding of the workings of the Sun would be high priority in the scientific world, and beyond. The outer-most layer of the interior of the Sun comprises of what is known as the convection zone. This chaotic zone produces innumerable pressure waves, which propagate through the Sun. These waves carry the energy of the Sun to its atmosphere and beyond, and are thought to be responsible for the infamous ‘coronal heating problem’. The magnetic nature of the Sun allows both acoustic and magnetic waves, or various combinations of the two, to exist. This is where the complexity lies, with so many different types of waves being produced and exchanging energy between themselves, it is extremely difficult to pinpoint which waves are responsible for the observations we make. Observations and mathematical/physical theories of ever improving quality are used to understand the details of waves in the Sun, however they often lack a bridge to connect them, which is where numerical simulations come in. The work presented here provides a combination of 1.5, 2.5 and 3 dimensional simulations looking to explain how a variety of waves propagate and carry energy through the internal and external layers of the Sun. When a wave reaches a layer in the Sun’s atmosphere where the sound and Alfvén speeds coincide, it splits into two ‘modes’, a fast and slow mode. Recent mathematical findings suggested an incoming shock wave would not only split into its fast and slow components, but that both wave modes would be smoothed as they exit this area. Numerical simulations herein show that only the slow wave is smoothed, with the fast wave propagating unhindered. Within the Sun’s atmosphere, various steep gradients of its physical components are found. These gradients have been proposed to act as barriers to incoming waves, which can be partially reflected off them. Multiple reflection sites suggests cavities can be created that acoustic waves can resonate within. Numerical simulations herein show a stark increase in the velocity of frequencies proposed to be characteristic of a cavity within the chromosphere. This suggests cavities can exist within the Sun’s atmosphere and the amplitude of velocity observations from within these areas must be partially attributed to the resonant effects of the cavities themselves. Following the onset of some solar flares, ripples are observed on the Solar surface emanating from the flare site. These ripples show strong anisotropies in their appearance, a characteristic not well studied. Numerical simulations herein show that these anisotropies can be attributed to both the strength and inclination of the magnetic field but even more so by the nature of the impacting source. A source with motion perpendicular to the solar surface causes constructive interference and higher amplitude ripples are created along the axis of motion