How can numerical simulations advance our understanding of our life-giving star?
On 29th January 2020, the world witnessed the highest resolution image of the Sun ever taken in the history of humankind. The “caramelized popcorn” picture at the heart of our nearest star flooded the digital and print media around the world. Thanks to the newly installed 4-meter Daniel K. Inouye Solar Telescope (DKIST) in Hawaii, we will now have an unprecedented view of our nearest star that would bring us closer to solving several long-standing mysteries of the solar atmosphere.
While there are several excellent articles and blogs on the internet about the latest image and DKIST, I would like to take this opportunity to demonstrate the potential of the state-of-the-art numerical simulations that are crucial towards a better understanding of our life-giving star. The atmosphere of the stars (like our Sun) can be very complex to interpret since they sample very different physical regimes as we move from the convection zone (below the visible surface of the solar photosphere) to the outermost million-degree corona. To understand its details it is necessary to simulate the whole (or a part of) atmosphere since the different layers interact strongly. The major advantage of performing numerical simulations is that we “know” the exact physics behind what we see in them since they are run by solving some mathematical equations that govern a physical system. Over the past two decades, developments in the field of numerical simulations in solar physics led to the formulation of radiative MHD codes such as MURaM, RADYN, and Bifrost to name a few, that have enhanced our understanding of solar physics (a recent review here: Radiation Hydrodynamics).
With the help of Bifrost MHD code, I managed to compute a synthetic intensity image of the solar surface similar to the “caramelized popcorn” image observed by DKIST. Bifrost solves the standard magnetohydrodynamic (MHD) equations in 3D geometry and yields several physical parameters such as the magnetic field, velocity, and temperature of the solar atmosphere, to name a few. These physical parameters can then be used to account for the energy transfer that takes place in the atmosphere of the Sun via the process of radiative transfer, which can then be translated into intensity radiated from the solar surface. Then we can synthetically generate a “caramelized popcorn” picture as shown in the left panel of the figure below. The spatial extent of the simulation box corresponds to 24,000 km by 24,000 km in the horizontal and vertical direction with a resolution of 31 km which is pretty close to the theoretical resolution of DKIST (~36 km at a wavelength of 789 nm.)
As can be seen, the synthetic image resembles the observation from DKIST to a significant extent including the sizes and shapes of the cell-like patterns and both samples the solar photosphere. Qualitatively, the contrast of both images looks very similar. It is also to be noted that the synthetic image is generated just by solving the MHD equations discussed above and has no direct input from actual solar observations. Despite the similarities, the computed image also reflects the differences, viz-a-viz, the bright patches that we see in the DKIST image in the upper right part of the field-of-view (FOV) are somewhat lacking in our synthetic image. This is mainly attributed to the fact that we simulated what is termed as the “quiet sun” where the magnetic activity is quite low; the bright network like patches, on the other hand, indicate regions of a relatively higher magnetic field strength. Nevertheless, for a vast majority of the FOV, the resemblance is noteworthy!
Achieving a coherent picture of the complex physical processes through confronting state-of-the-art 3D numerical simulations along with comprehensive observations of the entire solar atmosphere, is what most of today’s solar physicists strive for. By combining observations from the newly launched satellites such as Solar Orbiter and Parker Solar Probe along with ground-based telescopes like DKIST and the Swedish 1-m Solar Telescope (SST) we are going to gain an unprecedented new understanding of our life-giving star. With new data and new tools in our hands, we feel more competent than ever before. What an exciting time it is to be a solar physicist!
