2025 PDG seminars

Dynamics of the lower and upper solar atmosphere using MURaM

In this talk, I will discuss two cases of dynamics in the lower and upper solar atmosphere respectively using the realistic three-dimensional MHD code, MURaM. I will primarily talk about the first case, which is a comparison of vortex dynamics in the photosphere and lower chromosphere in three different magnetic field regions. We use a flux tube expansion model and fourier spectra analysis to find that there is increased potential of vortex-induced torsional Alfvén waves to travel higher in the atmosphere for weaker magnetic regions, whereas vortices would result in dissipation and heating due to the vortex interactions in narrow flux tubes for the strongly magnetized regions. 

For the second case, which is my present work, I will briefly talk about EUV emission line synthesis using the MURaM simulation and the CHIANTI database in the upper solar atmosphere. Overall, the goal of this talk is to provide a new understanding of the extent to which the current complex 3D MHD models capture and represent the physics of the solar atmosphere. 

Radiative Models of Coronal Condensation at Extreme Resolution

Radiation remains the primary vector by which the properties of solar plasma can be investigated. Atomic spectral lines, often forming in thin atmospheric layers, offer a powerful mechanism to probe the solar atmosphere, in particular its outer layers where conditions are typically outside of local thermodynamic equilibrium. Synthesising the radiation produced by numerical models also provides an essential lens through which to compare models and observations. Isolated solar structures such as filaments and prominences are of particular interest due to the complexity of spectral line formation within them and the diagnostic window this provides onto their formation and stability.

In recent years, magnetohydrodynamic models of these isolated structures have become significantly more advanced, whilst radiative treatments have primarily remained the same. We have recently introduced the DexRT code: a novel approach to multidimensional non-equilibrium radiative transfer using a technique termed radiance cascades to efficiently treat problems with intertangled layers of optically thin and thick material – a regime where current approaches can fail with dramatic so-called ray effects. Here, I will provide a brief description of the methods used in DexRT along with spectra of a variety of lines synthesised from complex magnetohydrodynamic models. I will also discuss the importance of considering both detailed radiative transfer and viewing angle effects when making predictions and comparisons to observations. 

Sun-as-a-star flare observations with high-resolution telescopes

Stellar flares cannot be spatially resolved, meaning we have to extract complex three-dimensional behavior from a one-dimensional disk-integrated spectral timeseries. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument in combination with spatially resolved observations including some large IRIS flares. In this talk, I will discuss how high-resolution observations with a limited field-of-view can be converted into approximations of disk-integrated spectra using the newly developed Numerical Sun-as-a-Star Integrator (NESSI). Additionally, I will discuss the impact of projectional effects on the study of such events with focus on the detection of coronal mass ejections. Our findings suggest common patterns in the disk-integrated spectra between flares of different strengths and locations that can be used to better interpret stellar flares without resolved context.

Improved atomic models to interpret the solar radiation emitted from the Transition region and chromosphere

We have long-standing issues and discrepancies between predicted and observed emissions. In the solar transition region and chromosphere, some are due to the inherent limitations of the physical models, but some are due to simplified atomic models.

We developed improved modelling of the ion balance, including physical effects which occur all the time and made them available via the CHIANTI v.11. I will briefly describe them and show how they improve the comparisons with observations of the Sun and other stars, with very simple 1D static atmospheric models.  I will then describe current models we are developing to explain some chromospheric lines, and the plans to include other effects such as photo-ionization.