2023 PDG PhD student seminars


Fisal Ahmed H Asiri

Slow body MHD waves in inhomogeneous magnetic flux tubes

Intense magnetic waveguides such as pores, sunspots, coronal loops, fibrils, etc. are ideal environments for the propagation of guided magnetohydrodynamic (MHD) waves. In general the theoretical description of wave propagation within these structures considers plasma parameters (density, pressure, temperature, magnetic field, etc.) to be constant. For an improved modelling of waves one needs to understand the modifications in wave properties in realistic waveguides such as sunspots, where the plasma shows a transversal density profile inhomogeneity. The present study investigates the propagation characteristics and the spatial structure of slow body MHD eigenmodes in a magnetic flux tube with circular cross section with inhomogeneous equilibrium density distribution under solar photospheric conditions in the short wavelength limit. The equilibrium  density profile inhomogeneity is represented by a local circular density enhancement or depletion whose strength, size and position can change. The task was addressed numerically with use of  the Fourier-Chebyshev Spectral method (FCS). The radial and azimuthal variation of eigenfunctions is obtained by solving a Helmholtz-type partial differential equation with Dirichlet boundary conditions. The inhomogeneous transverse equilibrium  density profile results in modified eigenvalues and eigenvectors. It was found that modification in the equilibrium density distribution leads to a decrease in the eigenvalues and the spatial structure of modes ceases to be global harmonic oscillations, as the modes migrate towards regions of lower density. Comparing the homogeneous case and the cases corresponding to depleted density enhancement, the dimensionless phase speed undergoes a significant drop in its value (at least 40%). In contrast to the density enhancement, the slow body modes investigated here preserve their morphology in terms of their spatial structure. We carry out a parametric analysis to determine the importance of the physical parameters on the dimensionless phase speed of slow body modes corresponding to the spatial structure of the total pressure perturbation under photospheric conditions. The presented model can be considered a first attempt to study theoretically the properties of slow magnetoacoustic body waves in magnetic flux tubes modelling  pores and the umbra of sunspots with local density inhomogeneities labeled as umbral dots.



Matthew Lennard

Today's Weather… IN SPACE

Solar activity is hugely responsible for changes in the climate of the Earth’s magnetosphere. Such changes caused by these events can be problematic for day-to-day technologies, upon which people are becoming more dependent. Current space weather modelling and forecasting is dependent on taking measurements during the occurrence of solar flares/CMEs giving only minutes to hours to help protect against such phenomena. In this talk I will discuss how the application of new machine learning tools are being combined with dynamical systems theory to predict the short term future of the Sun’s activity and the progress being made by our team in Sheffield on a new field dubbed ‘Magnetohydrology’.



Max McMurdo

Phase mixed Alfven waves in a partially ionised plasmas