Title: MHD Sausage Oscillations in Solar Coronal Loops
This seminar is organised by KU Leuven's Center for mathematical Plasma-Astrophysics (CmPA).
Speaker: Daye Lim (CmPA)
Abstract: Solar flare emissions commonly show flux variations called quasi-periodic pulsations (QPPs). The period of QPPs ranges from sub-seconds to a few tens of minutes. Magnetohydrodynamics (MHD) oscillations, and among them, sausage oscillations have been used for the interpretation of QPPs. In this seminar, I will present my previous works on sausage oscillations as follows.
Unlike the other oscillation modes, theoretically, the sausage oscillations have the cutoff wavelength of the fundamental radial harmonic in cylinders with a straight, untwisted magnetic field inside and outside the cylinder. We studied linear sausage oscillations of a cylinder embedded in a plasma with an azimuthal magnetic field. Dispersion relations demonstrate that the lowest radial harmonic of the sausage mode is in the trapped regime for all values of the parallel wave number. The results can be applied to the interpretation of long-period oscillations of emission intensity detected in solar coronal structures.
One interesting feature of QPPs is the often detected anharmonic modulation patterns, i.e., the shape of the oscillatory signal could be different from the harmonic one. Possible mechanisms, e.g., coalescence instabilities and the superposition of several different oscillation modes, have been suggested. We analyzed impulsively excited sausage oscillations of a plasma cylinder with a smooth radial profile of Alfvén speed and straight magnetic fields. It is shown that even in the long-wavelength limit, damping times of higher radial sausage harmonics could be significantly greater than the oscillation periods. Thus, higher radial harmonics could appear as anharmonic oscillatory patterns.
The seminar will be held in a hybrid mode, in person in building 200B, room 02.14 and transmitted online using the following permanent Blackboard Collaborate link, that can be used for all the upcoming seminars in this academic year:
https://eu.bbcollab.com/guest/6fc7259fc2ba4f29af721ccb6a19eefd
