School of Physics and Astronomy

Astronomy Unit Seminar -- Magnetohydrodynamic Seismology of the Solar Coronal Plasma with Kink Waves

25 October 2019

Time: 2:30 - 3:30pm
Speaker: Valery Nakariakov
Venue: G. O. Jones Building, Room 610

Standing transverse oscillations of the plasma loops of the solar corona have been intensively studied for the last 20 years as a tool for the diagnostics of the coronal magnetic field. Those oscillations are confidently interpreted as standing fast magnetoacoustic kink modes of the plasma non-uniformities. Statistical analysis demonstrates that the oscillations are excited by a mechanical displacement of the loop from an equilibrium by a low coronal eruption. Standing kink oscillations are observed to operate in two regimes: rapidly decaying, large amplitude oscillations and undamped small amplitude oscillations. In the former regime the decay is associated with the phenomenon of linear coupling of a collective kink oscillations with unresolved phase-mixed torsional Alfvenic motions - the effect of resonant absorption. In the latter regime the damping should be compensated by energy supply, which  allows  the  loop  to  perform  almost  monochromatic  oscillations  with  almost  constant  amplitude  and  phase.  Different loops oscillate with different periods that scale with the oscillating loop length. The oscillation amplitude does not show dependence on the loop length or the oscillation period. We developed a low-dimensional model explaining the undamped kink oscillations as a self-oscillatory process caused by the effect of negative friction, which is analogous to producing a tune by moving a bow across a violin string. The period of self-oscillations is determined by the frequency of the kink mode. The ubiquity of decayless kink oscillations makes them an excellent tool for MHD seismology.