Title:
Seismology of Coronal Loops: Two Decades of Resonant Damping
Marcel Goossens
Katholieke Universiteit Leuven
Abstract:
The detection of rapidly damped transverse
oscillations in coronal loops by Aschwanden et al. and Nakariakov
et al. in 1999 gave a strong impetus to the study of MHD waves and
their damping in coronal loops. The common interpretation of the
observations of the transverse oscillations is based on the kink
modes. This colloquium reviews the concept of resonant absorption
as damping mechanism for oscillations in coronal loops. I will
start with the 1988 paper by Hollweg and Yang and will review work
by different groups (UNH, St-Andrews, Leuven, Warwick, Sheffield,
UIB among others) on MHD oscillations in coronal loops. In
particular I will show that the kink mode of a uniform cylindrical
equilibrium model embedded in an exterior plasma, both with
constant density and constant magnetic field, is transformed into
a damped quasi-mode when the true discontinuity is replaced by
continuous variation. I will derive approximate expressions for
the period and damping rate for the thin tube thin boundary (TTTB)
equilibrium model and show that the Hollweg and Yang expression
is a particular case. When these expressions were used for the
interpretation of 11 loop oscillation events, the important
finding was that resonant absorption can explain the
observations, but that the loops need to have thick non-uniform
layers. I will briefly discuss numerical results for equilibrium
models of fully non-uniform loops.