“Three-D MHD numerical simulations of coronal loop
oscillations associated with flares”
Miyagoshi et al., PASJ 56, 207, 2004
Issues of coronal seismology
• The dynamic spectrum of the coronal plasma structure has resonances (“normal modes”)
• Several lines of observation show these
• There is controversy with damping
• Their excitation is fundamentally interesting
Literature
• There is quite a bit of observational literature
• Analytic theory has been developed (especially by B. Roberts et al.)
• The problems presented by the oscillations seem ideally suited to ideal MHD theory
• But there are non-ideal effects observed
Foregoing was for an empty loop (uniformhydrostatic potential-field medium.
What about an overdense loop?
Conclusions
• Oscillations found, at P = 2L/VA
• Oscillations damp at damp = -1/2
• Overdense loops oscillate more slowly, as expected
• Trapping effect is weak
Remaining questions
• How do these results compare with the analytic development of Roberts et al.?
• What modes are excited?
• How can this explanation of damping (the mere radiation of Alfven waves) be correct?
Periods
• Oscillations found, at P = 2L/VA
• Theory (Roberts, Edwin, Benz 1984) for “principal kink wave” gives
P = 2L/Ck, where
Ck = sqrt(2/(1+e/o))VA
Possible causes of damping
• Unexpectedly high viscosity
• Phase mixing
• Wave leakage at footpoints
• Resonant absorption
• Direct Alfven-wave losses?