Prominence oscillations and stability
Communicating the distant photospheric boundary
Received 29 July 1996 / Accepted 16 January 1997
The photosphere provides an important boundary condition for prominence support. The conservation of photospheric flux (sometimes called line tying) sets a serious constraint on the evolution of coronal magnetic fields. This boundary condition can only be communicated to the prominence by Alfvén and magneto-acoustic waves. As a result, the boundary condition as experienced by the prominence at height h lags behind a time (: Alfvénspeed) as compared to the instantaneous situation at the location of the photosphere.
In this paper I study vertical oscillations and stability of prominences, taking retardation effects into account. An equation of motion for a Kuperus-Raadu prominence is derived, describing the prominence as a line current and the photosphere as a perfectly conducting plate. Solving this equation of motion implies solving the full time-dependent Maxwell equations, thus guaranteeing a realistic field evolution under the assumption of photospheric line tying. In terms of the currents that flow, such a description is equivalent to the corresponding MHD picture.
The results indicate that the travel time is an important parameter of the system as it influences the decay or growth times of prominence oscillations greatly. A new kind of instability is found, whereby the prominence experiences oscillations growing in time, even in the nonlinear regime. This instability occurs when the travel time is comparable to or greater than the oscillation period. Also, forced oscillations can only be significant for rather precisely matched values of and the driving period.
Key words: Sun: prominences Sun: oscillations magnetic fields waves
Send offprint requests to: Nick Schutgens, Sterrekundig Instituut Utrecht, Utrecht University, postbus 80,000, 3508 TA Utrecht, The Netherlands
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998