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Astron. Astrophys. 323, 969-985 (1997)


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Prominence oscillations and stability

Communicating the distant photospheric boundary

N.A.J. Schutgens

Sterrekundig Instituut Utrecht, Utrecht University, The Netherlands

Received 29 July 1996 / Accepted 16 January 1997

Abstract

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 [FORMULA] ([FORMULA]: 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 [FORMULA] 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 [FORMULA] is comparable to or greater than the oscillation period. Also, forced oscillations can only be significant for rather precisely matched values of [FORMULA] 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

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