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Astron. Astrophys. 340, 287-299 (1998)


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On the steady state of nonlinear quasiresonant Alfvén oscillations in one-dimensional magnetic cavity

Luigi Nocera 1 and Michael S. Ruderman 2, 3

1 Institute of Atomic and Molecular Physics, National Research Council, Via Giardino 7, I-56127 Pisa, Italy
2 School of Mathematical and Computational Sciences, St Andrews University, St Andrews, Fife KY16 9SS, Scotland, UK
3 On leave of Institute for Problems in Mechanics, Russian Academy of Sciences, Vernadskii Prosp. 101, 117526 Moscow, Russia

Received 7 May 1998 / Accepted 21 September 1998

Abstract

We study the steady state of nonlinear, small-amplitude, quasiresonant Alfvén oscillations in a homogeneous dissipative hydromagnetic cavity which is forced by the shear motion of its boundaries. It is shown that, even in the case of strong nonlinearity, these oscillations can be represented, to leading order, by a sum of two solutions in the form of oppositely propagating waves with permanent shapes. An infinite set of nonlinear equations for the Fourier coefficients of these solutions is derived which, in general, admits multiple solutions, depending on the re-scaled total Reynolds number, R, and mistuning, [FORMULA], between the frequency of the boundary forcing and the first Alfvén eigenmode of the cavity. Two types of solutions are found. On the one hand, low-modal solutions set in over the entire parameter range studied, which can be represented, with a remarkable accuracy, by very few Fourier modes even at very large R. For a fixed [FORMULA] the time-averaged energy, [FORMULA], that can be stored in the cavity is saturated, as R increases, to a value which is approximately proportional to [FORMULA], [FORMULA] being the Alfvén Mach number of the boundary motions. The time-averaged absorbed power (the Poynting flux [FORMULA]) scales as [FORMULA]. For suitable values of R and [FORMULA] catastrophic transitions occur between these solutions, in which the average power released scales as R, provided [FORMULA]. The second type of solutions sets in for a narrow window of [FORMULA] and develops large gradients (shocks) which need to be represented by many Fourier modes. For a fixed [FORMULA] the build-up of these gradients takes place starting from a low-modal solution in either a continuous way, by increasing R, or in a sudden catastrophic way as R becomes smaller than a critical value. Both [FORMULA] and [FORMULA] are saturated as R is increased. It is suggested that both types of solutions can explain bright events in the solar atmosphere.

Key words: MHD – waves – Sun: chromosphere – Sun: corona – Sun: oscillations

Send offprint requests to: L. Nocera

© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998

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