TY - JOUR
T1 - Leaky and resonantly damped flux tube modes reconsidered
JF - Astronomy & Astrophysics
Y1 - 1999
A1 - Stenuit, H.
A1 - Tirry, W. J.
A1 - Keppens, R.
A1 - Goossens, M.
AB - In this research note the results for the eigenfrequencies of the uniform and non-uniform magnetic flux tubes of Stenuit et al. (1998) are reconsidered. In that paper it is shown that the eigenfrequencies may have a damping rate due to two mechanisms causing a loss of energy. In non-uniform flux tubes the eigenmodes can be damped by resonant absorption. The other mechanism is leakage of wave energy into the surroundings, which can occur for both uniform and non-uniform flux tubes. We point out that the dispersion relations obtained by Stenuit et al. are correct for leaky and undamped non-leaky modes, but are not correct for resonantly damped non-leaky modes.
VL - 342
SN - 0004-6361
U5 - 0430790f5711556975fefab59a39745b
ER -
TY - JOUR
T1 - Eigenfrequencies and optimal driving frequencies of 1D non-uniform magnetic flux tubes
JF - Astronomy & Astrophysics
Y1 - 1998
A1 - Stenuit, H.
A1 - Keppens, R.
A1 - Goossens, M.
AB - The eigenfrequencies and the optimal driving frequencies for flux tubes embedded in uniform but wave-carrying surroundings are calculated, based on matching conditions formulated in terms of the normal acoustic impedances at the Bur tube boundary. The requirement of the equality of the normal acoustic impedance of the transmitted wave field with the normal acoustic impedance of the outgoing wave field selects the eigenmodes, while the equality of the ingoing and the transmitted normal acoustic impedance selects the optimal driving frequencies (Keppens 1996). Even if the flux tube is uniform, the eigenfrequencies can be complex due to leakage of wave energy into the surroundings. The case of uniform flux tubes has been considered previously (e.g. Cally 1986), and serves as a testcase of our formalism. We extend Cally's results by taking a radial stratification of the flux tube into account. The non-uniformity of the flux tube can introduce another cause for energy loss, namely resonant absorption internal to the flux tube. When resonant absorption occurs. we must incorporate the appropriate jump conditions over the dissipative layer(s). This can be done using a simple numerical scheme as introduced by Stenuit et al. (1995).
VL - 331
SN - 0004-6361
U5 - 555175cde1513aa5bcf7da12e11638a2
ER -
TY - JOUR
T1 - Direct excitation of resonant torsional Alfven waves by footpoint motions
JF - Astronomy & Astrophysics
Y1 - 1997
A1 - Ruderman, M. S.
A1 - Berghmans, D.
A1 - Goossens, M.
A1 - Poedts, S.
AB - The present paper studies the heating of coronal loops by linear resonant Alfven waves that are excited by the motions of the photospheric footpoints of the magnetic field lines. The analysis is restricted to torsionally polarised footpoint motions in an axially symmetric system so that only torsional Alfven waves are excited. For this subclass of footpoint motions, the Alfven and cusp singularities are absent from the analysis which means that resonant coupling between global modes of the loop and localised oscillations is avoided. Instead, the focus is on the resonances due to the finite extent of the loop in the longitudinal direction: at the radii where Alfven waves travelling back and forth along the length of the loop are in phase with the footpoint motions, the oscillations grow unbounded in ideal MHD. Inclusion of electrical resistivity and viscosity as dissipation mechanisms prevents singular growth and we can look at the steady state in which the energy injected at the photospheric part of the loop is balanced by the energy dissipated at the dissipative layer around the resonance. In this sense, we show that the direct excitation of Alfven waves by torsionally polarised footpoint motions leads to a very efficient heating mechanism for coronal loops, even without resonant coupling to global modes.
VL - 320
SN - 0004-6361
U5 - fc0bbcb6e364d6fb58b67dc9bbd3818b
ER -
TY - RPRT
T1 - Alfven wave heating and dissipative instabilities of astrophysical plasmas
Y1 - 1992
A1 - Goedbloed, J. P.
A1 - Goossens, M.
A1 - Kerner, W.
A1 - Poedts, S.
A1 - Van der Linden, R. A. M.
A1 - Halberstadt, G.
A1 - Huysmans, G. T. A.
A1 - Stenuit, H.
A1 - Keppens, R.
A1 - Scheurwater, R.
A1 - Schwarz, E.
A1 - Keegan, B.
KW - Plasma physics
PB - FOM Rijnhuizen
CY - Nieuwegein, Netherlands
U5 - a1db73ce1cba0c6cff43094b235afa38
ER -