39 research outputs found
Fast magnetoacoustic waves in curved coronal loops. I, Trapped and leaky modes
A study of vertically polarised fast magnetoacoustic waves in a curved coronal loop is presented. The loop is modeled as a semi-circular magnetic slab in the zero plasma-β limit. The governing equations for linear waves are derived. We show that the wave mode behaviour depends on the slope of the equilibrium density profile, which is modeled as a piece-wise continuous power law curve of index α. For all profiles, except for α = −4, wave modes are not trapped in the loop and leak out into the external medium through wave tunneling. The particular case of α = −4, which corresponds to a linearly increasing Alfvén speed profile, is examined in more detail as this is the only model that can support trapped wave modes. We compare the results with a straight slab model and find similar behaviour. Coupling between sausage and kink wave modes has not been found in the model
Transverse oscillations in solar coronal loops induced by propagating Alfvenic pulses
The propagation and the evolution of Alfvenic pulses in the solar coronal
arcades is investigated by means of MHD numerical simulations. Significant
transverse oscillations in coronal loops, triggered by nearby flare events, are
often measured in EUV lines and are generally interpreted as standing kink
modes. However, the damping times of these oscillations are typically very
short (from one to a few periods) and the physical mechanism responsible for
the decay is still a matter of debate. Moreover, the majority of the observed
cases actually appears to be better modeled by propagating, rather than
standing, modes. Here we perform 2.5-D compressible MHD simulations of
impulsively generated Alfven waves propagating in a potential magnetic arcade
(assumed as a simplified 2-D loop model), taking into account the
stratification of the solar atmosphere with height from the photosphere to the
corona. The results show a strong spreading of the initially localized pulses
along the loop, due to the variations in the Alfven velocity with height, and
correspondingly an efficient damping of the amplitude of the oscillations. We
believe that simple explanations based on the effects of wave propagation in
highly inhomogeneous media may apply to the majority of the reported cases, and
that variations of the background density and Alfven speed along the loop
should be considered as key ingredients in future models.Comment: Accepted for publication in A&A on 26 October 2004; 10 pages, 8
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Identification of different types of kink modes in coronal loops: principles and application to TRACE results
We explore the possible observational signatures of different types of kink
modes (horizontal and vertical oscillations in their fundamental mode and
second harmonic) that may arise in coronal loops, with the aim of determining
how well the individual modes can be uniquely identified from time series of
images. A simple, purely geometrical model is constructed to describe the
different types of kink-mode oscillations. These are then `observed' from a
given direction. In particular, we employ the 3D geometrical parameters of 14
TRACE loops of transverse oscillations to try to identify the correct observed
wave mode. We find that for many combinations of viewing and loop geometry it
is not straightforward to distinguish between at least two types of kink modes
just using time series of images. We also considered Doppler signatures and
find that these can help obtain unique identifications of the oscillation modes
when employed in combination with imaging. We then compare the modeled spatial
signatures with the observations of 14 TRACE loops. We find that out of three
oscillations previously identified as fundamental horizontal mode oscillations,
two cases appear to be fundamental vertical mode oscillations (but possibly
combined with the fundamental horizontal mode), and one case appears to be a
combination of the fundamental vertical and horizontal modes, while in three
cases it is not possible to clearly distinguish between the fundamental mode
and the second-harmonic of the horizontal oscillation. In five other cases it
is not possible to clearly distinguish between a fundamental horizontal mode
and the second-harmonic of a vertical mode.Comment: 12 pages, 10 figures, 2 tables. A&A in pres
Excitation of standing kink oscillations in coronal loops
In this work we review the efforts that have been done to study the
excitation of the standing fast kink body mode in coronal loops. We mainly
focus on the time-dependent problem, which is appropriate to describe flare or
CME induced kink oscillations. The analytical and numerical studies in slab and
cylindrical loop geometries are reviewed. We discuss the results from very
simple one-dimensional models to more realistic (but still simple) loop
configurations. We emphasise how the results of the initial value problem
complement the eigenmode calculations. The possible damping mechanisms of the
kink oscillations are also discussed