152 research outputs found
Line-of-sight geometrical and instrumental resolution effects on intensity perturbations by sausage modes
Diagnostics of MHD waves in the solar atmosphere is a topic which often
encounters problems of interpretation, due partly to the high complexity of the
solar atmospheric medium. Forward modeling can significantly guide
interpretation, bridging the gap between numerical simulations and
observations, and increasing the reliability of mode identification for
application of MHD seismology. In this work we aim at determining the
characteristics of the fast MHD sausage mode in the corona on the modulation of
observable quantities such as line intensity and spectral line broadening.
Effects of line-of-sight angle, and spatial, temporal and spectral resolutions
are considered. We take a cylindrical tube simulating a loop in a low-{\beta}
coronal environment with an optically thin background, and let it oscillate
with the fast sausage mode. A parametric study is performed. Among other
results, we show that regardless of the ionisation state of the plasma, the
variation of spectral line broadening can be significant, even for low
intensity modulation. The nature of this broadening is not thermal but is
mostly turbulent. This places spectrometers in clear advantage over imaging
instruments for the detection of the sausage mode. The modulation of all
quantities is considerably affected by the line-of-sight angle, and especially
by the spatial and temporal resolution when these are on the order of the
mode's wavelength and period. This places high constraints on instrumentation.Comment: 16 pages, 20 figure
Coronal loop transverse oscillations excited by different driver frequencies
We analyse transverse oscillations of a coronal loop excited by continuous
monoperiodic motions of the loop footpoint at different frequencies in the
presence of gravity. Using the MPI-AMRVAC code, we perform three-dimensional
numerical magnetohydrodynamic simulations, considering the loop as a magnetic
flux tube filled in with denser, hotter, and gravitationally stratified plasma.
We show the resonant response of the loop to its external excitation and
analyse the development of the Kelvin-Helmholtz instability at different
heights. We also study the spatial distribution of plasma heating due to
transverse oscillations along the loop. The positions of the maximum heating
are in total agreement with those for the intensity of the Kelvin-Helmholtz
instability, and correspond to the standing wave anti-nodes in the resonant
cases. The initial temperature configuration and plasma mixing effect appear to
play a significant role in plasma heating by transverse footpoint motions. In
particular, the development of the Kelvin-Helmholtz instability in a hotter
loop results in the enhancement of the mean plasma temperature in the domain.Comment: Published in Ap
Spatial magneto-seismology : effect of density stratification on the first harmonic amplitude profile of transversal coronal loop oscillations
Context. The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO)and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona.
Aims. We show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma density stratification of that loop.
Methods. The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code pollux. Dispersion relations are derived and solved analytically. The results of the two methods are compared and verified.
Results. Density stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the density stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a density scale height
H = 50 Mm and loop half length L = 100 Mm is approximately 5.6Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops.
Conclusions. The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the
corona
Wave modes excited by photospheric p-modes and mode conversion in a multi-loop system
Context. Waves are ubiquitous in the solar corona and there are indications
that they are excited by photospheric p-modes. However, it is unclear how
p-modes in coronal loops are converted to sausage modes and transverse (kink)
modes, which are observed in the corona. Aims. We aim to investigate how those
wave modes are excited in the lower corona by photospheric acoustic waves.
Methods. We built 3D magnetohydrostatic loop systems with multiple inclinations
spanning from the photosphere to the lower corona. We then simulated these
atmospheres with the MANCHA code, in which we perturb the equilibrium with a
p-mode driver at the bottom of the domain. By splitting the velocity
perturbation into components longitudinal, normal, and azimuthal to the
magnetic flux surfaces we can study wave behavior. Results. In vertical flux
tubes, we find that deformed fast sausage surface waves and slow sausage body
waves are excited. In inclined flux tubes fast kink surface waves, slow sausage
body waves, and either a fast sausage surface wave or a plane wave are excited.
In addition, we calculate a wave conversion factor (0 C 1) from
acoustic to magnetic wave behavior by taking the ratio of the mean magnetic
energy flux to the sum of the mean magnetic and acoustic energy flux and
compare it to a commonly used theoretical conversion factor. We find that
between magnetic field inclinations of 10 to 30 those two
methods lie within 40%. For smaller inclinations the absolute deviation is
smaller than 0.1.Comment: 14 pages, 14 figure
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