35 research outputs found
Synthetic observations of internal gravity waves in the solar atmosphere
We study the properties of internal gravity waves (IGWs) detected in
synthetic observations that are obtained from realistic numerical simulation of
the solar atmosphere. We used four different simulations of the solar
magneto-convection performed using the CO5BOLD code. A magnetic-field-free
model and three magnetic models were simulated. The latter three models start
with an initial vertical, homogeneous field of 10, 50, and 100 G magnetic flux
density, representing different regions of the quiet solar surface. We used the
NICOLE code to compute synthetic spectral maps from all the simulated models
for the two magnetically insensitive neutral iron lines Fe I 5434 {\AA} and
5576 {\AA}. We find the signatures of the internal gravity waves in the
synthetic spectra to be consistent with observations of the real Sun. The phase
differences obtained using the spectral lines are significantly different from
the phase differences in the simulation. The phase coherency between two
atmospheric layers in the gravity wave regime is height dependent and is seen
to decrease with the travel distance between the observed layers. We conclude
that the energy flux of IGWs determined from the phase difference analysis may
be overestimated by an order of magnitude. Spectral lines that are weak and
less temperature sensitive may be better suited to detecting internal waves and
accurately determining their energy flux in the solar atmosphere.Comment: To appear in Astronomy & Astrophysics, 12 pages, 12 figures, abridged
abstrac
On the effect of oscillatory phenomena on Stokes inversion results
Stokes inversion codes are crucial in returning properties of the solar
atmosphere, such as temperature and magnetic field strength. However, the
success of such algorithms to return reliable values can be hindered by the
presence of oscillatory phenomena within magnetic wave guides. Returning
accurate parameters is crucial to both magnetohydrodynamics studies and solar
physics in general. Here, we employ a simulation featuring propagating MHD
waves within a flux tube with a known driver and atmospheric parameters. We
invert the Stokes profiles for the 6301 \unicode{0xc5} and 6302
\unicode{0xc5} line pair emergent from the simulations using the well-known
Stokes Inversions from Response functions (SIR) code to see if the atmospheric
parameters can be returned for typical spatial resolutions at ground-based
observatories. The inversions return synthetic spectra comparable to the
original input spectra, even with asymmetries introduced in the spectra from
wave propagation in the atmosphere. The output models from the inversions match
closely to the simulations in temperature, line-of-sight magnetic field and
line-of-sight velocity within typical formation heights of the inverted lines.
Deviations from the simulations are seen away from these height regions. The
inversion results are less accurate during passage of the waves within the line
formation region. The original wave period could be recovered from the
atmosphere output by the inversions, with empirical mode decomposition
performing better than the wavelet approach in this task.Comment: Accepted for publication in Phil. Trans. R. Soc. A, 20 pages, 4
figure
Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the solar
atmosphere in the past decades. Four of the most frequently used numerical
codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the
wave propagation characteristics in model runs from these four codes by
measuring the dispersion relation of acoustic-gravity waves at various heights.
We find considerable differences between the various models. The height
dependence of wave power, in particular of high-frequency waves, varies by up
to two orders of magnitude between the models, and the phase difference spectra
of several models show unexpected features, including phase
jumps.Comment: 19 pages, 15 figure
3D Simulations of Magnetohydrodynamic Waves in the Magnetized Solar Atmosphere
We present results of three-dimensional numerical simulations of
magnetohydrodynamic (MHD) wave propagation in a solar magnetic flux tube. Our
study aims at understanding the properties of a range of MHD wave modes
generated by different photospheric motions. We consider two scenarios observed
in the lower solar photosphere, namely, granular buffeting and vortex-like
motion, among the simplest mechanism for the generation of waves within a
strong, localized magnetic flux concentration. We show that granular buffeting
is likely to generate stronger slow and fast magnetoacoustic waves as compared
to swirly motions. Correspondingly, the energy flux transported differs as a
result of the driving motions. We also demonstrate that the waves generated by
granular buffeting are likely to manifest in stronger emission in the
chromospheric network. We argue that different mechanisms of wave generation
are active during the evolution of a magnetic element in the intergranular
lane, resulting in temporally varying emission at chromospheric heights.Comment: Appeared in ApJ, 11 pages, 12 figure
Wave propagation and energy transport in the magnetic network of the Sun
We investigate wave propagation and energy transport in magnetic elements,
which are representatives of small scale magnetic flux concentrations in the
magnetic network on the Sun. This is a continuation of earlier work by Hasan et
al. (2005). The new features in the present investigation include a
quantitative evaluation of the energy transport in the various modes and for
different field strengths, as well as the effect of the boundary-layer
thickness on wave propagation. We carry out 2-D MHD numerical simulations of
magnetic flux concentrations for strong and moderate magnetic fields. Waves are
excited in the tube and ambient medium by a transverse impulsive motion of the
lower boundary. The nature of the modes excited depends on the value of beta.
Mode conversion occurs in the moderate field case when the fast mode crosses
the beta=1 contour. In the strong field case the fast mode undergoes conversion
from predominantly magnetic to predominantly acoustic when waves are leaking
from the interior of the flux concentration to the ambient medium. We also
estimate the energy fluxes in the acoustic and magnetic modes. The main
conclusions of our work are twofold: firstly, for transverse, impulsive
excitation, flux tubes/sheets with strong fields are more efficient than those
with weak fields in providing acoustic flux to the chromosphere. However, there
is insufficient energy in the acoustic flux to balance the chromospheric
radiative losses in the network, even for the strong field case. Secondly, the
acoustic emission from the interface between the flux concentration and the
ambient medium decreases with the width of the boundary layer.Comment: Accepted for publication in A&A, 13 pages, 10 figures. v2: improved
placement and quality of figures, acknowledgments, acceptance dat