2,172 research outputs found
Hinode/EIS observations of propagating low-frequency slow magnetoacoustic waves in fan-like coronal loops
We report the first observation of multiple-periodic propagating disturbances
along a fan-like coronal structure simultaneously detected in both intensity
and Doppler shift in the Fe XII 195 A line with the EUV Imaging Spectrometer
(EIS) onboard Hinode. A new application of coronal seismology is provided based
on this observation. We analyzed the EIS sit-and-stare mode observation of
oscillations using the running difference and wavelet techniques. Two harmonics
with periods of 12 and 25 min are detected. We measured the Doppler shift
amplitude of 1-2 km/s, the relative intensity amplitude of 3%-5% and the
apparent propagation speed of 100-120 km/s. The amplitude relationship between
intensity and Doppler shift oscillations provides convincing evidence that
these propagating features are a manifestation of slow magnetoacoustic waves.
Detection lengths (over which the waves are visible) of the 25 min wave are
about 70-90 Mm, much longer than those of the 5 min wave previously detected by
TRACE. This difference may be explained by the dependence of damping length on
the wave period for thermal conduction. Based on a linear wave theory, we
derive an inclination of the magnetic field to the line-of-sight about 598
deg, a true propagation speed of 12825 km/s and a temperature of
0.70.3 MK near the loop's footpoint from our measurements.Comment: 4 pages and 4 figures, with 3 online figures and 1 online table;
Astron & Astrophys Letter, in pres
Three dimensional MHD Modeling of Vertical Kink Oscillations in an Active Region Plasma Curtain
Observations on 2011 August 9 of an X6.9-class flare in active region (AR)
11263 by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics
Observatory (SDO), were followed by a rare detection of vertical kink
oscillations in a large-scale coronal active region plasma curtain in EUV
coronal lines. The damped oscillations with periods in the range 8.8-14.9 min
were detected and analyzed recently. Our aim is to study the generation and
propagation of the MHD oscillations in the plasma curtain taking into account
realistic 3D magnetic and density structure of the curtain. We also aim at
testing and improving coronal seismology for more accurate determination of the
magnetic field than with standard method. We use the observed morphological and
dynamical conditions, as well as plasma properties of the coronal curtain based
on Differential Emission Measure (DEM) analysis to initialize a 3D MHD model of
its vertical and transverse oscillations by implementing the impulsively
excited velocity pulse mimicking the flare generated nonlinear fast
magnetosonic propagating disturbance interacting with the curtain obliquely.
The model is simplified by utilizing initial dipole magnetic field, isothermal
energy equation, and gravitationally stratified density guided by observational
parameters. Using the 3D MHD model, we are able to reproduce the details of the
vertical oscillations and study the process of their excitation by nonlinear
fast magnetosonic pulse, propagation, and damping, finding agreement with the
observations. We estimate the accuracy of simplified slab-based coronal
seismology by comparing the determined magnetic field strength to actual values
from the 3D MHD modeling results and demonstrate the importance of taking into
account more realistic magnetic geometry and density for improving coronal
seismology
Oscillations in active region fan loops: Observations from EIS/{\it Hinode} and AIA/SDO
Active region fan loops in AR 11076 were studied, in search of oscillations,
using high cadence spectroscopic observations from EIS on board Hinode combined
with imaging sequences from the AIA on board SDO. Spectra from EIS were
analyzed in two spectral windows, \FeXII 195.12 \AA and \FeXIII 202.04 \AA
along with the images from AIA in 171 \AA and 193 \AA channels. We find short
(3 min) and long (9 min) periods at two different locations.
Shorter periods show oscillations in all the three line parameters and the
longer ones only in intensity and Doppler shift but not in line width. Line
profiles at both these locations do not show any visible blue-shifted component
and can be fitted well with a single Gaussian function along with a polynomial
background. Results using co-spatial and co-temporal data from AIA/SDO do not
show any significant peak corresponding to shorter periods, but longer periods
are clearly observed in both 171 \AA and 193 \AA channels. Space-time analysis
in these fan loops using images from AIA/SDO show alternate slanted ridges of
positive slope, indicative of outward propagating disturbances. The apparent
propagation speeds were estimated to be 83.5 1.8 \kms and 100.5 4.2
\kms, respectively, in the 171 \AA and 193 \AA channels. Observed short period
oscillations are suggested to be caused by the simultaneous presence of more
than one MHD mode whereas the long periods are suggested as signatures of slow
magneto-acoustic waves. In case of shorter periods, the amplitude of
oscillation is found to be higher in EIS lines with relatively higher
temperature of formation. Longer periods, when observed from AIA, show a
decrease of amplitude in hotter AIA channels which might indicate damping due
to thermal conduction owing to their acoustic nature.Comment: Accepted for publication in Solar Physic
Propagating slow magnetoacoustic waves in coronal loops observed by Hinode/EIS
We present the first Hinode/EIS observations of 5 min quasi-periodic
oscillations detected in a transition-region line (He II) and five coronal
lines (Fe X, Fe XII, Fe XIII, Fe XIV, and Fe XV) at the footpoint of a coronal
loop. The oscillations exist throughout the whole observation, characterized by
a series of wave packets with nearly constant period, typically persisting for
4-6 cycles with a lifetime of 20-30 min. There is an approximate in-phase
relation between Doppler shift and intensity oscillations. This provides
evidence for slow magnetoacoustic waves propagating upwards from the transition
region into the corona. We find that the oscillations detected in the five
coronal lines are highly correlated, and the amplitude decreases with
increasing temperature. The amplitude of Doppler shift oscillations decrease by
a factor of about 3, while that of relative intensity decreases by a factor of
about 4 from Fe X to Fe XV. These oscillations may be caused by the leakage of
the photospheric p-modes through the chromosphere and transition region into
the corona, which has been suggested as the source for intensity oscillations
previously observed by TRACE. The temperature dependence of the oscillation
amplitudes can be explained by damping of the waves traveling along the loop
with multithread structure near the footpoint. Thus, this property may have
potential value for coronal seismology in diagnostic of temperature structure
in a coronal loop.Comment: 13 pages, 11 color figures, 4 tables, Astrophys.J, May 2009 - v696
issue, (in press
Observations of Dissipation of Slow Magneto-acoustic Waves in a Polar Coronal Hole
We focus on a polar coronal hole region to find any evidence of dissipation
of propagating slow magneto-acoustic waves. We obtained time-distance and
frequency-distance maps along the plume structure in a polar coronal hole. We
also obtained Fourier power maps of the polar coronal hole in different
frequency ranges in 171~\AA\ and 193~\AA\ passbands. We performed intensity
distribution statistics in time domain at several locations in the polar
coronal hole. We find the presence of propagating slow magneto-acoustic waves
having temperature dependent propagation speeds. The wavelet analysis and
Fourier power maps of the polar coronal hole show that low-frequency waves are
travelling longer distances (longer detection length) as compared to
high-frequency waves. We found two distinct dissipation length scales of wave
amplitude decay at two different height ranges (between 0--10 Mm and 10--70 Mm)
along the observed plume structure. The dissipation lengths obtained at higher
height range show some frequency dependence. Individual Fourier power spectrum
at several locations show a power-law distribution with frequency whereas
probability density function (PDF) of intensity fluctuations in time show
nearly Gaussian distributions. Propagating slow magneto-acoustic waves are
getting heavily damped (small dissipation lengths) within the first 10~Mm
distance. Beyond that waves are getting damped slowly with height. Frequency
dependent dissipation lengths of wave propagation at higher heights may
indicate the possibility of wave dissipation due to thermal conduction,
however, the contribution from other dissipative parameters cannot be ruled
out. Power-law distributed power spectra were also found at lower heights in
the solar corona, which may provide viable information on the generation of
longer period waves in the solar atmosphere.Comment: corrected typos and grammar, In press A&
Modulation of gyrosynchrotron emission in solar and stellar flares by slow magnetoacoustic oscillations
Gyrosynchrotron emission generated by non-thermal electrons in solar and stellar coronal flares can be efficiently modulated by slow magnetoacoustic oscillations in the flaring loops. The modulation mechanism is based upon perturbation of the efficiency in the Razin suppression of optically thin gyrosynchrotron emission. Modulation of the emission is in anti-phase with the density perturbation in the slow wave. The observed emission modulation depth can be up to an order of magnitude higher than the slow wave amplitude. This effect is more pronounced at lower frequencies. Observations with spatial resolution, together with analysis of the modulation frequency, are shown to be sufficient for providing the information needed to identify the mode
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