1,540 research outputs found
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
Direct Imaging by SDO AIA of Quasi-periodic Fast Propagating Waves of ~2000 km/s in the Low Solar Corona
Quasi-periodic, propagating fast mode magnetosonic waves in the corona were
difficult to observe in the past due to relatively low instrument cadences. We
report here evidence of such waves directly imaged in EUV by the new SDO AIA
instrument. In the 2010 August 1 C3.2 flare/CME event, we find arc-shaped wave
trains of 1-5% intensity variations (lifetime ~200 s) that emanate near the
flare kernel and propagate outward up to ~400 Mm along a funnel of coronal
loops. Sinusoidal fits to a typical wave train indicate a phase velocity of
2200 +/- 130 km/s. Similar waves propagating in opposite directions are
observed in closed loops between two flare ribbons. In the k- diagram
of the Fourier wave power, we find a bright ridge that represents the
dispersion relation and can be well fitted with a straight line passing through
the origin. This k- ridge shows a broad frequency distribution with
indicative power at 5.5, 14.5, and 25.1 mHz. The strongest signal at 5.5 mHz
(period 181 s) temporally coincides with quasi-periodic pulsations of the
flare, suggesting a common origin. The instantaneous wave energy flux of
estimated at the coronal base is comparable
to the steady-state heating requirement of active region loops.Comment: Accepted by Astrophysical Journal Letters, 5 figures, 6 pages. Press
Release at the AAS Solar Physics Division 2011 Meeting, Las Cruces, New
Mexico, June 15, see movies at
http://www.lmsal.com/press/apjl2011_magnetosoni
Propagating waves in polar coronal holes as seen by SUMER and EIS
To study the dynamics of coronal holes and the role of waves in the
acceleration of the solar wind, spectral observations were performed over polar
coronal hole regions with the SUMER spectrometer on SoHO and the EIS
spectrometer on Hinode. Using these observations, we aim to detect the presence
of propagating waves in the corona and to study their properties. The
observations analysed here consist of SUMER spectra of the Ne VIII 770 A line
(T = 0.6 MK) and EIS slot images in the Fe XII 195 A line (T = 1.3 MK). Using
the wavelet technique, we study line radiance oscillations at different heights
from the limb in the polar coronal hole regions. We detect the presence of long
period oscillations with periods of 10 to 30 min in polar coronal holes. The
oscillations have an amplitude of a few percent in radiance and are not
detectable in line-of-sight velocity. From the time distance maps we find
evidence for propagating velocities from 75 km/s (Ne VIII) to 125 km/s (Fe
XII). These velocities are subsonic and roughly in the same ratio as the
respective sound speeds. We interpret the observed propagating oscillations in
terms of slow magneto-acoustic waves. These waves can be important for the
acceleration of the fast solar wind.Comment: 5 pages, 7 figures Accepted as Astronomy and Astrophysics Lette
'EUV Waves' are Waves: First Quadrature Observations of an EUV Wave from STEREO
The nature of CME-associated low corona propagating disturbances,
'EUV waves', has been controversial since their discovery by EIT on
\textit{SOHO}. The low cadence, single viewpoint EUV images and the lack of
simultaneous inner corona white light observations has hindered the resolution
of the debate on whether they are true waves or just projections of the
expanding CME. The operation of the twin EUV imagers and inner corona
coronagraphs aboard \textsl{STEREO} has improved the situation dramatically.
During early 2009, the \textsl{STEREO} Ahead (STA) and Behind (STB) spacecraft
observed the Sun in quadrature having an angular separation.
An EUV wave and CME erupted from active region 11012, on February 13, when the
region was exactly at the limb for STA and hence at disk center for STB. The
\textit{STEREO} observations capture the development of a CME and its
accompanying EUV wave not only with high cadence but also in quadrature. The
resulting unprecentented dataset allowed us to separate the CME structures from
the EUV wave signatures and to determine without doubt the true nature of the
wave. It is a fast-mode MHD wave after all!Comment: ApJL, 2009, submitte
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
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