94 research outputs found
Solar Coronal Structures and Stray Light in TRACE
Using the 2004 Venus transit of the Sun to constrain a semi-empirical
point-spread function for the TRACE EUV solar telescope, we have measured the
effect of stray light in that telescope. We find that 43% of 171A EUV light
that enters TRACE is scattered, either through diffraction off the entrance
filter grid or through other nonspecular effects. We carry this result forward,
via known-PSF deconvolution of TRACE images, to identify its effect on analysis
of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the
effect of visible haze in the TRACE 171A images, enhances bright features, and
reveals that the smooth background component of the corona is considerably less
bright (and hence much more rarefied) than commonly supposed. Deconvolution
reveals that some prior conlclusions about the Sun appear to have been based on
stray light in the images. In particular, the diffuse background "quiet corona"
becomes consistent with hydrostatic support of the coronal plasma; feature
contrast is greatly increased, possibly affecting derived parameters such as
the form of the coronal heating function; and essentially all existing
differential emission measure studies of small features appear to be affected
by contamination from nearby features. We speculate on further implications of
stray light for interpretation of EUV images from TRACE and similar
instruments, and advocate deconvolution as a standard tool for image analysis
with future instruments such as SDO/AIA.Comment: Accepted by APJ; v2 reformatted to single-column format for online
readabilit
Deceleration and Dispersion of Large-scale Coronal Bright Fronts
One of the most dramatic manifestations of solar activity are large-scale
coronal bright fronts (CBFs) observed in extreme ultraviolet (EUV) images of
the solar atmosphere. To date, the energetics and kinematics of CBFs remain
poorly understood, due to the low image cadence and sensitivity of previous EUV
imagers and the limited methods used to extract the features. In this paper,
the trajectory and morphology of CBFs was determined in order to investigate
the varying properties of a sample of CBFs, including their kinematics and
pulse shape, dispersion, and dissipation. We have developed a semi-automatic
intensity profiling technique to extract the morphology and accurate positions
of CBFs in 2.5-10 min cadence images from STEREO/EUVI. The technique was
applied to sequences of 171A and 195A images from STEREO/EUVI in order to
measure the wave properties of four separate CBF events. Following launch at
velocities of ~240-450kms^{-1} each of the four events studied showed
significant negative acceleration ranging from ~ -290 to -60ms^{-2}. The CBF
spatial and temporal widths were found to increase from ~50 Mm to ~200 Mm and
~100 s to ~1500 s respectively, suggesting that they are dispersive in nature.
The variation in position-angle averaged pulse-integrated intensity with
propagation shows no clear trend across the four events studied. These results
are most consistent with CBFs being dispersive magnetoacoustic waves.Comment: 15 pages, 18 figure
Are "EIT Waves" Fast-Mode MHD Waves?
We examine the nature of large-scale, coronal, propagating wave fronts (``EIT
waves'') and find they are incongruous with solutions using fast-mode MHD
plane-wave theory. Specifically, we consider the following properties:
non-dispersive single pulse manifestions, observed velocities below the local
Alfven speed, and different pulses which travel at any number of constant
velocities, rather than at the ``predicted'' fast-mode speed. We discuss the
possibility of a soliton-like explanation for these phenomena, and show how it
is consistent with the above-mentioned aspects.Comment: to be published in the Astrophysical Journa
The Post-Eruptive Evolution of a Coronal Dimming
We discuss the post-eruptive evolution of a "coronal dimming" based on
observations of the EUV corona from the Solar and Heliospheric Observatory and
the Transition Region and Coronal Explorer. This discussion highlights the
roles played by magnetoconvection-driven magnetic reconnection and the global
magnetic environment of the plasma in the "filling" and apparent motion of the
region following the eruption of a coronal mass ejection (CME). A crucial
element in our understanding of the dimming region evolution is developed by
monitoring the disappearance and reappearance of bright TRACE "moss" around the
active region giving rise to the CME. We interpret the change in the TRACE moss
as a proxy of the changing coronal magnetic field topology behind the CME
front. We infer that the change in global magnetic topology also results in a
shift of energy balance in the process responsible for the production of the
moss emission while the coronal magnetic topology evolves from closed, to open
and back to closed again because, following the eruption, the moss reforms
around the active region in almost exactly its pre-event configuration. As a
result of the moss evolution, combining our discussion with recent
spectroscopic results of an equatorial coronal hole, we suggest that the
interchangeable use of the term "transient coronal hole" to describe a coronal
dimming is more than just a simple coincidence.Comment: In Press ApJ [May 2007] - 15 pages, 5 figures, 7 movies that are
available upon request [contact author
Coronal loop seismology using multiple transverse loop oscillation harmonics
Context. TRACE observations (23/11/1998 06:35:57−06:48:43 UT) in the 171 Å bandpass of an active region are studied. Coronal loop oscillations are observed after a violent disruption of the equilibrium.
Aims. The oscillation properties are studied to give seismological estimates of physical quantities, such as the density scale height.
Methods. A loop segment is traced during the oscillation, and the resulting time series is analysed for periodicities.
Results. In the loop segment displacement, two periods are found: 435.6 ± 4.5 s and 242.7 ± 6.4 s, consistent with the periods of the fundamental and 2nd harmonic fast kink oscillation. The small uncertainties allow us to estimate the density scale height in the loop to be 109 Mm, which is about double the estimated hydrostatical value of 50 Mm.
Because a loop segment is traced, the amplitude dependence along the loop is found for each of these oscillations. The obtained spatial information is used as a seismological tool to give details about the geometry of the observed loop
High cadence observations of a global coronal wave by EUVI/STEREO
We report a large-scale coronal wave (so-called "EIT wave") observed with
high cadence by EUVI onboard STEREO in association with the GOES B9.5 flare and
double CME event on 19 May 2007. The EUVI instruments provide us with the
unprecedented opportunity to study the {\it dynamics} of flare/CME associated
coronal waves imaged in the extreme ultraviolet. The coronal wave under study
reveals deceleration, indicative of a freely propagating MHD wave.
Complementary analysis of the associated flare and erupting filament/CME hint
at wave initiation by the CME expanding flanks, which drive the wave only over
a limited distance. The associated flare is very weak and occurs too late to
account for the wave initiation.Comment: Astrophysical Journal Letters (in press
Forced oscillations of coronal loops driven by external EIT waves
Aims. We study the generation of transversal oscillations in coronal loops represented as a straight thin flux tube under the effect of an external driver modelling the global coronal EIT wave. We investigate how the generated oscillations depend on the nature of the driver, and the type of interaction between the two systems.
Methods. We consider the oscillations of a magnetic straight cylinder with fixed-ends under the influence of an external driver modelling the force due to the global EIT wave. Given the uncertainties related to the nature of EIT waves, we first approximate the driver by an oscillatory force in time and later by a shock with a finite width.
Results. Results show that for a harmonic driver the dominant period in the generated oscillation belongs to the driver. Depending on the period of driver, compared to the natural periods of the loop, a mixture of standing modes harmonics can be initiated. In the case of a non-harmonic driver (modelling a shock wave), the generated oscillations in the loop are the natural periods only. The amplitude of oscillations is determined by the position of the driver along the tube. The full diagnosis of generated oscillations is achieved using simple numerical methods
STEREO quadrature observations of the 3D structure and driver of a global coronal wave
We present the first observations of a global coronal wave ("EIT wave") from
the two Solar Terrestrial Relations Observatory (STEREO) satellites in quadra-
ture. The wave's initiation site was at the disk center in STEREO-B and
precisely on the limb in STEREO-A. These unprecedented observations from the
STEREO Extreme Ultraviolet Imaging (EUVI) instruments enable us to gain insight
into the wave's kinematics, initiation and 3D structure. The wave propagates
globally over the whole solar hemisphere visible to STEREO-B with a constant
velocity of 263+/-16 km/s. From the two STEREO observations we derive a height
of the wave in the range of 80-100 Mm. Comparison of the wave kinematics with
the early phase of the erupting CME structure indicates that the wave is
initiated by the CME lateral expansion, and then propagates freely with a
velocity close to the fast magnetosonic speed in the quiet solar corona.Comment: ApJ Letters (accepted
Forced oscillations of coronal loops driven by external EIT waves
Aims. We study the generation of transversal oscillations in coronal loops represented as a straight thin flux tube under the effect of an external driver modelling the global coronal EIT wave. We investigate how the generated oscillations depend on the nature of the driver, and the type of interaction between the two systems.
Methods. We consider the oscillations of a magnetic straight cylinder with fixed-ends under the influence of an external driver modelling the force due to the global EIT wave. Given the uncertainties related to the nature of EIT waves, we first approximate the driver by an oscillatory force in time and later by a shock with a finite width.
Results. Results show that for a harmonic driver the dominant period in the generated oscillation belongs to the driver. Depending on the period of driver, compared to the natural periods of the loop, a mixture of standing modes harmonics can be initiated. In the case of a non-harmonic driver (modelling a shock wave), the generated oscillations in the loop are the natural periods only. The amplitude of oscillations is determined by the position of the driver along the tube. The full diagnosis of generated oscillations is achieved using simple numerical methods
Quadrature Observations of Wave and Non-Wave Components and Their Decoupling in an Extreme-Ultraviolet Wave Event
We report quadrature observations of an extreme-ultraviolet (EUV) wave event
on 2011 January 27 obtained by the Extreme Ultraviolet Imager (EUVI) onboard
\emph{Solar Terrestrial Relations Observatory} (\emph{STEREO}), and the
Atmospheric Imaging Assembly (AIA) onboard the \emph{Solar Dynamics
Observatory} (\emph{SDO}). Two components are revealed in the EUV wave event. A
primary front is launched with an initial speed of 440 km s. It
appears significant emission enhancement in the hotter channel but deep
emission reduction in the cooler channel. When the primary front encounters a
large coronal loop system and slows down, a secondary much fainter front
emanates from the primary front with a relatively higher starting speed of
550 km s. Afterwards the two fronts propagate independently with
increasing separation. The primary front finally stops at a magnetic
separatrix, while the secondary front travels farther before it fades out. In
addition, upon the arrival of the secondary front, transverse oscillations of a
prominence are triggered. We suggest that the two components are of different
natures. The primary front belongs to a non-wave coronal mass ejection (CME)
component, which can be reasonably explained with the field-line stretching
model. The multi-temperature behavior may be caused by considerable heating due
to the nonlinear adiabatic compression on the CME frontal loop. For the
secondary front, most probably it is a linear fast-mode magnetohydrodynamic
(MHD) wave that propagates through a medium of the typical coronal temperature.
X-ray and radio data provide us with complementary evidence in support of the
above scenario.Comment: 21 pages, 8 figures, accepted for publication in Ap
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