175 research outputs found
Automatic detection of limb prominences in 304 A EUV images
A new algorithm for automatic detection of prominences on the solar limb in 304 A EUV images is presented, and results of its application to SOHO/EIT data discussed. The detection is based on the method of moments combined with a
classifier analysis aimed at discriminating between limb prominences, active regions, and the quiet corona. This classifier analysis is based on a Support Vector Machine (SVM). Using a set of 12 moments of the radial intensity profiles, the algorithm performs well in discriminating between the above three categories of limb structures, with a misclassification rate of 7%. Pixels detected as belonging to a prominence are then used as starting point to reconstruct the whole prominence by morphological image processing techniques. It is planned that a catalogue of limb prominences identified in SOHO and STEREO data using this method will be made publicly available to the scientific community
Micro-Sigmoids as Progenitors of Coronal Jets - Is Eruptive Activity Self-Similarly Multi-Scaled?
Observations from the X-ray telescope (XRT) on Hinode are used to study the
nature of X-ray bright points, sources of coronal jets. Several jet events in
the coronal holes are found to erupt from small-scale, S-shaped bright regions.
This finding suggests that coronal micro-sigmoids may well be progenitors of
coronal jets. Moreover, the presence of these structures may explain numerous
observed characteristics of jets such as helical structures, apparent
transverse motions, and shapes. In analogy to large-scale sigmoids giving rise
to coronal mass ejections (CMEs), a promising future task would perhaps be to
investigate whether solar eruptive activity, from coronal jets to CMEs, is
self-similar in terms of properties and instability mechanisms.Comment: 8 pages, 5 figures, 1 tabl
On active region loops: Hinode/EIS observations
Coronal loops are fundamental building blocks of the solar active regions and
the corona. Therefore, a clear understanding of the physics of coronal loops
will help us understand the physics of active region heating in particular and
coronal heating in general. This requires a precise measurement of physical
quantities such as electron densities and filling factors, temperatures, and
flows in coronal loops. In this paper we have carried out an investigation of a
spatially well resolved coronal loop using the EIS onboard Hinode to measure
the above mentioned physical quantities. Based on this study we find that a
nano-flare model could explain most of the observed characteristics of this
loop.Comment: 27 pages, 7 figures, Accepted in Ap
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
One of the major discoveries of the Extreme ultraviolet Imaging Telescope
(EIT) on SOHO were intensity enhancements propagating over a large fraction of
the solar surface. The physical origin(s) of the so-called `EIT' waves is still
strongly debated. They are considered to be either wave (primarily fast-mode
MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in
understanding the nature of EUV waves lies with the limitations of the EIT
observations which have been used almost exclusively for their study. Their
limitations are largely overcome by the SECCHI/EUVI observations on-board the
STEREO mission. The EUVI telescopes provide high cadence, simultaneous
multi-temperature coverage, and two well-separated viewpoints. We present here
the first detailed analysis of an EUV wave observed by the EUVI disk imagers on
December 07, 2007 when the STEREO spacecraft separation was .
Both a small flare and a CME were associated with the wave cadence, and single
temperature and viewpoint coverage. These limitations are largely overcome by
the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes
provide high cadence, simultaneous multi-temperature coverage, and two
well-separated viewpoints. Our findings give significant support for a
fast-mode interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.Comment: Solar Physics, 2009, Special STEREO Issue, in pres
Analysis of the coronal green line profiles: evidence of excess blueshifts
The coronal green line (Fe XIV 5303 A) profiles were obtained from
Fabry-Perot interferometric observations of the solar corona during the total
solar eclipse of 21 June 2001 from Lusaka, Zambia. The instrumental width is
about 0.2 A and the spectral resolution is about 26000. About 300 line profiles
were obtained within a radial range of 1.0-1.5 R\odot and position angle
coverage of about 240\circ. The line profiles were fitted with single Gaussian
and their intensity, Doppler velocity, and line width have been obtained. Also
obtained are the centroids of the line profiles which give a measure of line
asymmetry. The histograms of Doppler velocity show excess blueshifts while the
centroids reveal a pre-dominant blue wing in the line profiles. It has been
found that the centroids and the Doppler velocities are highly correlated. This
points to the presence of multiple components in the line profiles with an
excess of blueshifted components. We have then obtained the(Blue-Red) wing
intensity which clearly reveals the second component, majority of which are
blueshifted ones. This confirms that the coronal green line profiles often
contain multicomponents with excess blueshifts which also depend on the solar
activity. The magnitude of the Doppler velocity of the secondary component is
in the range 20-40 km s-1 and they show an increase towards poles. The possible
explanations of the multicomponents could be the type II spicules which were
recently found to have important to the coronal heating or the nascent solar
wind flow, but the cause of the blue asymmetry in the coronal lines above the
limb remains unclear
Coronal Shock Waves, EUV Waves, and Their Relation to CMEs. III. Shock-Associated CME/EUV Wave in an Event with a Two-Component EUV Transient
On 17 January 2010, STEREO-B observed in extreme ultraviolet (EUV) and white
light a large-scale dome-shaped expanding coronal transient with perfectly
connected off-limb and on-disk signatures. Veronig et al. (2010, ApJL 716, 57)
concluded that the dome was formed by a weak shock wave. We have revealed two
EUV components, one of which corresponded to this transient. All of its
properties found from EUV, white light, and a metric type II burst match
expectations for a freely expanding coronal shock wave including correspondence
to the fast-mode speed distribution, while the transient sweeping over the
solar surface had a speed typical of EUV waves. The shock wave was presumably
excited by an abrupt filament eruption. Both a weak shock approximation and a
power-law fit match kinematics of the transient near the Sun. Moreover, the
power-law fit matches expansion of the CME leading edge up to 24 solar radii.
The second, quasi-stationary EUV component near the dimming was presumably
associated with a stretched CME structure; no indications of opening magnetic
fields have been detected far from the eruption region.Comment: 18 pages, 10 figures. Solar Physics, published online. The final
publication is available at http://www.springerlink.co
On the Nature and Genesis of EUV Waves: A Synthesis of Observations from SOHO, STEREO, SDO, and Hinode
A major, albeit serendipitous, discovery of the SOlar and Heliospheric
Observatory mission was the observation by the Extreme Ultraviolet Telescope
(EIT) of large-scale Extreme Ultraviolet (EUV) intensity fronts propagating
over a significant fraction of the Sun's surface. These so-called EIT or EUV
waves are associated with eruptive phenomena and have been studied intensely.
However, their wave nature has been challenged by non-wave (or pseudo-wave)
interpretations and the subject remains under debate. A string of recent solar
missions has provided a wealth of detailed EUV observations of these waves
bringing us closer to resolving their nature. With this review, we gather the
current state-of-art knowledge in the field and synthesize it into a picture of
an EUV wave driven by the lateral expansion of the CME. This picture can
account for both wave and pseudo-wave interpretations of the observations, thus
resolving the controversy over the nature of EUV waves to a large degree but
not completely. We close with a discussion of several remaining open questions
in the field of EUV waves research.Comment: Solar Physics, Special Issue "The Sun in 360",2012, accepted for
publicatio
Coronal Shock Waves, EUV waves, and their Relation to CMEs. II. Modeling MHD Shock Wave Propagation Along the Solar Surface, Using Nonlinear Geometrical Acoustics
We model the propagation of a coronal shock wave, using nonlinear geometrical
acoustics. The method is based on the Wentzel-Kramers-Brillouin (WKB) approach
and takes into account the main properties of nonlinear waves: i) dependence of
the wave front velocity on the wave amplitude, ii) nonlinear dissipation of the
wave energy, and iii) progressive increase in the duration of solitary shock
waves. We address the method in detail and present results of the modeling of
the propagation of shock-associated extreme-ultraviolet (EUV) waves as well as
Moreton waves along the solar surface in the simplest solar corona model. The
calculations reveal deceleration and lengthening of the waves. In contrast,
waves considered in the linear approximation keep their length unchanged and
slightly accelerate.Comment: 15 pages, 7 figures, accepted for publication in Solar Physic
Characteristics of EUV coronal jets observed with STEREO/SECCHI
In this paper we present the first comprehensive statistical study of EUV
coronal jets observed with the SECCHI imaging suites of the two STEREO
spacecraft. A catalogue of 79 polar jets is presented, identified from
simultaneous EUV and white-light coronagraph observations, taken during the
time period March 2007 to April 2008. The appearances of the coronal jets were
always correlated with underlying small-scale chromospheric bright points. A
basic characterisation of the morphology and identification of the presence of
helical structure were established with respect to recently proposed models for
their origin and temporal evolution. A classification of the events with
respect to previous jet studies shows that amongst the 79 events there were 37
Eiffel tower-type jet events commonly interpreted as a small-scale (about 35
arcsec) magnetic bipole reconnecting with the ambient unipolar open coronal
magnetic fields at its looptops, and 12 lambda-type jet events commonly
interpreted as reconnection with the ambient field happening at the bipoles
footpoints. Five events were termed micro-CME type jet events because they
resembled the classical coronal mass ejections (CMEs) but on much smaller
scales. A few jets are also found in equatorial coronal holes. The typical
lifetimes in the SECCHI/EUVI (Extreme UltraViolet Imager) field of view between
1.0 to 1.7 solar radius and in SECCHI/COR1 field of view between 1.4 to 4 solar
radius are obtained, and the derived speed are roughly estimated. In summary,
the observations support the assumption of continuous small-scale reconnection
as an intrinsic feature of the solar corona, with its role for the heating of
the corona, particle acceleration, structuring and acceleration of the solar
wind remaining to be explored in more details in further studies.Comment: 20 pages, 7 tables of figures, 2 tables of plots, an appendix with
list event
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