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Author name code: verwichte
ADS astronomy entries on 2022-09-14
author:"Verwichte, Erwin"
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Title: An automated method to detect, track and characterise bright
points in multi-instrument solar imagery.
Authors: Ramsey, Brad; Verwichte, Erwin; Morgan, Huw
2022cosp...44.2515R Altcode:
The photosphere, transition region, and corona are host to a plethora of
small, bright, transient phenomena, collectively known as bright points,
or brightenings. Given their ubiquity and frequency, bright points
are likely an important signature of plasma heating and/or transport
mechanisms. We present a novel and efficient wavelet-based method
that automatically detects and tracks the evolution of a large set of
bright points in solar imagery, from SDO, IRIS and SST. Co-locating
bright points across simultaneous multi-instrument observations
of the photosphere, transition region, and corona enables a more
comprehensive study of their characteristics. A statistical analysis of
their characteristics such as occurrence rates, lifetimes and sizes are
given, along with a more detailed study of individual events. Through
the study of a statistically significant set of bright points we
attempt to place constraints on the underlying physical mechanisms.
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Title: HiRISE - High-Resolution Imaging and Spectroscopy Explorer
- Ultrahigh resolution, interferometric and external occulting
coronagraphic science
Authors: Erdélyi, Robertus; Damé, Luc; Fludra, Andrzej; Mathioudakis,
Mihalis; Amari, T.; Belucz, B.; Berrilli, F.; Bogachev, S.; Bolsée,
D.; Bothmer, V.; Brun, S.; Dewitte, S.; de Wit, T. Dudok; Faurobert,
M.; Gizon, L.; Gyenge, N.; Korsós, M. B.; Labrosse, N.; Matthews,
S.; Meftah, M.; Morgan, H.; Pallé, P.; Rochus, P.; Rozanov, E.;
Schmieder, B.; Tsinganos, K.; Verwichte, E.; Zharkov, S.; Zuccarello,
F.; Wimmer-Schweingruber, R.
2022ExA...tmp...21E Altcode:
Recent solar physics missions have shown the definite role of waves and
magnetic fields deep in the inner corona, at the chromosphere-corona
interface, where dramatic and physically dominant changes occur. HiRISE
(High Resolution Imaging and Spectroscopy Explorer), the ambitious new
generation ultra-high resolution, interferometric, and coronagraphic,
solar physics mission, proposed in response to the ESA Voyage 2050
Call, would address these issues and provide the best-ever and most
complete solar observatory, capable of ultra-high spatial, spectral,
and temporal resolution observations of the solar atmosphere, from the
photosphere to the corona, and of new insights of the solar interior
from the core to the photosphere. HiRISE, at the L1 Lagrangian
point, would provide meter class FUV imaging and spectro-imaging,
EUV and XUV imaging and spectroscopy, magnetic fields measurements,
and ambitious and comprehensive coronagraphy by a remote external
occulter (two satellites formation flying 375 m apart, with a
coronagraph on a chaser satellite). This major and state-of-the-art
payload would allow us to characterize temperatures, densities, and
velocities in the solar upper chromosphere, transition zone, and inner
corona with, in particular, 2D very high resolution multi-spectral
imaging-spectroscopy, and, direct coronal magnetic field measurement,
thus providing a unique set of tools to understand the structure and
onset of coronal heating. HiRISE's objectives are natural complements
to the Parker Solar Probe and Solar Orbiter-type missions. We present
the science case for HiRISE which will address: i) the fine structure
of the chromosphere-corona interface by 2D spectroscopy in FUV at
very high resolution; ii) coronal heating roots in the inner corona by
ambitious externally-occulted coronagraphy; iii) resolved and global
helioseismology thanks to continuity and stability of observing at the
L1 Lagrange point; and iv) solar variability and space climate with,
in addition, a global comprehensive view of UV variability.
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Title: The Nature of High-frequency Oscillations Associated with
Short-lived Spicule-type Events
Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Doyle,
J. G.
2021arXiv211214486S Altcode:
We investigate high resolution spectroscopic and imaging observations
from the CRisp Imaging SpectroPolarimeter (CRISP) instrument to
study the dynamics of chromospheric spicule type events. It is
widely accepted that chromospheric fine structures are waveguides
for several types of magnetohydrodynamic (MHD) oscillations, which
can transport energy from the lower to upper layers of the Sun. We
provide a statistical study of 30 high frequency waves associated
with spicule type events. These high frequency oscillations have two
components of transverse motions: the plane of sky (POS) motion and
the line of sight (LOS) motion. We focus on single isolated spicules
and track the POS using time distance analysis and in the LOS direction
using Doppler information. We use moment analysis to find the relation
between the two motions. The composition of these two motions suggests
that the wave has a helical structure. The oscillations do not have
phase differences between points along the structure. This may be the
result of the oscillation being a standing mode, or that propagation
is mostly in the perpendicular direction. There is evidence of fast
magnetoacoustic wave fronts propagating across these structures. To
conclude, we hypothesize that the compression and rarefaction of
passing magnetoacoustic waves may influence the appearance of spicule
type events, not only by contributing to moving them in and out of
the wing of the spectral line but also through the creation of density
enhancements and an increase in opacity in the Halpha line.
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Title: The Nature of High-frequency Oscillations Associated with
Short-lived Spicule-type Events
Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Doyle,
J. G.
2021ApJ...921...30S Altcode:
We investigate high-resolution spectroscopic and imaging observations
from the CRisp Imaging SpectroPolarimeter (CRISP) instrument to
study the dynamics of chromospheric spicule-type events. It is
widely accepted that chromospheric fine structures are waveguides
for several types of magnetohydrodynamic (MHD) oscillations, which
can transport energy from the lower to upper layers of the Sun. We
provide a statistical study of 30 high-frequency waves associated
with spicule-type events. These high-frequency oscillations have two
components of transverse motions: the plane-of-sky (POS) motion and
the line-of-sight (LOS) motion. We focus on single isolated spicules
and track the POS using time-distance analysis and in the LOS direction
using Doppler information. We use moment analysis to find the relation
between the two motions. The composition of these two motions suggests
that the wave has a helical structure. The oscillations do not have
phase differences between points along the structure. This may be the
result of the oscillation being a standing mode, or that propagation
is mostly in the perpendicular direction. There is evidence of fast
magnetoacoustic wave fronts propagating across these structures. To
conclude, we hypothesize that the compression and rarefaction of passing
magnetoacoustic waves may influence the appearance of spicule-type
events, not only by contributing to moving them in and out of the
wing of the spectral line but also through the creation of density
enhancements and an increase in opacity in the Hα line.
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Title: Acoustic response to transverse oscillations in a solar
coronal loop
Authors: White, S. J.; Verwichte, E.
2021A&A...654A..33W Altcode:
Context. Magnetohydrodynamic (MHD) waves play an important role in
the dynamics and heating of the solar corona. Their investigation
also reveals information about the local conditions. Transverse
(Alfvénic) oscillations of loops commonly occur in response to solar
eruptions. It has been shown that these oscillations elicit an acoustic
response through wave coupling at the footpoint and the pondermotive
force. <BR /> Aims: We extend the modelling of wave coupling between
a transverse loop oscillation and slow magnetoacoustic waves through
line-tied footpoint boundary conditions by considering the effect
of transverse loop structuring and non-linearity. <BR /> Methods: We
combine analytical wave modelling with fully non-linear MHD simulations
to study the wave field of propagating slow waves in a two-dimensional
slab loop (arcade) model. <BR /> Results: We demonstrate that transverse
loop oscillations generate propagating slow waves from the footpoints
with the same periodicity but shorter wavelength determined by the
local sound speed. The degree of wave coupling is proportional to the
square root of the plasma-β. The slow wave field is anti-symmetric
in the direction of transverse wave polarisation. We show through
synthetic diagnostics that this has important consequences for their
observability in terms of the orientation of the loop with respect to
the observer. We also show that for the interpretation of intensity
oscillations associated with typical loop oscillations the ponderomotive
response also needs to be taken into account. The modelling presented
here allows for the successful identification of the slow waves and
pondermotive response in a previous observational study.
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Title: Multiwavelength Imaging and Spectral Analysis of Jet-like
Phenomena in a Solar Active Region Using IRIS and AIA
Authors: Dafydd Humphries, Llŷr; Verwichte, Erwin; Kuridze, David;
Morgan, Huw
2020arXiv201004042D Altcode:
High-resolution observations of dynamic phenomena give insight into
properties and processes that govern the low solar atmosphere. We
present the analysis of jet-like phenomena emanating from a penumbral
foot-point in active region (AR) 12192 using imaging and spectral
observations from the Interface Region Imaging Spectrograph (IRIS)
and the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory. These jets are associated with line-of-sight
(LoS) Doppler speeds of $\pm$ 10-22 km s$^{-1}$ and bright fronts
which seem to move across the Plane-of-Sky (PoS) at speeds of 23-130
km s$^{-1}$. Such speeds are considerably higher than the expected
sound speed in the chromosphere. The jets have signatures which are
visible both in the cool and hot channels of IRIS and AIA. Each jet
lasts on average 15 minutes and occur 5-7 times over a period of 2
hours. Possible mechanisms to explain this phenomenon are suggested,
the most likely of which involve p-mode or Alfv\' en wave shock trains
impinging on the transition region (TR) and corona as a result of
steepening photospheric wavefronts or gravity waves.
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Title: Unveiling the magnetic nature of chromospheric vortices
Authors: Murabito, Mariarita; Shetye, Juie; Stangalini, Marco;
Verwichte, Erwin; Arber, Tony; Ermolli, Ilaria; Giorgi, Fabrizio;
Goffrey, Tom
2020A&A...639A..59M Altcode: 2020arXiv200613776M
Context. Vortex structures in the Sun's chromosphere are believed to
channel energy between different layers of the solar atmosphere. <BR
/> Aims: We investigate the nature and dynamics of two small-scale
quiet-Sun rotating structures in the chromosphere. <BR /> Methods:
We analysed two chromospheric structures that show clear rotational
patterns in spectropolarimetric observations taken with the
Interferometric Bidimensional Spectrometer at the Ca II 8542 Å
line. <BR /> Results: We present the detection of spectropolarimetric
signals that manifest the magnetic nature of rotating structures in
the chromosphere. Our observations show two long-lived structures
of plasma that each rotate clockwise inside a 10 arcsec<SUP>2</SUP>
quiet-Sun region. Their circular polarisation signals are five to
ten times above the noise level. Line-of-sight Doppler velocity and
horizontal velocity maps from the observations reveal clear plasma flows
at and around the two structures. A magnetohydrodynamics simulation
shows these two structures are plausibly magnetically connected. Wave
analysis suggests that the observed rotational vortex pattern could be
due to a combination of slow actual rotation and a faster azimuthal
phase speed pattern of a magnetoacoustic mode. <BR /> Conclusions:
Our results imply that the vortex structures observed in the Sun's
chromosphere are magnetic in nature and that they can be connected
locally through the chromosphere <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038360/olm">https://www.aanda.org</A>
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Title: Multiwavelength Imaging and Spectral Analysis of Jet-like
Phenomena in a Solar Active Region Using IRIS and AIA
Authors: Humphries, Llŷr Dafydd; Verwichte, Erwin; Kuridze, David;
Morgan, Huw
2020ApJ...898...17H Altcode:
High-resolution observations of dynamic phenomena give insights into
the properties and processes that govern the low solar atmosphere. We
present an analysis of jet-like phenomena emanating from a penumbral
footpoint in active region (AR) 12192 using imaging and spectral
observations from the Interface Region Imaging Spectrograph (IRIS)
and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory. These jets are associated with line-of-sight Doppler
speeds of ±10-22 km s<SUP>-1</SUP> and bright fronts that seem to move
across the plane-of-sky at speeds of 23-130 km s<SUP>-1</SUP>. Such
speeds are considerably higher than the expected sound speed in the
chromosphere. The jets have signatures that are visible both in the
cool and hot channels of IRIS and AIA. Each jet lasts on average
15 minutes and occurs 5-7 times over a period of 2 hr. Possible
mechanisms to explain this phenomenon are suggested, the most likely
of which involve p-mode or Alfvén wave shock trains impinging on the
transition region and corona as a result of steepening photospheric
wavefronts or gravity waves.
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Title: First direct observation of a torsional Alfvén oscillation
at coronal heights
Authors: Kohutova, P.; Verwichte, E.; Froment, C.
2020A&A...633L...6K Altcode:
Context. Torsional Alfvén waves are promising candidates for
the transport of energy across different layers of the solar
atmosphere. They have been predicted theoretically for decades. Previous
detections of Alfvén waves so far have however mostly relied
on indirect signatures. <BR /> Aims: We present the first direct
observational evidence of a fully resolved torsional Alfvén oscillation
of a large-scale structure occurring at coronal heights. <BR /> Methods:
We analysed IRIS imaging and spectral observation of a surge resulting
from magnetic reconnection between active region prominence threads and
surrounding magnetic field lines. <BR /> Results: The IRIS spectral
data provide clear evidence of an oscillation in the line-of-sight
velocity with a 180° phase difference between the oscillation
signatures at opposite edges of the surge flux tube. This together
with an alternating tilt in the Si IV and Mg II k spectra across the
flux tube and the trajectories traced by the individual threads of
the surge material provide clear evidence of torsional oscillation of
the flux tube. <BR /> Conclusions: Our observation shows that magnetic
reconnection leads to the generation of large-scale torsional Alfvén
waves. <P />Movies attached to Figs. 1 and 2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201937144/olm">https://www.aanda.org</A>
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Title: First direct observation of a torsional Alfvén oscillation
at coronal heights
Authors: Kohutova, P.; Verwichte, E.; Froment, C.
2019arXiv191203954K Altcode:
Torsional Alfvén waves are promising candidates for transport of
energy across different layers of the solar atmosphere and have been
theoretically predicted for decades. Previous detections of Alfvén
waves so far have however mostly relied on indirect signatures. We
present a first direct observational evidence of a fully resolved
torsional Alfvén oscillation of a large-scale structure occurring at
coronal heights. We analyse IRIS imaging and spectral observation of
a surge resulting from magnetic reconnection between active region
prominence threads and surrounding magnetic fieldlines. The IRIS
spectral data provides clear evidence of an oscillation in the
line-of-sight velocity with a 180° phase difference between the
oscillation signatures at opposite edges of the surge flux tube. This
together with an alternating tilt in the Si IV and Mg II k spectra
across the flux tube and the trajectories traced by the individual
threads of the surge material provides clear evidence of torsional
oscillation of the flux tube. Our observation shows that magnetic
reconnection leads to the generation of large-scale torsional Alfvén
waves.
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Title: Formation of coronal rain triggered by impulsive heating
associated with magnetic reconnection
Authors: Kohutova, P.; Verwichte, E.; Froment, C.
2019A&A...630A.123K Altcode: 2019arXiv191007746K
Context. Coronal rain consists of cool plasma condensations
formed in coronal loops as a result of thermal instability. The
standard models of coronal rain formation assume that the heating
is quasi-steady and localised at the coronal loop footpoints. <BR
/> Aims: We present an observation of magnetic reconnection in the
corona and the associated impulsive heating triggering formation
of coronal rain condensations. <BR /> Methods: We analyse combined
SDO/AIA and IRIS observations of a coronal rain event following a
reconnection between threads of a low-lying prominence flux rope and
surrounding coronal field lines. <BR /> Results: The reconnection
of the twisted flux rope and open field lines leads to a release of
magnetic twist. Evolution of the emission of one of the coronal loops
involved in the reconnection process in different AIA bandpasses
suggests that the loop becomes thermally unstable and is subject to
the formation of coronal rain condensations following the reconnection
and that the associated heating is localised in the upper part of the
loop leg. <BR /> Conclusions: In addition to the standard models of
thermally unstable coronal loops with heating localised exclusively
in the footpoints, thermal instability and subsequent formation of
condensations can be triggered by the impulsive heating associated
with magnetic reconnection occurring anywhere along a magnetic
field line. <P />The movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936253/olm">https://www.aanda.org</A>
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Title: Multiwavelength High-resolution Observations of Chromospheric
Swirls in the Quiet Sun
Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Judge,
Philip G.; Doyle, J. G.; Arber, Tony; Scullion, Eamon; Wedemeyer, Sven
2019ApJ...881...83S Altcode:
We report observations of small-scale swirls seen in the solar
chromosphere. They are typically 2 Mm in diameter and last around
10 minutes. Using spectropolarimetric observations obtained by the
CRisp Imaging Spectro-Polarimeter at the Swedish 1 m Solar Telescope,
we identify and study a set of swirls in chromospheric Ca II 8542 Å
and Hα lines as well as in the photospheric Fe I line. We have three
main areas of focus. First, we compare the appearance, morphology,
dynamics, and associated plasma parameters between the Ca II and Hα
channels. Rotation and expansion of the chromospheric swirl pattern
are explored using polar plots. Second, we explore the connection to
underlying photospheric magnetic concentration (MC) dynamics. MCs are
tracked using the SWAMIS tracking code. The swirl center and MC remain
cospatial and share similar periods of rotation. Third, we elucidate
the role swirls play in modifying chromospheric acoustic oscillations
and found a temporary reduction in wave period during swirls. We use
cross-correlation wavelets to examine the change in period and phase
relations between different wavelengths. The physical picture that
emerges is that a swirl is a flux tube that extends above an MC in a
downdraft region in an intergranular lane. The rotational motion of
the MC matches the chromospheric signatures. We could not determine
whether a swirl is a gradual response to the photospheric motion or
an actual propagating Alfvénic wave.
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Title: Simulating the Dynamics of Coronal Plasma Condensations
Authors: Kohutova, Petra; Verwichte, Erwin
2018IAUS..335...23K Altcode:
We present numerical MHD simulations of the dynamics of cool plasma
condensations in a coronal loop. We address 2 mechanisms for how coronal
rain leads to the excitation of coronal loop oscillations. We find that
the combined effect of pressure gradients in the coronal loop plasma
and magnetic tension force resulting from changes in magnetic field
geometry explains observed sub-ballistic motion of coronal rain and
longitudinal oscillations of the individual condensations. We also
find that the condensations can excite sustained, small amplitude,
vertically polarised transverse loop oscillations.
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Title: Excitation and Evolution of Transverse Loop Oscillations by
Coronal Rain
Authors: Verwichte, Erwin; Kohutova, Petra; Antolin, Patrick; Rowlands,
George; Neukirch, Thomas
2018IAUS..335...36V Altcode:
We present evidence of the excitation of vertically polarised transverse
loop oscillations triggered by a catastrophic cooling of a coronal
loop with two thirds of the loop mass comprising of cool rain mass. The
nature and excitation of oscillations associated with coronal rain is
not well understood. We consider observations of coronal rain using data
from IRIS, SOT/Hinode and AIA/SDO in a bid to elucidate the excitation
mechanism and evolution of wave characteristics. We apply an analytical
model of wave-rain interaction, that predicts the inertial excitation
amplitude of transverse loop oscillations as a function of the rain
mass, to deduce the relative rain mass. It is consistent with the
evolution of the oscillation period showing the loop losing a third
of its mass due to falling coronal rain in a 10-15 minute time period.
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Title: Excitation and evolution of transverse loop oscillations by
coronal rain
Authors: Verwichte, Erwin; Kohutova, Petra
2018cosp...42E3542V Altcode:
Coronal rain is composed of cool dense blobs that form in solar coronal
loops and are a manifestation of catastrophic cooling linked to thermal
instability. The nature and excitation of oscillations associated
with coronal rain is not well understood. We present an overview of
observations of coronal rain oscillations seen in data from IRIS,
SOT/Hinode and AIA/SDO as well as ground-based observatories. We
follow this with detailed analytical and numerical MHD modelling to
elucidate the excitation mechanism and evolution of wave characteristics
in rain. We will discuss the possibility for rain blobs to excite
transverse oscillations due to gravitational or inertial forces as
well as acoustic oscillations due to a piston effect. In particular,
we shall highlight the first evidence of the excitation of vertically
polarised transverse loop oscillations triggered by a catastrophic
cooling at the loop top and consistent with inertial excitation
due to two thirds of the loop mass comprising of cool rain mass. The
seismological capability of the oscillation is exploited to deduce the
relative rain mass. It is shown to be consistent with the evolution
of the oscillation period showing the loop losing a third of its mass
due to falling coronal rain in a 10-15 minute time period.
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Title: Modeling wave energy transport using a 3D MHD simulation
of AR12683
Authors: Tarr, Lucas; Linton, Mark; Reardon, Kevin; Shetye, Juie;
Verwichte, Erwin
2018tess.conf20544T Altcode:
We use the 3D Magnetohydrodynamic code LARE to model wave transport
within NOAA Active Region 12683. The simulation extends vertically
from the photosphere to the low corona and includes gravitational
stratification and a steep transition region to a high temperature
corona. AR12683 consists of a leading negative polarity sunspot
followed by an extended region of plage, and had little X-ray
activity during the time period we study. Given that, we use a
potential field, extrapolated from HMI data, for the initial 3D
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</use> </g> </svg> observations taken with the Goode
Solar Telescope at Big Bear Solar Observatory revealed interesting
propagating disturbances near the sunspot, which will be reported
on elsewhere. In this work, we focus on the general properties of
wave propagation through the region by introducing compressive wave
packets at the lower boundary, mimicking convective forcing. We then
use the techniques developed in Tarr, Linton, & Leake, ApJ 2017,
to track wave energy, mode conversion, and shock formation as the wave
packets propagate through the simulation. By varying the wave packet
injection location, we can isolate photospheric source locations for
disturbances seen higher up in the chromosphere and low corona, and
understand the important role of mode conversion in the transfer of
wave energy throughout the realistically structured atmosphere.
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Title: Excitation of vertical coronal loop oscillations by impulsively
driven flows
Authors: Kohutova, P.; Verwichte, E.
2018A&A...613L...3K Altcode:
Context. Flows of plasma along a coronal loop caused by the pressure
difference between loop footpoints are common in the solar corona. <BR
/> Aims: We aim to investigate the possibility of excitation of loop
oscillations by an impulsively driven flow triggered by an enhanced
pressure in one of the loop footpoints. <BR /> Methods: We carry out
2.5D magnetohydrodynamic (MHD) simulations of a coronal loop with an
impulsively driven flow and investigate the properties and evolution of
the resulting oscillatory motion of the loop. <BR /> Results: The action
of the centrifugal force associated with plasma moving at high speeds
along the curved axis of the loop is found to excite the fundamental
harmonic of a vertically polarised kink mode. We analyse the dependence
of the resulting oscillations on the speed and kinetic energy of the
flow. <BR /> Conclusions: We find that flows with realistic speeds of
less than 100 km s<SUP>-1</SUP> are sufficient to excite oscillations
with observable amplitudes. We therefore propose plasma flows as a
possible excitation mechanism for observed transverse loop oscillations.
---------------------------------------------------------
Title: Excitation of vertical coronal loop oscillations by plasma
condensations
Authors: Kohutova, P.; Verwichte, E.
2017A&A...606A.120K Altcode:
Context. Coronal rain composed of downfalling cool plasma condensations
occurs in thermally unstable loops as a consequence of catastrophic
cooling. Such loops contain significant quantities of dense plasma out
of hydrostatic equilibrium. Transverse oscillations traced by coronal
rain blobs are often observed in rainy loops. <BR /> Aims: We aim to
investigate the possibility of excitation of loop oscillations by the
presence of condensation plasma. <BR /> Methods: We carried out 2.5D
magnetohydrodynamic simulations of a coronal loop containing a cool
and a dense condensation region near the loop apex and investigated
the properties and evolution of the resulting oscillatory motion of the
loop. <BR /> Results: The presence of dense condensation region at the
apex of the coronal loop is found to excite fundamental harmonic of
a vertically polarised kink mode. As the condensations fall towards
the loop footpoints under the influence of gravity, the fundamental
mode period decreases as a result of the change in distribution of
mass along the loop. <BR /> Conclusions: We propose coronal rain as
a possible excitation mechanism for transverse loop oscillations.
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Title: Dynamics of plasma condensations in a gravitationally
stratified coronal loop
Authors: Kohutova, P.; Verwichte, E.
2017A&A...602A..23K Altcode:
Context. Coronal rain composed of cool plasma condensations falling from
coronal heights is a phenomenon occurring in footpoint-heated coronal
loops as a result of thermal instability. High-resolution coronal rain
observations suggest that condensations move with less than free-fall
speed and can sometimes undergo longitudinal oscillations. <BR /> Aims:
We investigate the evolution and dynamics of plasma condensations in
a gravitationally stratified coronal loop. <BR /> Methods: We carried
out 2.5 dimensional magnetohydrodynamic simulations of a cool plasma
condensation in a gravitationally stratified coronal loop and analysed
its evolution, kinematics, and the evolution of the forces acting on
the condensation. We further propose a one-dimensional analytical model
of the condensation dynamics. <BR /> Results: The motion of plasma
condensations is found to be strongly affected by the pressure of
the coronal loop plasma. Maximum downward velocities are in agreement
with recent coronal rain observations. A high coronal magnetic field
or low condensation mass can lead to damped oscillatory motion of
the condensations that are caused by the pressure gradient force and
magnetic tension force that results from bending of the magnetic field
in the lower part of the coronal loop. Period and damping scaling
time of the oscillatory motion seen in the simulations are consistent
with values predicted by the model. <BR /> Conclusions: The combined
effect of pressure gradients in the coronal loop plasma and magnetic
tension force that results from changes in magnetic field geometry can
explain observed sub-ballistic motion and longitudinal oscillations
of coronal rain.
---------------------------------------------------------
Title: Observational Signatures of a Kink-unstable Coronal Flux Rope
Using Hinode/EIS
Authors: Snow, B.; Botha, G. J. J.; Régnier, S.; Morton, R. J.;
Verwichte, E.; Young, P. R.
2017ApJ...842...16S Altcode: 2017arXiv170505114S
The signatures of energy release and energy transport for a
kink-unstable coronal flux rope are investigated via forward
modeling. Synthetic intensity and Doppler maps are generated from
a 3D numerical simulation. The CHIANTI database is used to compute
intensities for three Hinode/EIS emission lines that cover the
thermal range of the loop. The intensities and Doppler velocities at
simulation-resolution are spatially degraded to the Hinode/EIS pixel
size (1″), convolved using a Gaussian point-spread function (3″),
and exposed for a characteristic time of 50 s. The synthetic images
generated for rasters (moving slit) and sit-and-stare (stationary
slit) are analyzed to find the signatures of the twisted flux and the
associated instability. We find that there are several qualities of a
kink-unstable coronal flux rope that can be detected observationally
using Hinode/EIS, namely the growth of the loop radius, the increase in
intensity toward the radial edge of the loop, and the Doppler velocity
following an internal twisted magnetic field line. However, EIS cannot
resolve the small, transient features present in the simulation,
such as sites of small-scale reconnection (e.g., nanoflares).
---------------------------------------------------------
Title: Excitation and evolution of vertically polarised transverse
loop oscillations by coronal rain
Authors: Verwichte, E.; Kohutova, P.
2017A&A...601L...2V Altcode:
Context. Coronal rain is composed of cool dense blobs that form in solar
coronal loops and are a manifestation of catastrophic cooling linked
to thermal instability. The nature and excitation of oscillations
associated with coronal rain is not well understood. <BR /> Aims:
We consider observations of coronal rain in a bid to elucidate the
excitation mechanism and evolution of wave characteristics. <BR />
Methods: We analyse IRIS and Hinode/SOT observations of an oscillating
coronal rain event on the 17th Aug. 2014 and determine the wave
characteristics as a function of time using tried and tested time-space
analysis techniques. <BR /> Results: We exploit the seismological
capability of the oscillation to deduce the relative rain mass from
the oscillation amplitude. This is consistent with the evolution of the
oscillation period showing the loop losing a third of its mass due to
falling coronal rain in a 10-15 min time period. <BR /> Conclusions:
We present the first evidence of the excitation of vertically polarised
transverse loop oscillations triggered by catastrophic cooling at
the loop top and consistent with two thirds of the loop mass being
comprised of cool rain mass.
---------------------------------------------------------
Title: Kinematics of coronal rain in a transversely oscillating loop:
Ponderomotive force and rain-excited oscillations
Authors: Verwichte, E.; Antolin, P.; Rowlands, G.; Kohutova, P.;
Neukirch, T.
2017A&A...598A..57V Altcode:
Context. Coronal rain is composed of cool dense blobs that form in solar
coronal loops and are a manifestation of catastrophic cooling linked to
thermal instability. Once formed, rain falls towards the solar surface
at sub-ballistic speeds, which is not well understood. Pressure forces
seem to be the prime candidate to explain this. In many observations
rain is accompanied by transverse oscillations and the interaction
between rain and these oscillations needs to be explored. <BR /> Aims:
Therefore, an alternative kinematic model for coronal rain kinematics in
transversely oscillating loops is developed to understand the physical
nature of the observed sub-ballistic falling motion of rain. This
model explicitly explores the role of the ponderomotive force arising
from the transverse oscillation on the rain motion and the capacity
of rain to excite wave motion. <BR /> Methods: An analytical model is
presented that describes a rain blob guided by the coronal magnetic
field supporting a one-dimensional shear Alfvén wave as a point mass on
an oscillating string. The model includes gravity and the ponderomotive
force from the oscillation acting on the mass and the inertia of the
mass acting on the oscillation. <BR /> Results: The kinematics of
rain in the limit of negligible rain mass are explored and falling and
trapped regimes are found, depending on wave amplitude. In the trapped
regime for the fundamental mode, the rain blob bounces back and forth
around the loop top at a long period that is inversely proportional
to the oscillation amplitude. The model is compared with several
observational rain studies, including one in-depth comparison with
an observation that shows rain with up-and-down bobbing motion. The
role of rain inertia in exciting transverse oscillations is explored
in inclined loops. <BR /> Conclusions: It is found that the model
requires displacement amplitudes of the transverse oscillation that
are typically an order of magnitude larger than observed to explain
the measured sub-ballistic motion of the rain. Therefore, it is
concluded that the ponderomotive force is not the primary reason for
understanding sub-ballistic motion, but it plays a role in cases of
large loop oscillations. The appearance of rain causes the excitation
of small-amplitude transverse oscillations that may explain observed
events and provide a seismological tool to measure rain mass.
---------------------------------------------------------
Title: Analysis of Coronal Rain Observed by IRIS, HINODE/SOT, and
SDO/AIA: Transverse Oscillations, Kinematics, and Thermal Evolution
Authors: Kohutova, P.; Verwichte, E.
2016ApJ...827...39K Altcode:
Coronal rain composed of cool plasma condensations falling from coronal
heights along magnetic field lines is a phenomenon occurring mainly in
active region coronal loops. Recent high-resolution observations have
shown that coronal rain is much more common than previously thought,
suggesting its important role in the chromosphere-corona mass cycle. We
present the analysis of MHD oscillations and kinematics of the coronal
rain observed in chromospheric and transition region lines by the
Interface Region Imaging Spectrograph (IRIS), the Hinode Solar Optical
Telescope (SOT), and the Solar Dynamics Observatory (SDO) Atmospheric
Imaging Assembly (AIA). Two different regimes of transverse oscillations
traced by the rain are detected: small-scale persistent oscillations
driven by a continuously operating process and localized large-scale
oscillations excited by a transient mechanism. The plasma condensations
are found to move with speeds ranging from few km s<SUP>-1</SUP> up to
180 km s<SUP>-1</SUP> and with accelerations largely below the free-fall
rate, likely explained by pressure effects and the ponderomotive force
resulting from the loop oscillations. The observed evolution of the
emission in individual SDO/AIA bandpasses is found to exhibit clear
signatures of a gradual cooling of the plasma at the loop top. We
determine the temperature evolution of the coronal loop plasma using
regularized inversion to recover the differential emission measure
(DEM) and by forward modeling the emission intensities in the SDO/AIA
bandpasses using a two-component synthetic DEM model. The inferred
evolution of the temperature and density of the plasma near the apex is
consistent with the limit cycle model and suggests the loop is going
through a sequence of periodically repeating heating-condensation
cycles.
---------------------------------------------------------
Title: Erratum: The Effect of Loop Curvature on Coronal Loop Kink
Oscillations
Authors: Van Doorsselaere, Tom; Verwichte, Erwin; Terradas, Jaume
2014SSRv..184..275V Altcode: 2014SSRv..tmp...40V
No abstract at ADS
---------------------------------------------------------
Title: 3D Reconstruction of Coronal Loops by the Principal Component
Analysis
Authors: Nisticò, Giuseppe; Verwichte, Erwin; Nakariakov, Valery
2013Entrp..15.4520N Altcode:
Knowing the three dimensional structure of plasma filaments in the
uppermost part of the solar atmosphere, known as coronal loops, and
especially their length, is an important parameter in the wave-based
diagnostics of this part of the Sun. The combination of observations of
the Sun from different points of observations in space, thanks to the
most recent missions, including the Solar Dynamics Observatory (SDO)
and the Solar TErrestrial RElations Observatory (STEREO), allows us
to infer information about the geometrical shape of coronal loops in
3D space. Here, we propose a new method to reconstruct the loop shape
starting from stereoscopically determined 3D points, which sample the
loop length, by principal component analysis. This method is shown
to retrieve in an easy way the main parameters that define the loop,
e.g., the minor and major axes, the loop plane, the azimuthal and
inclination angles, for the special case of a coplanar loop.
---------------------------------------------------------
Title: Anti-phase Signature of Flare Generated Transverse Loop
Oscillations
Authors: White, R. S.; Verwichte, E.; Foullon, C.
2013ApJ...774..104W Altcode:
Transverse loop oscillations observed by the Atmospheric Imaging
Assembly instrument on the Solar Dynamics Observatory spacecraft are
studied after an impulsive solar flare eruption on 2012 May 8. We have
found that a transversely oscillating coronal loop seen in the 171 Å
bandpass oscillates in anti-phase with respect to adjacent larger loops
seen in the 193 Å and 211 Å bandpasses. These unusual oscillations
are analyzed to investigate the excitation mechanism responsible for
their initial inwardly directed anti-phase behavior. The transverse
oscillations are analyzed by constructing space-time diagrams from cuts
made parallel to the projected loop displacements. The displacement
time oscillation profiles are background subtracted and fitted with a
damped cosine curve that includes a linear change in the period with
time. The local magnetic topology of the active region is modeled using
potential field source surface extrapolation. It reveals that the
loops are anchored in different topological regions with foot point
locations identified on either side of the EUV flare peak emission
source. In this context, the oscillation characteristics indicate that
the excitation mechanism is closely linked to the local magnetic field
topology and the reconnection generated wave dynamics in the active
region rather than following an external flare blast wave. We discuss
how observations such as these may serve to identify reconnection
processes in similar quadrupolar active regions.
---------------------------------------------------------
Title: Kelvin-Helmholtz Instability of the CME Reconnection Outflow
Layer in the Low Corona
Authors: Foullon, Claire; Verwichte, Erwin; Nykyri, Katariina;
Aschwanden, Markus J.; Hannah, Iain G.
2013ApJ...767..170F Altcode:
New capabilities for studying the Sun allow us to image for the first
time the magnetic Kelvin-Helmholtz (KH) instability developing at the
surface of a fast coronal mass ejecta (CME) less than 150 Mm above the
solar surface. We conduct a detailed observational investigation of this
phenomenon, observed off the east solar limb on 2010 November 3, in the
EUV with SDO/AIA. In conjunction with STEREO-B/EUVI, we derive the CME
source surface position. We ascertain the timing and early evolution
of the CME outflow leading to the instability onset. We perform image
and spectral analysis, exploring the CME plasma structuring and its
parabolic flow pattern. As we evaluate and validate the consistency
of the observations with theoretical considerations and predictions,
we take the view that the ejecta layer corresponds to a reconnection
outflow layer surrounding the erupting flux rope, accounting for
the timing, high temperature (~11.6 MK), and high flow shear (~680 km
s<SUP>-1</SUP>) on the unstable CME northern flank and for the observed
asymmetry between the CME flanks. From the irregular evolution of the
CME flow pattern, we infer a shear gradient consistent with expected
spatial flow variations across the KH-unstable flank. The KH phenomenon
observed is tied to the first stage of a linked flare-CME event.
---------------------------------------------------------
Title: Statistical seismology of transverse waves in the solar corona
Authors: Verwichte, E.; Van Doorsselaere, T.; White, R. S.; Antolin, P.
2013A&A...552A.138V Altcode:
Context. Observations show that transverse oscillations commonly occur
in solar coronal loops. The rapid damping of these waves has been
attributed to resonant absorption. The oscillation characteristics
carries information of the structuring of the corona. However,
self-consistent seismological methods that extract information
from individual oscillations are limited because there are fewer
observables than unknown parameters in the model, and the problem
is underdetermined. Furthermore, it has been shown that one-to-one
comparisons of the observed scaling of period and damping times with
wave damping theories are misleading. <BR /> Aims: We aim to investigate
whether seismological information can be gained from the observed
scaling laws in a statistical sense. <BR /> Methods: A statistical
approach is used whereby scaling laws are produced by forward modelling
using distributions of values for key loop cross-sectional structuring
parameters. We study two types of observations: 1) transverse loops
oscillations as seen mainly with TRACE and SDO and 2) running transverse
waves seen with the Coronal Multichannel Polarimeter (CoMP). <BR />
Results: We demonstrate that the observed period-damping time scaling
law does provide information about the physical damping mechanism,
if observations are collected from as wide range of periods as
possible and a comparison with theory is performed in a statistical
sense. The distribution of the ratio of damping time over period,
i.e. the quality factor, has been derived analytically and fitted to
the observations. A minimum value for the quality factor of 0.65 has
been found. From this, a constraint linking the ranges of possible
values for the density contrast and inhomogeneity layer thickness is
obtained for transverse loop oscillations. If the layer thickness is
not constrained, then the density contrast is at most equal to 3. For
transverse waves seen by CoMP, it is found that the ratio of maximum
to minimum values for these two parameters has to be less than 2.06;
i.e., the sampled values for the layer thickness and Alfvén travel
time come from a relatively narrow distribution. <BR /> Conclusions:
Now that more and more transverse loop oscillations have been analysed,
a statistical approach to coronal seismology becomes possible. Using
the observed data cloud, we have found restrictions to the loop's
density contrast and inhomogeneity layer thickness. Surprisingly, for
running waves, narrow distributions for loop parameters have been found.
---------------------------------------------------------
Title: Coronal Alfvén Speed Determination: Consistency between
Seismology Using AIA/SDO Transverse Loop Oscillations and Magnetic
Extrapolation
Authors: Verwichte, E.; Van Doorsselaere, T.; Foullon, C.; White, R. S.
2013ApJ...767...16V Altcode:
Two transversely oscillating coronal loops are investigated in
detail during a flare on the 2011 September 6 using data from the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory. We compare two independent methods to determine the
Alfvén speed inside these loops. Through the period of oscillation
and loop length, information about the Alfvén speed inside each
loop is deduced seismologically. This is compared with the Alfvén
speed profiles deduced from magnetic extrapolation and spectral
methods using AIA bandpass. We find that for both loops the two
methods are consistent. Also, we find that the average Alfvén
speed based on loop travel time is not necessarily a good measure
to compare with the seismological result, which explains earlier
reported discrepancies. Instead, the effect of density and magnetic
stratification on the wave mode has to be taken into account. We discuss
the implications of combining seismological, extrapolation, and spectral
methods in deducing the physical properties of coronal loops.
---------------------------------------------------------
Title: Decaying and decayless transverse oscillations of a coronal
loop
Authors: Nisticò, G.; Nakariakov, V. M.; Verwichte, E.
2013A&A...552A..57N Altcode:
<BR /> Aims: We investigate kink oscillations of loops observed in an
active region with the Atmospheric Imaging Assembly (AIA) instrument on
board the Solar Dynamics Observatory (SDO) spacecraft before and after
a flare. <BR /> Methods: The oscillations were depicted and analysed
with time-distance maps, extracted from the cuts taken parallel or
perpendicular to the loop axis. Moving loops were followed in time
with steadily moving slits. The period of oscillations and its time
variation were determined by best-fitting harmonic functions. <BR />
Results: We show that before and well after the occurrence of the flare,
the loops experience low-amplitude decayless oscillations. The flare
and the coronal mass ejection associated to it trigger large-amplitude
oscillations that decay exponentially in time. The periods of the kink
oscillations in both regimes (about 240 s) are similar. An empirical
model of the phenomenon in terms of a damped linear oscillator
excited by a continuous low-amplitude harmonic driver and by an
impulsive high-amplitude driver is found to be consistent with the
observations. <P />Two movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: First observation of a transverse vertical oscillation during
the formation of a hot post-flare loop
Authors: White, R. S.; Verwichte, E.; Foullon, C.
2012A&A...545A.129W Altcode:
<BR /> Aims: We report and analyse the first observation of a
transverse oscillation in a hot coronal loop with the Atmospheric
Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO),
following a linked coronal-flare mass-ejection event on the 3 November
2010. The oscillating coronal loop is observed off the east solar
limb and exclusively in the 131 Å and 94 Å bandpasses, indicating
a loop plasma of temperature in the range of 9-11 MK. Furthermore,
the loop is not observed to cool into the other AIA channels, but just
disappears from all bandpasses at the end of the oscillation. <BR />
Methods: A time series analysis of the loop oscillation is conducted by
taking several cuts at different positions along the loop, estimating
the transverse displacements over time for two strands in the loop and
fitting those with a damped cosine curve. Intensity time variations,
both along the loop and for a series of cut cross-sections, are
investigated. Using a three-dimensional loop geometry obtained from a
comparison of STEREO-B/EUVI and AIA images, we model different modes of
transverse oscillations in the uniformly filled loop. <BR /> Results:
Our time series analysis reveals a period of 302 ± 14 s (291 ± 9 s)
and a damping time of 306 ± 43 s (487 ± 125 s) for the first (second)
loop strand. A spatial phase shift along the loop of approximately 180°
suggests that we observe a higher order harmonic. Intensity oscillations
are consistent with an interpretation in terms of a vertically polarised
mode. Our forward modelling suggests that the loop oscillates as either
a second or third order harmonic of this mode. <BR /> Conclusions:
This is the first observation of a transverse loop oscillation observed
exclusively in the hot coronal lines. The loop oscillation is vertically
polarised and is dominated by a higher order harmonic mode. We conclude
that the excitation mechanism of this 5 min period oscillation is
directly connected with the reconnection processes that form the post
flare loop, which differs from the blast wave excitation mechanism
often proposed as the cause of cooler transverse loop oscillations.
---------------------------------------------------------
Title: Nonlinear evolution of torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; Verwichte, E.;
Van Doorsselaere, T.
2012A&A...544A.127V Altcode:
<BR /> Aims: We study the efficiency of the energy transfer to shorter
scales in the field-aligned direction - the parallel nonlinear cascade
- that accompanies the propagation of torsional Alfvén waves along
open magnetic fields in the solar and stellar coronae, and compare it
with the same effects for the shear Alfvén wave. The evolution of the
torsional Alfvén wave is caused by the back reaction of nonlinearly
induced compressive perturbations on the Alfvén wave. <BR /> Methods:
The evolution of upwardly propagating torsional Alfvén waves is
considered in terms of the second-order thin flux-tube approximation
in a straight untwisted and non-rotating magnetic flux-tube. The
Cohen-Kulsrud equation for weakly nonlinear torsional waves is
derived. In the model, the effect of the cubic nonlinearity on the
propagation of long-wavelength axisymmetric torsional waves is compared
with the similar effect that accompanies the propagation of plane
linearly-polarised (shear) Alfvén waves of small amplitude. <BR />
Results: The solution to the Cohen-Kulsrud type equation for torsional
waves shows that their evolution is independent of the plasma-β,
which is in contrast to the shear Alfvén wave. In a finite-β plasma,
the nonlinear evolution of torsional Alfvén waves is slower and the
parallel nonlinear cascade is less efficient than those of shear Alfvén
waves. These results have important implications for the analysis of
possible heating of the plasma and its acceleration in the upper layers
of solar and stellar coronae. In particular, one-dimensional models of
coronal heating and wave acceleration, which use shear Alfvén waves
instead of torsional Alfvén waves, over-estimate the efficiency of
these processes.
---------------------------------------------------------
Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP
Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.;
Verwichte, E.; Vissers, G.
2012ASPC..455..253A Altcode: 2012arXiv1202.0787A
The tropical wisdom that when it is hot and dense we can expect
rain might also apply to the Sun. Indeed, observations and numerical
simulations have showed that strong heating at footpoints of loops,
as is the case for active regions, puts their coronae out of thermal
equilibrium, which can lead to a phenomenon known as catastrophic
cooling. Following local pressure loss in the corona, hot plasma
locally condenses in these loops and dramatically cools down to
chromospheric temperatures. These blobs become bright in Hα and
Ca ii H in time scales of minutes, and their dynamics seem to be
subject more to internal pressure changes in the loop rather than to
gravity. They thus become trackers of the magnetic field, which results
in the spectacular coronal rain that is observed falling down coronal
loops. In this work we report on high resolution observations of coronal
rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at
the Swedish Solar Telescope (SST). A statistical study is performed in
which properties such as velocities and accelerations of coronal rain
are derived. We show how this phenomenon can constitute a diagnostic
tool for the internal physical conditions inside loops. Furthermore, we
analyze transverse oscillations of strand-like condensations composing
coronal rain falling in a loop, and discuss the possible nature of the
wave. This points to the important role that coronal rain can play in
the fields of coronal heating and coronal seismology.
---------------------------------------------------------
Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
Gottfried; Aurass, Henri; Neukirch, Thomas
2012ExA....33..237M Altcode: 2011ExA...tmp..124M
Energetic particles are critical components of plasma populations
found throughout the universe. In many cases particles are accelerated
to relativistic energies and represent a substantial fraction of
the total energy of the system, thus requiring extremely efficient
acceleration processes. The production of accelerated particles
also appears coupled to magnetic field evolution in astrophysical
plasmas through the turbulent magnetic fields produced by diffusive
shock acceleration. Particle acceleration is thus a key component
in helping to understand the origin and evolution of magnetic
structures in, e.g. galaxies. The proximity of the Sun and the range
of high-resolution diagnostics available within the solar atmosphere
offers unique opportunities to study the processes involved in particle
acceleration through the use of a combination of remote sensing
observations of the radiative signatures of accelerated particles, and
of their plasma and magnetic environment. The SPARK concept targets the
broad range of energy, spatial and temporal scales over which particle
acceleration occurs in the solar atmosphere, in order to determine how
and where energetic particles are accelerated. SPARK combines highly
complementary imaging and spectroscopic observations of radiation from
energetic electrons, protons and ions set in their plasma and magnetic
context. The payload comprises focusing-optics X-ray imaging covering
the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
LaBr<SUB>3</SUB> scintillators, and photometry and source localisation
at far-infrared wavelengths. The plasma environment of the regions
of acceleration and interaction will be probed using soft X-ray
imaging of the corona and vector magnetography of the photosphere
and chromosphere. SPARK is designed for solar research. However,
in addition it will be able to provide exciting new insights into the
origin of particle acceleration in other regimes, including terrestrial
gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
existence of axions.
---------------------------------------------------------
Title: Transverse coronal loop oscillations seen in unprecedented
detail by AIA/SDO
Authors: White, Rebecca.; Verwichte, Erwin.; Soler, Roberto.; Goossens,
Marcel; Van Doorsselaere, Tom.; Arregui, Inigo.
2012decs.confE..18W Altcode:
We present an observational study of transverse oscillations of eleven
coronal loops observed in three separate events using data from the
Solar Dynamics Observatory (SDO) which provides unprecedented temporal
and spatial resolution of the solar corona. We study oscillatory events
using the Atmospheric Imaging Assembly (AIA) instrument on board SDO,
primarily in the 171 Angstrom bandpass to obtain information on loop
lengths, periods and damping times. Where possible, data from SDO/AIA
has been complimented with data from STEREO in order to obtain an
estimation of the 3D loop geometry. Local coronal plasma properties
are often difficult to measure using direct methods, however they
can be probed using the diagnostic power of MHD waves. In particular,
coronal loop oscillations interpreted as the fast MHD kink mode provide
an excellent tool for investigating such properties using the technique
of coronal seismology. By probing the local coronal plasma, important
information on the physical conditions in the vicinity of events such as
solar flares and CMEs can be determined. Further to the observational
study, analytic and Bayesian seismology inversion techniques are
applied to the transverse loop oscillations under the thin tube, thin
boundary approximations and under the assumption that they are damped
via the mechanism of resonant absorption. This technique allows a 3D
parameter space to be constructed that relates the density contrast,
the loop inhomogeneity length scale and the Alfven travel time.
---------------------------------------------------------
Title: Transverse coronal loop oscillations seen in unprecedented
detail by AIA/SDO
Authors: White, R. S.; Verwichte, E.
2012A&A...537A..49W Altcode:
<BR /> Aims: Detailed analysis of 11 transverse coronal loop
oscillations in three events observed with the Atmospheric Imaging
Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO)
spacecraft. Detailed analysis includes analysis of the displacement time
series, intensity variations and comparing EUVI and AIA data to estimate
the 3D loop geometry. <BR /> Methods: Time distance images extracted
from cuts made perpendicular to the oscillations are obtained. A
Gaussian plus background fitting technique is used to extract the time
series which is then fitted with a damped cosine curve. Intensity
variations are extracted along the time series points. EUVI/STEREO
data is compared to AIA/SDO data to obtain three-dimensional models
of the loop geometry. <BR /> Results: Time series analysis revealed
periods between 1.7 and 10 min and damping times between 2.9 and 13
min. Intensity variations are reliably observed for six of the loops
and a comparison between EUVI/STEREO and AIA/SDO data is performed
to simulate the polarisation of the kink mode. We conclude that the
intensity variations are due to variations in the line of sight column
depth of a horizontally polarised transverse loop oscillation. Coronal
seismology of the kink mode was applied to determine the range of the
internal Alfvén speed and the magnetic field strength for each loop.
---------------------------------------------------------
Title: Transverse Oscillations of Loops with Coronal Rain Observed
by Hinode/Solar Optical Telescope
Authors: Antolin, P.; Verwichte, E.
2011ApJ...736..121A Altcode: 2011arXiv1105.2175A
The condensations composing coronal rain, falling down along
loop-like structures observed in cool chromospheric lines such as
Hα and Ca II H, have long been a spectacular phenomenon of the solar
corona. However, considered a peculiar sporadic phenomenon, it has not
received much attention. This picture is rapidly changing due to recent
high-resolution observations with instruments such as the Hinode/Solar
Optical Telescope (SOT), CRISP of the Swedish 1-m Solar Telescope, and
the Solar Dynamics Observatory. Furthermore, numerical simulations
have shown that coronal rain is the loss of thermal equilibrium
of loops linked to footpoint heating. This result has highlighted
the importance that coronal rain can play in the field of coronal
heating. In this work, we further stress the importance of coronal rain
by showing the role it can play in the understanding of the coronal
magnetic field topology. We analyze Hinode/SOT observations in the Ca
II H line of a loop in which coronal rain puts in evidence in-phase
transverse oscillations of multiple strand-like structures. The periods,
amplitudes, transverse velocities, and phase velocities are calculated,
allowing an estimation of the energy flux of the wave and the coronal
magnetic field inside the loop through means of coronal seismology. We
discuss the possible interpretations of the wave as either standing or
propagating torsional Alfvén or fast kink waves. An estimate of the
plasma beta parameter of the condensations indicates a condition that
may allow the often observed separation and elongation processes of the
condensations. We also show that the wave pressure from the transverse
wave can be responsible for the observed low downward acceleration of
coronal rain.
---------------------------------------------------------
Title: Damped large amplitude transverse oscillations in an EUV
solar prominence, triggered by large-scale transient coronal waves
Authors: Hershaw, J.; Foullon, C.; Nakariakov, V. M.; Verwichte, E.
2011A&A...531A..53H Altcode:
<BR /> Aims: We investigate two successive trains of large amplitude
transverse oscillations in an arched EUV prominence, observed
with SoHO/EIT on the north-east solar limb on 30 July 2005. The
oscillatory trains are triggered by two large scale coronal waves,
associated with an X-class and a C-class flare occurring in the same
remote active region. <BR /> Methods: The oscillations are tracked
within rectangular slits parallel to the solar limb at different
heights, which are taken to move with the apparent height profile
of the prominence to account for solar rotation. Time series for
the two prominence arch legs are extracted using Gaussian fitting
on the 195 Å absorption features, and fitted to a damped cosine
curve to determine the oscillatory parameters. <BR /> Results:
Differing energies of the two triggering flares and associated
waves are found to agree with the velocity amplitudes, of 50.6 ±
3.2 and 15.9 ± 8.0 km s<SUP>-1</SUP> at the apex, for the first
and second oscillatory trains respectively, as estimated in the
transverse direction. The period of oscillation is similar for both
trains, with an average of 99 ± 11 min, indicating a characteristic
frequency as predicted by magnetohydrodynamics. Increasing velocity
amplitude with height during the first oscillatory train, and in-phase
starting motions of the two legs regardless of height, for each train,
demonstrate that the prominence exhibits a global kink mode to a
first approximation. However, discrepancies between the oscillatory
characteristics of the two legs and an apparent dependence of period
upon height, suggest that the prominence actually oscillates as a
collection of separate but interacting threads. Damping times of around
two to three cycles are observed. Combining our results with those
of previously analysed loop oscillations, we find an approximately
linear dependence of damping time upon period for kink oscillations,
supporting resonant absorption as the damping mechanism despite
limitations in testing this theory.
---------------------------------------------------------
Title: Linear coupling between fast and slow MHD waves due to
line-tying effects
Authors: Terradas, J.; Andries, J.; Verwichte, E.
2011A&A...527A.132T Altcode: 2010arXiv1011.0936T
Context. Oscillations in coronal loops are usually interpreted in terms
of uncoupled magnetohydrodynamic (MHD) waves. Examples of these waves
are standing transverse motions, interpreted as the kink MHD modes, and
propagating slow modes, commonly reported at the loop footpoints. <BR
/> Aims: Here we study a simple system in which fast and slow MHD
waves are coupled. The goal is to understand the fingerprints of
the coupling when boundary conditions are imposed. <BR /> Methods:
The reflection problem of a fast and slow MHD wave interacting with a
rigid boundary, representing the line-tying effect of the photosphere,
is analytically investigated. Both propagating and standing waves
are analysed and the time-dependent problem of the excitation of these
waves is considered. <BR /> Results: An obliquely incident fast MHD wave
on the photosphere inevitably generates a slow mode. The frequency of
the generated slow mode at the photosphere is exactly the same as the
frequency of the incident fast MHD mode, but its wavelength is much
smaller, assuming that the sound speed is slower than the Alfvén
speed. <BR /> Conclusions: The main signatures of the generated slow
wave are density fluctuations at the loop footpoints. We have derived
a simple formula that relates the velocity amplitude of the transverse
standing mode with the density enhancements at the footpoints due to
the driven slow modes. Using these results it is shown that there is
possible evidence in the observations of the coupling between these
two modes.
---------------------------------------------------------
Title: Magnetic Kelvin-Helmholtz Instability at the Sun
Authors: Foullon, Claire; Verwichte, Erwin; Nakariakov, Valery M.;
Nykyri, Katariina; Farrugia, Charles J.
2011ApJ...729L...8F Altcode:
Flows and instabilities play a major role in the dynamics of magnetized
plasmas including the solar corona, magnetospheric and heliospheric
boundaries, cometary tails, and astrophysical jets. The nonlinear
effects, multi-scale and microphysical interactions inherent to the
flow-driven instabilities, are believed to play a role, e.g., in plasma
entry across a discontinuity, generation of turbulence, and enhanced
drag. However, in order to clarify the efficiency of macroscopic
instabilities in these processes, we lack proper knowledge of their
overall morphological features. Here we show the first observations
of the temporally and spatially resolved evolution of the magnetic
Kelvin-Helmholtz instability in the solar corona. Unprecedented
high-resolution imaging observations of vortices developing at the
surface of a fast coronal mass ejecta are taken by the new Solar
Dynamics Observatory, validating theories of the nonlinear dynamics
involved. The new findings are a cornerstone for developing a unifying
theory on flow-driven instabilities in rarefied magnetized plasmas,
which is important for understanding the fundamental processes at work
in key regions of the Sun-Earth system.
---------------------------------------------------------
Title: Nonlinear long-wavelength torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; van Doorsselaere,
T.; Verwichte, E.
2011A&A...526A..80V Altcode:
<BR /> Aims: We investigate the nonlinear phenomena accompanying
long-wavelength torsional waves in solar and stellar coronae. <BR
/> Methods: The second order thin flux-tube approximation is used
to determine perturbations of a straight untwisted and non-rotating
magnetic flux-tube, nonlinearly induced by long-wavelength axisymmetric
magnetohydrodynamic waves of small, but finite amplitude. <BR />
Results: Propagating torsional waves induce compressible perturbations
oscillating with double the frequency of the torsional waves. In
contrast with plane shear Alfvén waves, the amplitude of compressible
perturbations is independent of the plasma-β and is proportional
to the torsional wave amplitude squared. Standing torsional waves
induce compressible perturbations of two kinds, that grow with
the characteristic time inversely proportional to the sound speed,
and that oscillate at double the frequency of the inducing torsional
wave. The growing density perturbation saturates at the level, inversely
proportional to the sound speed.
---------------------------------------------------------
Title: The First Measurement of the Adiabatic Index in the Solar
Corona Using Time-dependent Spectroscopy of Hinode/EIS Observations
Authors: Van Doorsselaere, Tom; Wardle, Nick; Del Zanna, Giulio;
Jansari, Kishan; Verwichte, Erwin; Nakariakov, Valery M.
2011ApJ...727L..32V Altcode:
We use observations of a slow magnetohydrodynamic wave in the corona
to determine for the first time the value of the effective adiabatic
index, using data from the Extreme-ultraviolet Imaging Spectrometer
on board Hinode. We detect oscillations in the electron density,
using the CHIANTI atomic database to perform spectroscopy. From
the time-dependent wave signals from multiple spectral lines the
relationship between relative density and temperature perturbations is
determined, which allows in turn to measure the effective adiabatic
index to be γ<SUB>eff</SUB> = 1.10 ± 0.02. This confirms that the
thermal conduction along the magnetic field is very efficient in the
solar corona. The thermal conduction coefficient is measured from
the phase lag between the temperature and density, and is shown to be
compatible with Spitzer conductivity.
---------------------------------------------------------
Title: Oscillatory processes in solar flares
Authors: Nakariakov, V. M.; Inglis, A. R.; Zimovets, I. V.; Foullon,
C.; Verwichte, E.; Sych, R.; Myagkova, I. N.
2010PPCF...52l4009N Altcode: 2010arXiv1010.0063N
Electromagnetic (radio, visible-light, UV, EUV, x-ray and gamma-ray)
emission generated by solar and stellar flares often contains pronounced
quasi-periodic pulsations (QPPs). Physical mechanisms responsible
for the generation of long-period QPP (with periods longer than
1 s) are likely to be associated with MHD processes. The observed
modulation depths, periods and anharmonicity of QPP suggest that
they can be linked with some kind of MHD auto-oscillations, e.g. an
oscillatory regime of magnetic reconnection. Such regimes, of both
spontaneous and induced nature, have been observed in resistive-MHD
numerical simulations. The oscillations are essentially nonlinear and
non-stationary. We demonstrate that a promising novel method for their
analysis is the empirical mode decomposition technique.
---------------------------------------------------------
Title: Periodic Spectral Line Asymmetries in Solar Coronal Structures
from Slow Magnetoacoustic Waves
Authors: Verwichte, E.; Marsh, M.; Foullon, C.; Van Doorsselaere,
T.; De Moortel, I.; Hood, A. W.; Nakariakov, V. M.
2010ApJ...724L.194V Altcode:
Recent spectral observations of upward moving quasi-periodic intensity
perturbations in solar coronal structures have shown evidence of
periodic line asymmetries near their footpoints. These observations
challenge the established interpretation of the intensity perturbations
in terms of propagating slow magnetoacoustic waves. We show that slow
waves inherently have a bias toward enhancement of emission in the
blue wing of the emission line due to in-phase behavior of velocity
and density perturbations. We demonstrate that slow waves cause line
asymmetries when the emission line is averaged over an oscillation
period or when a quasi-static plasma component in the line of sight
is included. Therefore, we conclude that slow magnetoacoustic waves
remain a valid explanation for the observed quasi-periodic intensity
perturbations.
---------------------------------------------------------
Title: From Large-scale Loops to the Sites of Dense Flaring Loops:
Preferential Conditions for Long-period Pulsations in Solar Flares
Authors: Foullon, C.; Fletcher, L.; Hannah, I. G.; Verwichte, E.;
Cecconi, B.; Nakariakov, V. M.; Phillips, K. J. H.; Tan, B. L.
2010ApJ...719..151F Altcode:
Long-period quasi-periodic pulsations (QPPs) of solar flares are
a class apart from shorter period events. By involving an external
resonator, the mechanism they call upon differs from traditional QPP
models, but has wider applications. We present a multi-wavelength
analysis of spatially resolved QPPs, with periods around 10 minutes,
observed in the X-ray spectrum primarily at energies between 3 and 25
keV. Complementary observations obtained in Hα and radio emission in
the kHz to GHz frequency range, together with an analysis of the X-ray
plasma properties provide a comprehensive picture that is consistent
with a dense flaring loop subject to periodic energization and
thermalization. The QPPs obtained in Hα and type III radio bursts,
with similar periods as the QPPs in soft X-rays, have the longest
periods ever reported for those types of data sets. We also report 1-2
GHz radio emission, concurrent with but unrestricted to the QPP time
intervals, which is multi-structured at regularly separated narrowband
frequencies and modulated with ~18 minute periods. This radio emission
can be attributed to the presence of multiple "quiet" large-scale loops
in the background corona. Large scale but shorter inner loops below
may act as preferential resonators for the QPPs. The observations
support interpretations consistent with both inner and outer loops
subject to fast kink magnetohydrodynamic waves. Finally, X-ray imaging
indicates the presence of double coronal sources in the flaring sites,
which could be the particular signatures of the magnetically linked
inner loops. We discuss the preferential conditions and the driving
mechanisms causing the repeated flaring.
---------------------------------------------------------
Title: Spatial Seismology of a Large Coronal Loop Arcade from TRACE
and EIT Observations of its Transverse Oscillations
Authors: Verwichte, E.; Foullon, C.; Van Doorsselaere, T.
2010ApJ...717..458V Altcode:
We present a study of transverse loop oscillations in a large coronal
loop arcade, using observations from the Transition Region And Coronal
Explorer (TRACE) and Extreme-ultraviolet Imaging Telescope (EIT). For
the first time we reveal the presence of long-period transverse
oscillations with periods between 24 minutes and 3 hr. One loop bundle,
690 Mm long and with an oscillation period of 40 minutes, is analyzed
in detail and its oscillation characteristics are determined in an
automated manner. The oscillation quality factor is similar to what
has been found earlier for oscillations in much shorter loops. This
indicates that the damping mechanism of transverse loop oscillations
is independent of loop length or period. The displacement profile
along the whole length of the oscillating loop is determined for the
first time and consistently between TRACE and EIT. By comparing the
observed profile with models of the three-dimensional geometry of the
equilibrium and perturbed loop, we test the effect of longitudinal
structuring (spatial seismology) and find that the observations cannot
unambiguously distinguish between structuring and non-planarity of
the equilibrium loop. Associated intensity variations with a similar
periodicity are explained in terms of variations in the line-of-sight
column depth. Also, we report intensity oscillations at the loop
footpoint, which are in anti-phase with respect to the intensity
oscillations in the loop body. Lastly, this observation offers the
first opportunity to use the transverse oscillations of the arcade to
model the Alfvén speed profile in the global corona.
---------------------------------------------------------
Title: The Effect of Loop Curvature on Coronal Loop Kink Oscillations
Authors: van Doorsselaere, Tom; Verwichte, Erwin; Terradas, Jaume
2009SSRv..149..299V Altcode:
We will review analytical and numerical efforts in modelling the
influence of curvature on coronal loop oscillations. We will mainly
focus our attention on fast kink mode oscillations. A curved slab model
will be presented, where it becomes clear that curvature introduces wave
leakage into the system, because of changes in the equilibrium. The
importance of leakage will be assessed through the use of a slab and
cylindrical model where lateral leakage is allowed. A full analytical
model for a semi-toroidal loop will be constructed for a system with
no leaking waves but with an inhomogeneous layer that introduces
damping due to the process of resonant absorption. The model for a
semi-toroidal loop will be extended to also include leakage, and will
be studied numerically. The numerical results will be compared to the
analytical model.
---------------------------------------------------------
Title: Coronal Seismology by Means of Kink Oscillation Overtones
Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.;
Verwichte, E.; Erdélyi, R.
2009SSRv..149....3A Altcode:
The detection of overtones of coronal loop kink oscillations has been
an important advance in the development of coronal seismology. It has
significantly increased the potential of coronal seismology and has
thus initiated important theoretical and observational improvements. New
detections of overtones have been made and a reduction of the error bars
has been obtained. The efforts of theoreticians to extend eigenmode
studies to more general coronal loop models is no longer a matter
of checking the robustness of the model but now also allows for the
estimation of certain equilibrium parameters. The frequencies of
the detected (longitudinal) overtones are in particular sensitive to
changes in the equilibrium properties along the loop, especially the
density and the magnetic field expansion. Also, attempts have been
made to use the limited longitudinal resolution in combination with
the theoretical eigenmodes as an additional seismological tool.
---------------------------------------------------------
Title: Ultra-long-period Oscillations in EUV Filaments Near to
Eruption: Two-wavelength Correlation and Seismology
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2009ApJ...700.1658F Altcode:
We investigate whether or not ultra-long-period oscillations in EUV
filaments can be related to their eruption. We report new observations
of long-period (~10-30 hr) oscillatory motions in an apparently
quiescent filament, as it crosses the solar disk in a 12 minute cadence
SOHO/Extreme-Ultraviolet Imaging Telescope (EIT) 195 Å uninterrupted
data set. This data set is chosen to explore characteristics of the
filament oscillations depending on its eruptive behavior, which is
observed while the filament is still on the disk. The periods are
found to increase in a near-stable regime prior to eruption. For the
two sequences reported so far, we compare and link the EUV filament
oscillations with pulsations in full-disk solar EUV irradiance from
SOHO/CELIAS/SEM 304 Å flux measurements. In intervals with stationary
periods, we find that the 304 Å pulsations and the 195 Å filament
oscillations have similar periodicities, but are phase-shifted by
about a quarter of period. The two-wavelength correlation serves to
show that, when the filament is the dominant dynamical feature but
can no longer be tracked on the disk, the full-disk irradiance may
provide a mean to identify the period increase prior to the filament
eruption. We use the periods thus obtained to estimate the height
increase of filaments' suspending coronal magnetic field lines,
based on a magnetohydrodynamic (MHD) wave interpretation of the
oscillations. The results are consistent with changes in prominence
heights detected off-limb and thus support the seismological tool
employed. Other interpretations connected with thermal overstability or
MHD piston effect are possible. These theoretical predictions however
do not explain the quarter-period shift between the two EUV-wavelength
signals. In any case, the detected variations may provide a powerful
diagnostic tool for the forecasting of prominence eruptions.
---------------------------------------------------------
Title: Seismology of a Large Solar Coronal Loop from EUVI/STEREO
Observations of its Transverse Oscillation
Authors: Verwichte, E.; Aschwanden, M. J.; Van Doorsselaere, T.;
Foullon, C.; Nakariakov, V. M.
2009ApJ...698..397V Altcode:
The first analysis of a transverse loop oscillation observed by both
Solar TErrestrial RElations Observatories (STEREO) spacecraft is
presented, for an event on the 2007 June 27 as seen by the Extreme
Ultraviolet Imager (EUVI). The three-dimensional loop geometry is
determined using a three-dimensional reconstruction with a semicircular
loop model, which allows for an accurate measurement of the loop
length. The plane of wave polarization is found from comparison with
a simulated loop model and shows that the oscillation is a fundamental
horizontally polarized fast magnetoacoustic kink mode. The oscillation
is characterized using an automated method and the results from
both spacecraft are found to match closely. The oscillation period
is 630 ± 30 s and the damping time is 1000 ± 300 s. Also, clear
intensity variations associated with the transverse loop oscillations
are reported for the first time. They are shown to be caused by the
effect of line-of-sight integration. The Alfvén speed and coronal
magnetic field derived using coronal seismology are discussed. This
study shows that EUVI/STEREO observations achieve an adequate accuracy
for studying long-period, large-amplitude transverse loop oscillations.
---------------------------------------------------------
Title: Propagating transverse waves in soft X-ray coronal jets
Authors: Vasheghani Farahani, S.; Van Doorsselaere, T.; Verwichte,
E.; Nakariakov, V. M.
2009A&A...498L..29V Altcode:
Aims: The theoretical model for magnetohydrodynamic (MHD) modes guided
by a field-aligned plasma cylinder with a steady flow is adapted
to interpret transverse waves observed in solar coronal hot jets,
discovered with Hinode/XRT in terms of fast magnetoacoustic kink
modes. <BR />Methods: Dispersion relations for linear magnetoacoustic
perturbations of a plasma jet of constant cross-section surrounded by
static magnetised plasma are used to determine the phase and group
speeds of guided transverse waves and their relationship with the
physical parameters of the jet and the background plasma. The structure
of the perturbations in the macroscopic parameters of the plasma inside
and outside the jet, and the phase relations between them are also
established. <BR />Results: We obtained a convenient expansion for
the long wave-length limit of the phase and group speeds and have
shown that transverse waves observed in soft-X-ray solar coronal
jets are adequately described in terms of fast magnetoacoustic kink
modes by a magnetic cylinder model, which includes the effect of a
steady flow. In the observationally determined range of parameters,
the waves are not found to be subject to either the Kelvin-Helmholtz
instability or the negative energy wave instability, and hence they
are likely to be excited at the source of the jet.
---------------------------------------------------------
Title: Seismological demonstration of perpendicular density
structuring in the solar corona
Authors: Van Doorsselaere, T.; Brady, C. S.; Verwichte, E.; Nakariakov,
V. M.
2008A&A...491L...9V Altcode:
The peculiarities of the propagating transverse waves observed in the
solar corona with the Coronal Multi-channel Polarimeter (CoMP) indicate
the existence of fine field structuring in the coronal density. We
present results of numerical simulations studying the evolution of a
localised transverse magneto-hydrodynamic wave in a uniform magnetic
field. We consider two initial low plasma-beta equilibria: one with
a homogeneous density, and one with a field-aligned dense structure
(such as a loop or a plume). The perpendicular localisation of the
wave strongly determines the angular distribution of the energy
propagation. If the perpendicular scale of the wave is significantly
smaller than the parallel scale (e.g. wavelength), as established by
CoMP, the wave develops as an oblique fast magneto-acoustic wave. In
an unstructured medium, the energy of such a wave is transferred
mainly across the magnetic field. However, it is possible to channel
the energy of the transverse wave along the magnetic field in the
presence of a field-aligned density enhancement. We conclude that the
CoMP results provide an independent seismological proof that the corona
is structured in density in the perpendicular direction.
---------------------------------------------------------
Title: Damping of Slow MHD Coronal Loop Oscillations by Shocks
Authors: Verwichte, E.; Haynes, M.; Arber, T. D.; Brady, C. S.
2008ApJ...685.1286V Altcode:
The damping of slow magnetoacoustic coronal loop oscillations by shock
dissipation is investigated. Observations of large-amplitude slow-mode
observations made by SUMER show a clear dependency of the damping
rate on the oscillation amplitude. Fully nonlinear MHD simulations
of slow-mode oscillations in the presence of thermal conduction are
performed that show that shock dissipation is an important damping
mechanism at large amplitudes, enhancing the damping rate by up to 50%
above the rate given by thermal conduction alone. A comparison between
the numerical simulations and the SUMER observations shows that although
the shock dissipation model can indeed produce an enhanced damping
rate that is a function of the oscillation amplitude, the dependency
that we found is not as strong as that for the observations, even
after observational corrections and the inclusion of enhanced linear
dissipation were considered.
---------------------------------------------------------
Title: Damping of Slow MHD Coronal Loop Oscillations by Shocks
Authors: Verwichte, E.; Haynes, M.; Arber, T.; Brady, C.
2008ESPM...12.3.58V Altcode:
The damping of slow magnetoacoustic coronal loop oscillations by shock
dissipation is investigated. Observations of large amplitude slow mode
observations by SUMER show a clear dependency of the damping rate on
the oscillation amplitude. Fully nonlinear MHD simulations of slow mode
oscillations in the presence of thermal conduction are performed and
the importance of various damping mechanisms a a function of amplitudes
are investigated.
---------------------------------------------------------
Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.;
Young, P. R.
2008ESPM...12.2.81V Altcode:
Last year, Tomczyk et al. (2007) have conclusively proven that low
amplitude (1km/s) waves are ubiquitously present in the corona. A few
months later, this was followed up with the discovery that chromospheric
spicules carry tremendous wave power (De Pontieu et al., 2007). These
new developments form a basis for coronal seismological magnetic field
mapping of the corona and chromosphere. However, this must be based
upon a confident identification of the observed wave mode. <P />Using
basic MHD wave theory, we demonstrate that the only way to interpret
the observed wave motions is in terms of fast magnetoacoustic kink
waves. We prove that the interpretation in terms of Alfven waves,
as was put forward in the original articles, does not explain the
observed phenomenology, while the fast magnetoacoustic kink waves
reproduce the required observational constraints. <P />To underline the
potential of magnetic field mapping, we discuss a recent observation
of coronal loop kink oscillations observed with Hinode/EIS. Because
of its spectroscopic and imaging capabilities, we are able to measure
the loop density, simultaneously with the loop length. This allows us
to determine the magnetic field with unprecedented accuracy.
---------------------------------------------------------
Title: Coronal magnetic field measurement using loop oscillations
observed by Hinode/EIS
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Young, P. R.;
Verwichte, E.
2008A&A...487L..17V Altcode:
We report the first spectroscopic detection of a kink MHD oscillation
of a solar coronal structure by the Extreme-Ultraviolet Imaging
Spectrometer (EIS) on the Japanese Hinode satellite. The detected
oscillation has an amplitude of 1 km s<SUP>-1</SUP> in the Doppler
shift of the FeXII 195 Å spectral line (1.3~MK), and a period of
296~s. The unique combination of EIS's spectroscopic and imaging
abilities enables us to measure simultaneously the mass density and
length of the oscillating loop. This enables us to measure directly
the magnitude of the local magnetic field, the fundamental coronal
plasma parameter, as 39 ± 8~G, with unprecedented accuracy. This
proof of concept makes EIS an exclusive instrument for the full scale
implementation of the MHD coronal seismological technique.
---------------------------------------------------------
Title: Coronal loop seismology using multiple transverse loop
oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2008IAUS..247..140V Altcode: 2007IAUS..247..140V
TRACE observations (23/11/1998 06:35:57-06:48:43UT) in the 171 Å
bandpass of an active region are studied. Coronal loop oscillations are
observed after a violent disruption of the equilibrium. The oscillation
properties are studied to give seismological estimates of physical
quantities, such as the density scale height. A loop segment is traced
during the oscillation, and the resulting time series is analysed
for periodicities. 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. The eigenfunction is used to do spatial coronal seismology,
but that method does not give any conclusive results.
---------------------------------------------------------
Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2008ApJ...676L..73V Altcode:
Recently, the omnipresence of waves has been discovered in the corona
using the CoMP instrument. We demonstrate that the observational
findings can be explained in terms of guided kink magnetoacoustic
modes. The interpretation of the observations in terms of Alfvén waves
is shown to be inconsistent with MHD wave theory. The implications of
the interpretation in terms of kink waves are discussed.
---------------------------------------------------------
Title: Coronal loop slow mode oscillations driven by the kink
instability
Authors: Haynes, M.; Arber, T. D.; Verwichte, E.
2008A&A...479..235H Altcode:
Aims:To establish the dominant wave modes generated by an internal,
m=1 kink instability in a short coronal flux tube. <BR />Methods:
The 3D MHD numerical simulations are performed using Lare3d to model
the kink instability and the subsequent wave generation. The initial
conditions are a straight, zero net current flux tube containing a twist
higher than the kink instability threshold. <BR />Results: It is shown
that the kink instability initially sets up a 1{st} harmonic (1{st}
overtone) that is converted through the rearrangement of the magnetic
field into two out-of-phase fundamental slow modes. These slow modes are
in the two entwined flux tubes created during the kink instability. <BR
/>Conclusions: The long-lived oscillations established after a kink
instability provide a possible way to identify whether sudden, short
coronal loop brightenings may have resulted from a confined kink
instability. The mode oscillation structure changes from the 1{st}
harmonic to fundamental due to field line relaxation. The subsequent
decay in the fundamental mode is comparable to observations and is
caused by shock dissipation. This result has important consequences
for the damping of the slow mode oscillations observed by SUMER.
---------------------------------------------------------
Title: Coronal loop seismology using multiple transverse loop
oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
2007A&A...473..959V Altcode:
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. <BR />Aims:
The oscillation properties are studied to give seismological estimates
of physical quantities, such as the density scale height. <BR />Methods:
A loop segment is traced during the oscillation, and the resulting
time series is analysed for periodicities. <BR />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.
---------------------------------------------------------
Title: Quasi-periodic modulation of solar and stellar flaring emission
by magnetohydrodynamic oscillations in a nearby loop
Authors: Nakariakov, V. M.; Foullon, C.; Verwichte, E.; Young, N. P.
2006A&A...452..343N Altcode:
We propose a new model for quasi-periodic modulation of solar
and stellar flaring emission. Fast magnetoacoustic oscillations
of a non-flaring loop can interact with a nearby flaring active
region. This interaction occurs when part of the oscillation situated
outside the loop reaches the regions of steep gradients in magnetic
field within an active region and produces periodic variations of
electric current density. The modulation depth of these variations is
a few orders of magnitude greater than the amplitude of the driving
oscillation. The variations of the current can induce current-driven
plasma micro-instabilities and thus anomalous resistivity. This can
periodically trigger magnetic reconnection, and hence acceleration
of charged particles, producing quasi-periodic pulsations of X-ray,
optical and radio emission at the arcade footpoints.
---------------------------------------------------------
Title: Seismology of curved coronal loops with vertically polarised
transverse oscillations
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...452..615V Altcode:
Aims.Using a model of vertically polarised fast magnetoacoustic waves
in curved coronal loops, the method of coronal seismology is applied
to observations of transverse loop oscillations. <BR /> Methods: . A
coronal loop is modeled as a curved magnetic slab in the zero plasma-β
limit. For an arbitrary piece-wise continuous power law equilibrium
density profile, the dispersion relation governing linear vertically
polarised fast magnetoacoustic kink waves is derived. The ways in which
this model can be used for coronal seismology are explored and applied
to two observational examples. <BR /> Results: . The Alfvén speed and
equilibrium density profile are determined from observations. It is
shown that the mechanism of lateral leakage of fast magnetoacoustic
kink oscillations described in this model is efficient. In fact, the
damping is so efficient that in order to match predicted values with
observational ones, either the loop needs to be highly contrasted or the
transverse Alfvén speed profile needs to be close to linear. Possible
improvements to make the modeling of lateral wave leakage in loops more
realistic, allowing a lower damping efficiency, are discussed. <BR />
---------------------------------------------------------
Title: Leakage of waves from coronal loops by wave tunneling
Authors: Brady, C. S.; Verwichte, E.; Arber, T. D.
2006A&A...449..389B Altcode:
To better understand the decay of vertically polarised fast kink modes
of coronal loops by the mechanism of wave tunneling, simulations
are performed of fast kink modes in straight flux slabs which have
Alfvén speed profiles which include a tunneling region. The decay
rates are found to be determined by the mode number of the trapped
mode and the thickness of the tunneling region. Two analytical models
are suggested to explain the observed decay. The first is a extension
of the work of Roberts (1981, Sol. Phys., 69, 39) to include a finite
thickness tunneling region, and the second is a simpler model which
yields an analytical solution for the relationship between decay rate,
period and the thickness of the tunneling region. The decay rates for
these straight slabs are found to be slower than in observations and
those found in a previous paper on the the subject by Brady & Arber
(2005, A&A, 438, 733) using curved flux slabs. It is found that the
difference between the straight slabs used here and the curved slabs
used in Brady & Arber (2005, A&A, 438, 733) can be represented
as a geometric correction to the decay rate.
---------------------------------------------------------
Title: Fast magnetoacoustic waves in curved coronal
loops. II. Tunneling modes
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...449..769V Altcode:
Aims. Fast magnetoacoustic waves in curved coronal loops are
investigated and the role of lateral leakage in wave damping, which
includes the mechanism of wave tunneling, is explored. Methods. A
coronal loop is modeled as a curved, magnetic slab in the zero
plasma-β limit. In this model and for an arbitrary piece-wise
continuous power law equilibrium density profile, the wave equation
governing linear vertically polarised fast magnetoacoustic waves is
solved analytically. An associated dispersion relation is derived
and the frequencies and eigenfunctions of the wave modes are
characterised. Results. For some equilibria, the waves are shown
to be all damped due to lateral leakage. It is demonstrated that
waves either leak straight out into the external medium or have to
overcome an evanescent barrier, which is linked to wave tunneling. The
wave solutions consist of alternating vertically polarised kink
and sausage branches. Fast kink oscillations may have a non-zero
density perturbation when averaged across the loop. The calculated
damping rate of fast magnetoacoustic kink oscillations is shown to be
consistent with related numerical simulations and show that lateral
leakage may explain the observed damping of (vertically polarised)
fast magnetoacoustic kink oscillations.
---------------------------------------------------------
Title: Automated Detection of EUV Prominences
Authors: Foullon, C.; Verwichte, E.
2006SoPh..234..135F Altcode:
We present methods to detect automatically off-limb prominences in
the extreme ultraviolet (EUV), using synoptic images taken by the
extreme-ultraviolet imaging telescope (EIT) on board SOHO. The 304 Å
line is essential for the detection of EUV prominences, but the optimal
detection is achieved through a combined image processing of the four
synoptic EIT images. In addition, the difference between consecutive
304 Å images serves to identify erupted prominences. Representation
maps of the quiescent EUV prominences for a given Carrington
rotation are generated and used for further analysis of the detected
structures. Longitudinal profiles of long-lived prominences are
investigated for three examples at different latitudes, in conjunction
with on-disk intensity profiles in the EUV. The observations coincide
with theoretically predicted apparent longitudinal profiles, which
can be distinguished from the profile of a prominence rising before
eruption. The developed algorithms may be relevant to study the 3D
geometry of features seen in the EUV and may facilitate the analysis
of data from the future STEREO mission.
---------------------------------------------------------
Title: Fast magnetoacoustic waves in curved coronal loops
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
2006A&A...446.1139V Altcode:
A study of vertically polarised fast magnetoacoustic waves in a curved
coronal loop is presented. The loop is modeled as a semi-circular
magnetic slab in the zero plasma-β limit. The governing equations for
linear waves are derived. We show that the wave mode behaviour depends
on the slope of the equilibrium density profile, which is modeled as
a piece-wise continuous power law curve of index α. For all profiles,
except for α=-4, wave modes are not trapped in the loop and leak out
into the external medium through wave tunneling. The particular case of
α=-4, which corresponds to a linearly increasing Alfvén speed profile,
is examined in more detail as this is the only model that can support
trapped wave modes. We compare the results with a straight slab model
and find similar behaviour. Coupling between sausage and kink wave
modes has not been found in the model.
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005ESASP.600E.101V Altcode: 2005dysu.confE.101V; 2005ESPM...11..101V
No abstract at ADS
---------------------------------------------------------
Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005ESASP.600E..33F Altcode: 2005ESPM...11...33F; 2005dysu.confE..33F
No abstract at ADS
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005ESASP.596E..38V Altcode: 2005ccmf.confE..38V
No abstract at ADS
---------------------------------------------------------
Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005ESASP.596E..46F Altcode: 2005ccmf.confE..46F
No abstract at ADS
---------------------------------------------------------
Title: X-ray quasi-periodic pulsations in solar flares as
magnetohydrodynamic oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
2005A&A...440L..59F Altcode:
We report the first observation at high spatial resolution of
long-period quasi-periodic pulsations (QPP) of X-ray radiation during
solar flares, made possible with the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), supported by complementary data at other
wavelengths from space-based and ground-based telescopes. Evidence
for the presence of a transequatorial loop possibly responsible for
the detected periodicity connected with its kink mode is found. Our
findings suggest that QPP can be interpreted as a periodic pumping
of electrons in a compact flaring loop, modulated by oscillations
in a magnetically linked and larger loop acting as a long-period
magnetohydrodynamic resonator.
---------------------------------------------------------
Title: Coronal Waves and Oscillations
Authors: Nakariakov, Valery M.; Verwichte, Erwin
2005LRSP....2....3N Altcode:
Wave and oscillatory activity of the solar corona is confidently
observed with modern imaging and spectral instruments in the
visible light, EUV, X-ray and radio bands, and interpreted in terms
of magnetohydrodynamic (MHD) wave theory. The review reflects
the current trends in the observational study of coronal waves
and oscillations (standing kink, sausage and longitudinal modes,
propagating slow waves and fast wave trains, the search for torsional
waves), theoretical modelling of interaction of MHD waves with plasma
structures, and implementation of the theoretical results for the mode
identification. Also the use of MHD waves for remote diagnostics of
coronal plasma — MHD coronal seismology — is discussed and the
applicability of this method for the estimation of coronal magnetic
field, transport coefficients, fine structuring and heating function
is demonstrated.
---------------------------------------------------------
Title: Transverse waves in a post-flare supra-arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2005A&A...430L..65V Altcode:
Observations of propagating transverse waves in an open magnetic field
structure with the Transition Region And Coronal Explorer (TRACE)
are presented. Waves associated with dark tadpole-like sunward
moving structures in the post-flare supra-arcade of NOAA active
region 9906 on the 21st of April 2002 are analysed. They are seen as
quasi-periodic transverse displacements of the dark tadpole tails, with
periods in the range of 90-220 s. Their phase speeds and displacement
amplitudes decrease as they propagate sunwards. At heights of 90 and
60 Mm above the post-flare loop footpoints the phase speeds are in
the ranges 200-700 km s<SUP>-1</SUP> and 90-200 km s<SUP>-1</SUP>
respectively. Furthermore, for consecutive tadpoles the phase speeds
decrease and periods increase as a function of time. The waves are
interpreted as propagating fast magnetoacoustic kink waves guided by
a vertical, evolving, open structure.
---------------------------------------------------------
Title: Detection of Ultra-Long Oscillations in an EUV Filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2004ESASP.575..394F Altcode: 2004soho...15..394F
No abstract at ADS
---------------------------------------------------------
Title: Transverse Oscillations in a Coronal Loop Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
2004ESASP.575..460V Altcode: 2004soho...15..460V
No abstract at ADS
---------------------------------------------------------
Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
2004ESASP.575..126V Altcode: 2004soho...15..126V
No abstract at ADS
---------------------------------------------------------
Title: Detection of ultra-long-period oscillations in an EUV filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
2004A&A...427L...5F Altcode:
We report the first detection of long-period (8-27 h) oscillatory
intensity variations in a coronal filament. The filament is observed
continuously as it crosses the solar disk in a 12-min-cadence SoHO/EIT
195 Å, uninterrupted data set. Cyclic intensity variations are found to
be correlated along the filament, while the most pronounced oscillations
are detected at its southern end for nearly 6 days. The dominant period
of these oscillations is 12.1 h and the amplitude of the intensity
variations reaches approximately 10% of the background intensity. The
ultra-long-period oscillations may be interpreted in terms of slow
string MHD modes or may be connected with thermal over-stability
associated with peculiarities of the cooling/heating function and with
the effect of neutrals. These theoretical predictions however do not
explain the spatial structure of the oscillations along the filament.
---------------------------------------------------------
Title: Characteristics of transverse oscillations in a coronal
loop arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
2004SoPh..223...77V Altcode:
TRACE observations from 15 April 2001 of transverse oscillations in
coronal loops of a post-flare loop arcade are investigated. They
are considered to be standing fast kink oscillations. Oscillation
signatures such as displacement amplitude, period, phase and damping
time are deduced from 9 loops as a function of distance along the loop
length. Multiple oscillation modes are found with different amplitude
profile along the loop length, suggesting the presence of a second
harmonic. The damping times are consistent with the hypothesis of
phase mixing and resonant absorption, although there is a clear bias
towards longer damping times compared with previous studies. The
coronal magnetic field strength and coronal shear viscosity in the
loop arcade are derived.
---------------------------------------------------------
Title: Coronal seismology: Seismology of the corona of the Sun
Authors: Nakariakov, V. M.; Verwichte, E.
2004A&G....45d..26N Altcode:
Seismology now includes study of the Sun's corona, a promising research
target both in its own right and for its role in the relationship
between the Sun and the Earth and its links to the solar magnetic
field. In addition, the corona, as a natural plasma, is itself an
objective for fundamental physics. Observations using spacecraft such
as SOHO and TRACE highlight the interrelationship between the magnetic
field and corona.
---------------------------------------------------------
Title: The automated analysis of solar active regions
Authors: Verwichte, Erwin
2003ESASP.535..169V Altcode: 2003iscs.symp..169V
Active regions are primary sources of solar activity. Developments
in the automated identification of solar active regions and the
characterisation of their coronal emission are presented. Central
to this effort is the image data archive of the Extreme-ultraviolet
Imaging Telescope(EIT), whose increasing size favours the application
of automated analysis schemes. The image complexity requires
additional information. This paper focuses on the developments of
an automated detection scheme for active regions using magnetogram
observations, which serves as a stepping stone for the detection of
associated emission in EUV images. The ultimate goal of this work is
to classify active regions on the basis of their coronal emission and
to characterise their main constituents, i.e. coronal loops.
---------------------------------------------------------
Title: La météo de l'espace d'hier et d'aujourd'hui
Authors: Foullon, Claire; Verwichte, Erwin
2002C&T...118..119F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Influences Data Analysis Center: current status of
expanding activities
Authors: Clette, F.; van der Linden, R.; Cugnon, P.; Berghmans,
D.; Foullon, C.; Wouters, L.; Verwichte, E.; Hochedez, J. -F.;
Vanlommel, P.
2002ESASP.506..125C Altcode: 2002ESPM...10..125C; 2002svco.conf..125C
Over the last 24 months, the activities of the SIDC, which is the
European Regional Warning Center of the ISES, have steadily expanded. A
7-day/week service has been implemented, the SIDC Web interface has
been reworked and expanded and the number of registered users increased
further. New image data have been added, including new photospheric
and chromospheric CCD images from the Uccle Station. We summarize here
the new services provided to the community and some statistics about
the success rate of our forecasts. We also outline the orientations
of the future SIDC development.
---------------------------------------------------------
Title: Multiscale activity observed by EIT/SoHO
Authors: Hochedez, J. -F.; Jacques, L.; Verwichte, E.; Berghmans,
D.; Wauters, L.; Clette, F.; Cugnon, P.
2002ESASP.477..115H Altcode: 2002scsw.conf..115H
The Extreme ultraviolet Imaging Telescope (EIT) of SoHO provides a
high-resolution and large sampling of the solar corona in time, space
and brightness. To extract the wealth of its physics, it is valuable to
adopt a multiscale approach. The Mexican Hat (MH) Continuous Wavelet
Transform (CWT) is used for the first time to derive statistically
the distribution of scales over 4 Mm. The global behaviour of the
small scales offers a powerful way to monitor coronal activity. This
is demonstrated with the May 1998 "CME Watch" data. This benefit
is of space weather relevance and could improve forecasting of the
solar activity.
---------------------------------------------------------
Title: The solar influences data analysis centre
Authors: Berghmans, D.; Clette, F.; Cugnon, P.; Gabryl, J. -R.;
Hochedez, J. -F.; Van der Linden, R. A. M.; Verwichte, E.
2002JASTP..64..757B Altcode: 2002JATP...64..757B
Since 1981, the Royal Observatory of Belgium has operated the
Sunspot Index Data Centre, the World Data Centre for the Sunspot
Index. Recently, the Space Weather Forecast Centre of Paris-Meudon
was transferred and added to the activities of the SIDC. Moreover,
a complete archive of all images of the SOHO instrument EIT has
become available at the SIDC. Given all these extensions, the new
style SIDC has become a `Solar Influences Data Centre' that analyses
solar activity and provides services on three different time scales:
1. Fast warnings and real time monitoring. As the Regional Warning
Centre (RWC) for Western Europe of the International Space Environment
Service (ISES), the SIDC collects and redistributes solar, geomagnetic,
and ionospheric data in Western Europe. Short-term predictions (3 days)
and alerts are produced on a daily basis. 2. Forecasts and middle term
analysis. The SIDC takes care of the calculation of a sunspot index,
called the International Sunspot Number. We compute and broadcast the
daily, monthly, yearly international sunspot numbers, with middle range
predictions (up to 12 months). 3. Post-event analysis and long-term
solar cycle analysis. Since the launch of SOHO, EIT offers a global view
of the EUV corona over the whole rising phase of the solar activity
cycle. Such a long-duration data series is unprecedented and allows
the study of the evolution over the solar cycle of objects classes
such as active regions, coronal holes, coronal mass ejections or flares.
---------------------------------------------------------
Title: Wide bandgap EUV and VUV imagers for the Solar Orbiter
Authors: Hochedez, Jean-François; Lemaire, Philippe; Pace, Emanuele;
Schühle, Udo; Verwichte, Erwin
2001ESASP.493..245H Altcode: 2001sefs.work..245H
No abstract at ADS
---------------------------------------------------------
Title: Slow magneto-acoustic waves in coronal loops
Authors: Verwichte, E.; Nakariakov, V. M.; Berghmans, D.; Hochedez,
J. -F.
2001ESASP.493..395V Altcode: 2001sefs.work..395V
No abstract at ADS
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops: EIT and TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
Poedts, S.; Nakariakov, V. M.
2001A&A...370..591R Altcode:
On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board
of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition
Region And Coronal Explorer) instruments produced simultaneous high
cadence image sequences of the same active region (AR 8218). TRACE
achieved a 25 s cadence in the Fe Ix (171 Å) bandpass while EIT
achieved a 15 s cadence (operating in “shutterless mode”, SoHO JOP
80) in the Fe Xii (195 Å) bandpass. These high cadence observations
in two complementary wavelengths have revealed the existence of weak
transient disturbances in an extended coronal loop system. These
propagating disturbances (PDs) seem to be a common phenomenon in
this part of the active region. The disturbances originate from small
scale brightenings at the footpoints of the loops and propagate along
the loops. The projected propagation speeds roughly vary between 65
and 150 km s<SUP>-1</SUP> for both instruments which is close to and
below the expected sound speed in the coronal loops. The measured slow
magnetoacoustic propagation speeds seem to suggest that the transients
are sound (or slow) wave disturbances. This work differs from previous
studies in the sense that it is based on a multi-wavelength observation
of an entire loop bundle at high cadence by two EUV imagers. The
observation of sound waves along the same path shows that they propagate
along the same loop, suggesting that loops contain sharp temperature
gradients and consist of either concentric shells or thin loop threads,
at different temperatures.
---------------------------------------------------------
Title: Long Term Variations in the Extreme UV Corona: the EIT/SoHO
perspective
Authors: Hochedez, J. -F.; Clette, F.; Verwichte, E.; Berghmans, D.;
Cugnon, P.
2001IAUS..203..501H Altcode:
Since the start of the SOHO mission, EIT offered a global view of
the extreme ultraviolet corona constinuously over the whole rising
phase of the solar activity cycle. Such a long-duration data serie
is unprecedented. We present here the current results of an ongoing
investigation of the entire EIT data set. In this process, numerous
classes of magnetic regions of all sizes (active regions, coronal
holes, bright points, plumes, transition region network, filaments)
as well as many different classes of dynamic events (CME's, flares,
jets, blinkers, macrospicules) will be identified in EIT images made
in its four bandpasses. The changes in the class properties (location,
size, area, topology, lifetime, integrated flux) or in the relationship
between different object classes can then be monitored over the fast
rise of magnetic activity towards the current maximum. We describe here
the image processing techniques developed for this search as well as
early results.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; Verwichte, E.; Berghmans, D.; Robbrecht, E.
2000A&A...362.1151N Altcode:
A theoretical model interpreting propagating disturbances of EUV
emission intensity, recently observed in coronal loops, is constructed
in terms of slow magnetoacoustic waves. The model is one-dimensional
and incorporates effects of nonlinearity, dissipation due to finite
viscosity and thermal conduction, and gravitational stratification of
plasma in the loop. It has been found that, for the observationally
detected parameters of the waves, the main factors influencing the
wave evolution are dissipation and stratification. The upwardly
propagating waves of observed periods (5-20 min) are found to decay
significantly in the vicinity of the loop apex, explaining the rarity
of observational detection of downwardly propagating waves. The model
provides a theoretical basis for development of MHD seismology of the
coronal loops.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in coronal loops: EIT vs TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
Poedts, S.
2000AIPC..537..271R Altcode: 2000wdss.conf..271R
On May 13, 1998 the EIT (Extreme-Ultraviolet Imaging Telescope) and
TRACE (Transition Region And Coronal Explorer) instruments produced
simultaneous high cadence image sequences of the same active region
(AR 8218). TRACE achieved a 25 sec cadence in the Fe IX/X (171 Å)
bandpass while EIT achieved a 15 sec cadence (operating in `shutterless
mode,' SOHO JOP 80) in the Fe XII (195 Å) bandpass. These high
cadence observations in two complementary wavelengths have revealed
the existence of weak transient disturbances in an extended coronal
loop system. These propagating disturbances (PDs) seem to be a
common phenomenon in this part of the active region. The disturbances
originate from small scale brightenings at the footpoints of the loops
and propagate along the loops. The apparent propagation speeds roughly
vary between 65 and 150 km s<SUP>-1</SUP> which is close to the expected
sound speed of the coronal loops. The measured propagation speeds seem
to suggest that the transients are sound (or slow) wave disturbances. .
---------------------------------------------------------
Title: Mid-Term Variations in the Extreme UV Corona: the EIT/SOHO
Perspective
Authors: Hochedez, J. F.; Clette, Frederic; Verwichte, Erwin;
Berghmans, David; Cugnon, Pierre
2000ESASP.463...79H Altcode: 2000sctc.proc...79H
No abstract at ADS
---------------------------------------------------------
Title: Future Diamond UV Imagers For Solar Physics
Authors: Hochedez, J. -F.; Verwichte, E.; Bergonzo, P.; Guizard, B.;
Mer, C.; Tromson, D.; Sacchi, M.; Dhez, P.; Hainaut, O.; Lemaire,
P.; Vial, J. -C.
2000PSSAR.181..141H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Realisation of 3-dimensional data sets.
Authors: Brown, D.; Galsgaard, K.; Ireland, J.; Verwichte, E.;
Walsh, R.
1999joso.proc..211B Altcode:
The visualisation of three-dimensional objects on two dimensions is a
very common problem, but is a tricky one to solve. Every discipline
has its way of solving it. The artist uses light-shade interaction,
perspective, special colour coding. The architect produces projections
of the object. The cartographer uses both colour-coding and shading to
represent height elevations. There have been many attempts in the last
century by the entertainment industry to produce a three-dimensional
illusion, in the fifties it was fashionable to have 3d movies which
utilize the anaglyph method. Nowadays one can buy "Magic Eye" postcards
which show a hidden three dimensional picture if you stare at it half
cross-eyed. This poster attempts to demonstrate how some of these
techniques can be applied to three-dimensional data sets that can
occur in solar physics.
---------------------------------------------------------
Title: On the visualization of three-dimensional datasets
Authors: Verwichte, Erwin; Galsgaard, Klaus
1998SoPh..183..445V Altcode:
The effective visualization of three-dimensional (3D) datasets,
both observationally and computationally sourced, is becoming
common in solar physics. We present example plots of data from
a 3D magnetohydrodynamical simulation, where depth perception is
simulated using chromo-stereoscopy. The depth information is coded
into the images using colours. When such images are viewed with double
prism refraction ChromaDepthTM 3D glasses, a pronounced 3D effect is
achieved. This visualization method is especially suited for working
with and presenting computationally derived 3D datasets.
---------------------------------------------------------
Title: Dissipative instability of the MHD tangential discontinuity in
magnetized plasmas with anisotropic viscosity and thermal conductivity
Authors: Ruderman, M. S.; Verwichte, E.; Erdélyi, R.; Goossens, M.
1996JPlPh..56..285R Altcode:
The stability of the MHD tangential discontinuity is studied in
compressible plasmas in the presence of anisotropic viscosity and
thermal conductivity. The general dispersion equation is derived,
and solutions to this dispersion equation and stability criteria are
obtained for the limiting cases of incompressible and cold plasmas. In
these two limiting cases the effect of thermal conductivity vanishes,
and the solutions are only influenced by viscosity. The stability
criteria for viscous plasmas are compared with those for ideal plasmas,
where stability is determined by the Kelvin—Helmholtz velocity
V<SUB>KH</SUB> as a threshold for the difference in the equilibrium
velocities. Viscosity turns out to have a destabilizing influence when
the viscosity coefficient takes different values at the two sides of
the discontinuity. Viscosity lowers the threshold velocity V below
the ideal Kelvin—Helmholtz velocity VKH, so that there is a range
of velocities between V and V<SUB>KH</SUB> where the overstability is
of a dissipative nature.
---------------------------------------------------------
Title: Jupiter, een turbulent wereld.
Authors: Goertz, H.; Verwichte, E.
1992Zenit..19..502G Altcode:
No abstract at ADS
