Author name code: verwichte ADS astronomy entries on 2022-09-14 author:"Verwichte, Erwin" ------------------------------------------------------------------------ Title: An automated method to detect, track and characterise bright points in multi-instrument solar imagery. Authors: Ramsey, Brad; Verwichte, Erwin; Morgan, Huw Bibcode: 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. 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. Bibcode: 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. Title: The Nature of High-frequency Oscillations Associated with Short-lived Spicule-type Events Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Doyle, J. G. Bibcode: 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. Title: The Nature of High-frequency Oscillations Associated with Short-lived Spicule-type Events Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Doyle, J. G. Bibcode: 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. Title: Acoustic response to transverse oscillations in a solar coronal loop Authors: White, S. J.; Verwichte, E. Bibcode: 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.
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.
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.
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. 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 Bibcode: 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. 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 Bibcode: 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.
Aims: We investigate the nature and dynamics of two small-scale quiet-Sun rotating structures in the chromosphere.
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.
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 arcsec2 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.
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

Movies are available at https://www.aanda.org 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 Bibcode: 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-1 and bright fronts that seem to move across the plane-of-sky 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 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. Title: First direct observation of a torsional Alfvén oscillation at coronal heights Authors: Kohutova, P.; Verwichte, E.; Froment, C. Bibcode: 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.
Aims: We present the first direct observational evidence of a fully resolved torsional Alfvén oscillation of a large-scale structure occurring at coronal heights.
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.
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.
Conclusions: Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfvén waves.

Movies attached to Figs. 1 and 2 are available at https://www.aanda.org Title: First direct observation of a torsional Alfvén oscillation at coronal heights Authors: Kohutova, P.; Verwichte, E.; Froment, C. Bibcode: 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. Title: Formation of coronal rain triggered by impulsive heating associated with magnetic reconnection Authors: Kohutova, P.; Verwichte, E.; Froment, C. Bibcode: 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.
Aims: We present an observation of magnetic reconnection in the corona and the associated impulsive heating triggering formation of coronal rain condensations.
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.
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.
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.

The movie associated to Fig. 1 is available at https://www.aanda.org 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 Bibcode: 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. Title: Simulating the Dynamics of Coronal Plasma Condensations Authors: Kohutova, Petra; Verwichte, Erwin Bibcode: 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. Title: Excitation and Evolution of Transverse Loop Oscillations by Coronal Rain Authors: Verwichte, Erwin; Kohutova, Petra; Antolin, Patrick; Rowlands, George; Neukirch, Thomas Bibcode: 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. Title: Excitation and evolution of transverse loop oscillations by coronal rain Authors: Verwichte, Erwin; Kohutova, Petra Bibcode: 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. Title: Modeling wave energy transport using a 3D MHD simulation of AR12683 Authors: Tarr, Lucas; Linton, Mark; Reardon, Kevin; Shetye, Juie; Verwichte, Erwin Bibcode: 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 magnetic field. <svg xmlns:xlink="http://www.w3.org/1999/xlink" width="3.215ex" height="2.509ex" style="vertical-align: -0.671ex;" viewBox="0 -791.3 1384.4 1080.4" role="img" focusable="false" xmlns="http://www.w3.org/2000/svg" aria-labelledby="MathJax-SVG-1-Title" <title id="MathJax-SVG-1-Title" H_\alpha <defs aria-hidden="true" <path stroke-width="1" id="E1-MJMATHI-48" d="M228 637Q194 637 192 641Q191 643 191 649Q191 673 202 682Q204 683 219 683Q260 681 355 681Q389 681 418 681T463 682T483 682Q499 682 499 672Q499 670 497 658Q492 641 487 638H485Q483 638 480 638T473 638T464 637T455 637Q416 636 405 634T387 623Q384 619 355 500Q348 474 340 442T328 395L324 380Q324 378 469 378H614L615 381Q615 384 646 504Q674 619 674 627T617 637Q594 637 587 639T580 648Q580 650 582 660Q586 677 588 679T604 682Q609 682 646 681T740 680Q802 680 835 681T871 682Q888 682 888 672Q888 645 876 638H874Q872 638 869 638T862 638T853 637T844 637Q805 636 794 634T776 623Q773 618 704 340T634 58Q634 51 638 51Q646 48 692 46H723Q729 38 729 37T726 19Q722 6 716 0H701Q664 2 567 2Q533 2 504 2T458 2T437 1Q420 1 420 10Q420 15 423 24Q428 43 433 45Q437 46 448 46H454Q481 46 514 49Q520 50 522 50T528 55T534 64T540 82T547 110T558 153Q565 181 569 198Q602 330 602 331T457 332H312L279 197Q245 63 245 58Q245 51 253 49T303 46H334Q340 38 340 37T337 19Q333 6 327 0H312Q275 2 178 2Q144 2 115 2T69 2T48 1Q31 1 31 10Q31 12 34 24Q39 43 44 45Q48 46 59 46H65Q92 46 125 49Q139 52 144 61Q147 65 216 339T285 628Q285 635 228 637Z" </path> <path stroke-width="1" id="E1-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z" </path> </defs> <g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)" aria-hidden="true" <use xlink:href="#E1-MJMATHI-48" x="0" y="0" </use> <use transform="scale(0.707)" xlink:href="#E1-MJMATHI-3B1" x="1175" y="-213" </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. Title: Excitation of vertical coronal loop oscillations by impulsively driven flows Authors: Kohutova, P.; Verwichte, E. Bibcode: 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.
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.
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.
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.
Conclusions: We find that flows with realistic speeds of less than 100 km s-1 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. Bibcode: 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.
Aims: We aim to investigate the possibility of excitation of loop oscillations by the presence of condensation plasma.
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.
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.
Conclusions: We propose coronal rain as a possible excitation mechanism for transverse loop oscillations. Title: Dynamics of plasma condensations in a gravitationally stratified coronal loop Authors: Kohutova, P.; Verwichte, E. Bibcode: 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.
Aims: We investigate the evolution and dynamics of plasma condensations in a gravitationally stratified coronal loop.
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.
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.
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. Bibcode: 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. Bibcode: 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.
Aims: We consider observations of coronal rain in a bid to elucidate the excitation mechanism and evolution of wave characteristics.
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.
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.
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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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-1 up to 180 km s-1 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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-1) 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. Bibcode: 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.
Aims: We aim to investigate whether seismological information can be gained from the observed scaling laws in a statistical sense.
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).
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.
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. Bibcode: 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. Bibcode: 2013A&A...552A..57N Altcode:
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.
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.
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.

Two movies are available in electronic form at http://www.aanda.org 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. Bibcode: 2012A&A...545A.129W Altcode:
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.
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.
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.
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. Bibcode: 2012A&A...544A.127V Altcode:
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.
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.
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. Bibcode: 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 Bibcode: 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 LaBr3 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. Bibcode: 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. Bibcode: 2012A&A...537A..49W Altcode:
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.
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.
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. Bibcode: 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. Bibcode: 2011A&A...531A..53H Altcode:
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.
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.
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-1 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 2011A&A...526A..80V Altcode:
Aims: We investigate the nonlinear phenomena accompanying long-wavelength torsional waves in solar and stellar coronae.
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.
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. Bibcode: 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 γeff = 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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.

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. Bibcode: 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-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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. Bibcode: 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.
Aims: The oscillation properties are studied to give seismological estimates of physical quantities, such as the density scale height.
Methods: A loop segment is traced during the oscillation, and the resulting time series is analysed for periodicities.
Results: In the loop segment displacement, two periods are found: 435.6 ± 4.5~s and 242.7 ± 6.4~s, consistent with the periods of the fundamental and 2nd harmonic fast kink oscillation. The small uncertainties allow us to estimate the density scale height in the loop to be 109~Mm, which is about double the estimated hydrostatical value of 50~Mm. Because a loop segment is traced, the amplitude dependence along the loop is found for each of these oscillations. The obtained spatial information is used as a seismological tool to give details about the geometry of the observed loop. 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. Bibcode: 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. Bibcode: 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.
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.
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.
Title: Leakage of waves from coronal loops by wave tunneling Authors: Brady, C. S.; Verwichte, E.; Arber, T. D. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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-1 and 90-200 km s-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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-1 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 VKH 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 VKH where the overstability is of a dissipative nature. Title: Jupiter, een turbulent wereld. Authors: Goertz, H.; Verwichte, E. Bibcode: 1992Zenit..19..502G Altcode: No abstract at ADS