Extreme Ultraviolet Imaging Telescope Waves Research Articles

Fine Structures of an EUV Wave Event from Multi-viewpoint Observations

Abstract In this study, we investigate an extreme ultraviolet (EUV) wave event on 2010 February 11, which occurred as a limb event from the Earth viewpoint and a disk event from the Solar Terrestrial Relations Observatory-Behind viewpoint. We use the data obtained by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory in various EUV channels. The EUV wave event was launched by a partial prominence eruption. Compared with some EUV wave events in previous works, this EUV wave event contains a faster wave with a speed of ∼445 ± 6 km s−1, which we call a coronal Moreton wave, and a slower wave with a speed of ∼298 ± 5 km s−1, which we call the Extreme Ultraviolet Imaging Telescope (EIT) wave. The coronal Moreton wave is identified as a fast-mode wave and the EIT wave is identified as an apparent propagation due to successive field-line stretching. We also observe a stationary front associated with the fast-mode EUV wave. This stationary front is explained as mode conversion from the coronal Moreton wave to a slow-mode wave near a streamer.

Three-dimensional ray-tracing simulation of fast magnetoacoustic waves in a stratified solar atmosphere

In the solar atmosphere, there exist two frequently-observed phenomena, Moreton waves and EIT (extreme-ultraviolet imaging telescope) waves, whose physical nature is still under debate. In this work, we perform a three-dimensional ray-tracing simulation for the propagation of fast magnetoacoustic waves. We build a stratified solar atmosphere model and take partial ionization into consideration to give an exact description of chromosphere and transition region. The calculated result is compared with a flare event observation in which both Moreton waves and EIT waves were present. In agreement with observations, the calculated wavefront show different kinematical characteristics in different propagation directions during different times. Moreton waves and EIT waves have higher propagation speeds near the active region where the magnetic field strength is strong. The result suggests that both Moreton waves and EIT waves of this event can be interpreted as the fast magnetoacoustic waves propagating at different heights in the solar atmosphere.

Understanding the Physical Nature of Coronal \u201cEIT Waves\u201d

For almost 20 years the physical nature of globally propagating waves in the solar corona (commonly called “EIT waves”) has been controversial and subject to debate. Additional theories have been proposed over the years to explain observations that did not agree with the originally proposed fast-mode wave interpretation. However, the incompatibility of observations made using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory with the fast-mode wave interpretation was challenged by differing viewpoints from the twin Solar Terrestrial Relations Observatory spacecraft and data with higher spatial and temporal resolution from the Solar Dynamics Observatory. In this article, we reexamine the theories proposed to explain EIT waves to identify measurable properties and behaviours that can be compared to current and future observations. Most of us conclude that the so-called EIT waves are best described as fast-mode large-amplitude waves or shocks that are initially driven by the impulsive expansion of an erupting coronal mass ejection in the low corona.

Solar Energetic Particles and Associated EIT Disturbances in Solar Cycle 23

We explore the link between solar energetic particles (SEPs) observed at 1 AU and large-scale disturbances propagating in the solar corona, named after the Extreme ultraviolet Imaging Telescope (EIT) as EIT waves, which trace the lateral expansion of a coronal mass ejection (CME). A comprehensive search for SOHO/EIT waves was carried out for 179 SEP events during Solar Cycle 23 (1997-2006). 87% of the SEP events were found to be accompanied by EIT waves. In order to test if the EIT waves play a role in the SEP acceleration, we compared their extrapolated arrival time at the footpoint of the Parker spiral with the particle onset in the 26 eastern SEP events that had no direct magnetic connection to the Earth. We find that the onset of proton events was generally consistent with this scenario. However, in a number of cases the first near-relativistic electrons were detected too early. Furthermore, the electrons had in general only weakly anisotropic pitch-angle distributions. This poses a problem for the idea that the SEPs were accelerated by the EIT wave or in any other spatially confined region in the low corona. The presence of weak electron anisotropies in SEP events from the eastern hemisphere suggests that transport processes in interplanetary space, including cross-field diffusion, play a role in giving the SEPs access to a broad range of helio-longitudes.

Recent Developments of NEMO: Detection of EUV Wave Characteristics

The recent developments in space instrumentation for solar observations and telemetry have caused the necessity of advanced pattern recognition tools for the different classes of solar events. The Extreme ultraviolet Imaging Telescope (EIT) of solar corona on-board SOHO spacecraft has uncovered a new class of eruptive events which are often identified as signatures of Coronal Mass Ejection (CME) initiations on solar disk. It is evident that a crucial task is the development of an automatic detection tool of CMEs precursors. The Novel EIT wave Machine Observing (NEMO) (http://sidc.be/nemo) code is an operational tool that detects automatically solar eruptions using EIT image sequences. NEMO applies techniques based on the general statistical properties of the underlying physical mechanisms of eruptive events on the solar disc. In this work, the most recent updates of NEMO code - that have resulted to the increase of the recognition efficiency of solar eruptions linked to CMEs - are presented. These updates provide calculations of the surface of the dimming region, implement novel clustering technique for the dimmings and set new criteria to flag the eruptive dimmings based on their complex characteristics. The efficiency of NEMO has been increased significantly resulting to the extraction of dimmings observed near the solar limb and to the detection of small-scale events as well. As a consequence, the detection efficiency of CMEs precursors and the forecasts of CMEs have been drastically improved. Furthermore, the catalogues of solar eruptive events that can be constructed by NEMO may include larger number of physical parameters associated to the dimming regions.

Growing electric field parallel to magnetic field due to transverse kinetic drift waves in inhomogeneous corona

The transverse drift wave, which is unstable due to purely kinetic effects, and driven by the density and magnetic field gradients, is discussed in context of its application to the solar corona. The gradients of the two quantities are opposite to each other, as required by the equilibrium pressure balance, and they are in the plane perpendicular to the magnetic field vector. The transverse drift wave has such properties that it propagates strictly perpendicularly to both the magnetic field vector and the mentioned gradients. It is electromagnetic, with the perturbed electric field in the direction of the equilibrium magnetic field, while the perturbed magnetic field vector is in the direction of the equilibrium gradients. Such an orientation of the electric field implies a possibility of acceleration of coronal plasma particles along the background magnetic field, in both directions, outward and inward. In the case of locally open magnetic structures, the outwardly moving particles should contribute to the solar wind. Those moving inwards eventually arrive in the lower solar atmosphere where the mean free path is short and, due to collisions, they should disperse their energy to the surrounding plasma and contribute to heating. It is also shown that accelerated particles can additionally be stochastically heated by the wave. This completely new stochastic heating mechanism is found here for the first time. It takes place provided that the particles are simultaneously accelerated by the wave to large enough velocities in the parallel direction. The model is applicable to any inhomogeneous coronal environment, like magnetic loops, coronal holes and the so-called EIT waves, named after the Extreme-ultraviolet Imaging Telescope (EIT) used for their first detection.

Slow EIT waves as gravity modes

The EIT waves [named after the extreme-ultraviolet imaging telescope (EIT) onboard the solar and heliospheric observatory (SOHO)] are in the literature usually described as fast magneto-acoustic (FMA) modes. However, observations show that a large percentage of these events propagate with very slow speeds that may be as low as 20 km/s. This is far below the FMA wave speed which cannot be below the sound speed, the latter being typically larger than 102 km/s in the corona. In the present study, it is shown that, to account for such low propagation speed, a different wave model should be used, based on the theory of gravity waves, both internal (IG) and surface (SG) ones. The gravity modes are physically completely different from the FMA mode, as they are essentially dispersive and in addition the IG wave is a transverse mode. Both the IG and the SG mode separately can provide proper propagation velocities in the whole low speed range.

Large-scale Bright Fronts in the Solar Corona: A Review of “EIT waves”

``EIT waves" are large-scale coronal bright fronts (CBFs) that were first observed in 195 \AA\ images obtained using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}. Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that propagate pseudo-radially across the solar disk at velocities of 100--700 km s$^{-1}$ with front widths of 50-100 Mm. As their speed is greater than the quiet coronal sound speed ($c_s\leq$200 km s$^{-1}$) and comparable to the local Alfv\'{e}n speed ($v_A\leq$1000 km s$^{-1}$), they were initially interpreted as fast-mode magnetoacoustic waves ($v_{f}=(c_s^2 + v_A^2)^{1/2}$). Their propagation is now known to be modified by regions where the magnetosonic sound speed varies, such as active regions and coronal holes, but there is also evidence for stationary CBFs at coronal hole boundaries. The latter has led to the suggestion that they may be a manifestation of a processes such as Joule heating or magnetic reconnection, rather than a wave-related phenomena. While the general morphological and kinematic properties of CBFs and their association with coronal mass ejections have now been well described, there are many questions regarding their excitation and propagation. In particular, the theoretical interpretation of these enigmatic events as magnetohydrodynamic waves or due to changes in magnetic topology remains the topic of much debate.

The Dependence of the EIT Wave Velocity on the Magnetic Field Strength

"EIT waves" are a wavelike phenomenon propagating in the corona, which were initially observed in the extreme ultraviolet (EUV) wavelength by the EUV Imaging Telescope (EIT). Their nature is still elusive, with the debate between fast-mode wave model and non-wave model. In order to distinguish between these models, we investigate the relation between the EIT wave velocity and the local magnetic field in the corona. It is found that the two parameters show significant negative correlation in most of the EIT wave fronts, {\it i.e.}, EIT wave propagates more slowly in the regions of stronger magnetic field. Such a result poses a big challenge to the fast-mode wave model, which would predict a strong positive correlation between the two parameters. However, it is demonstrated that such a result can be explained by the fieldline stretching model, \emph{i.e.,} that "EIT waves" are apparently-propagating brightenings, which are generated by successive stretching of closed magnetic field lines pushed by the erupting flux rope during coronal mass ejections (CMEs).

GLOBAL SIMULATION OF AN EXTREME ULTRAVIOLET IMAGING TELESCOPE WAVE

We use the observation of an Extreme Ultraviolet Imaging Telescope (EIT) wave in the lower solar corona, seen with the two Solar Terrestrial Relations Observatory (STEREO) spacecraft in extreme ultraviolet light on 2007 May 19, to model the same event with a three-dimensional (3D) time-depending magnetohydrodynamic (MHD) code that includes solar coronal magnetic fields derived with Wilcox Solar Observatory magnetogram data, and a solar wind outflow accelerated with empirical heating functions. The model includes a coronal mass ejection (CME) of Gibson and Low flux rope type above the reconstructed active region with parameters adapted from observations to excite the EIT wave. We trace the EIT wave running as circular velocity enhancement around the launching site of the CME in the direction tangential to the sphere produced by the wave front, and compute the phase velocities of the wave front. We find that the phase velocities are in good agreement with theoretical values for a fast magnetosonic wave, derived with the physical parameters of the model, and with observed phase speeds of an incident EIT wave reflected by a coronal hole and running at about the same location. We also produce in our 3D MHD model the observed reflection of the EIT wave at the coronal hole boundary, triggered by the magnetic pressure difference between the wave front hitting the hole and the boundary magnetic fields of the coronal hole, and the response of the coronal hole, which leads to the generation of secondary reflected EIT waves radiating away in different directions than the incident EIT wave. This is the first 3D MHD model of an EIT wave triggered by a CME that includes realistic solar magnetic field, with results comparing favorably to STEREO Extreme Ultraviolet Imager observations.

A NEW VIEW OF CORONAL WAVES FROMSTEREO

On 2007 December 7, there was an eruption from AR 10977, which also hosted a sigmoid. An EUV Imaging Telescope (EIT) wave associated with this eruption was observed by EUVI on board the Solar Terrestrial Relations Observatory (STEREO). Using EUVI images in the 171 A and the 195 A passbands from both STEREO A and B, we study the morphology and kinematics of this EIT wave. In the early stages, images of the EIT wave from the two STEREO spacecrafts differ markedly. We determine that the EUV fronts observed at the very beginning of the eruption likely include some intensity contribution from the associated coronal mass ejection (CME). Additionally, our velocity measurements suggest that the EIT wave front may propagate at nearly constant velocity. Both results offer constraints on current models and understanding of EIT waves.

THE RELATION BETWEEN EIT WAVES AND CORONAL MASS EJECTIONS

More and more evidence indicates that "EIT waves" are strongly related to coronal mass ejections (CMEs). However, it is still not clear how the two phenomena are related to each other. We investigate a CME event on 1997 September 9, which was well observed by both EUV imaging telescope (EIT) and the high-cadence MK3 coronagraph at Mauna Loa Solar Observatory, and compare the spatial relation between the "EIT wave" fronts and the CME leading loops. It is found that "EIT wave" fronts are co-spatial with the CME leading loops, and the expanding EUV dimmings are co-spatial with the CME cavity. It is also found that the CME stopped near the boundary of a coronal hole, a feature common to observations of "EIT waves". It is suggested that "EIT waves"/dimmings are the EUV counterparts of the CME leading loop/cavity, based on which we propose that, as in the case of "EIT waves", CME leading loops are apparently-moving density enhancements that are generated by successive stretching (or opening-up) of magnetic loops.

A Fresh View of the Extreme‐Ultraviolet Corona from the Application of a New Image‐Processing Technique

The Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) has provided (and continues to provide) the solar physics community with an unprecedented view of the extreme-ultraviolet (EUV) transition region and corona. In particular, it has observed EUV bright points, coronal holes, loops, and arcades, as well as dynamical events such as flares, EIT waves, and mass ejections. However, the multiscale nature of the observed solar features has not been fully exploited so far. In this paper, we introduce a technique to enhance the EUV features based on their multiscale nature, show a few examples of features not revealed with standard image-processing techniques (and therefore not previously noticed in the EIT data set), and make the solar physics community aware of the availability of the full EIT data set as processed with this novel technique.

Coronal "Wave": Magnetic Footprint of a Coronal Mass Ejection?

We investigate the properties of two classical EUV Imaging Telescope (EIT) coronal waves. The two source regions of the associated coronal mass ejections (CMEs) possess opposite helicities, and the coronal waves display rotations in opposite senses. We observe deep core dimmings near the flare site and also widespread diffuse dimming, accompanying the expansion of the EIT wave. We also report a new property of these EIT waves, namely, that they display dual brightenings: persistent ones at the outermost edge of the core dimming regions and simultaneously diffuse brightenings constituting the leading edge of the coronal wave, surrounding the expanding diffuse dimmings. We show that such behavior is consistent with a diffuse EIT wave being the magnetic footprint of a CME. We propose a new mechanism where driven magnetic reconnections between the skirt of the expanding CME magnetic field and quiet-Sun magnetic loops generate the observed bright diffuse front. The dual brightenings and the widespread diffuse dimming are identified as innate characteristics of this process.

Multiscale optical flow probing of dynamics in solar EUV images

Context. Movies of the solar atmosphere reveal motion and variations in brightness. In particular, sequences of coronal images exhibit the plane- of- the- sky component of the velocity combined with other variations in the signal. The present work analyses solar extremeultraviolet images, as recorded by the Extreme ultraviolet Imaging Telescope ( EIT) on board the Solar and Heliospheric Observatory ( SoHO) and by the Transition Region and Coronal Explorer ( TRACE). Aims. Our aim is to simultaneously estimate the apparent motion vector and the variation in brightness from two successive images. Methods. We present a multiscale optical- flow algorithm derived from a local gradient- based technique that estimates the deformation parameters. Our algorithm is symmetric in the sense that it computes the exact same estimation if the two images are swapped. This also regularises the optical flow when two local image patterns do not match, e. g. in case of temporal sub- sampling. Independently our algorithm regularises the optical flow against aperture effects occurring typically along coronal loops. Results. We demonstrate a new di. erential rotation measurement technique and the identification of coronal events as regions exhibiting a significant brightness variation or an outstanding velocity field. Space weather services have motivated this study. The range of potential interests includes, but also extends beyond, on- disc signatures of flares and coronal mass ejection ( CME). It encompasses, for example, studies of bright points and filaments, coronal seismology, and EIT wave investigations.

A three‐dimensional analysis of global propagation of magnetohydrodynamic (MHD) waves in a structured solar atmosphere

A three‐dimensional compressible magnetohydrodynamic (MHD) simulation in the spherical coordinates is presented for the propagation of MHD waves in a structured solar atmosphere by using the fractional step method. An initial magnetic field profile that has the characteristics of the open configuration at the pole and closed configuration at the equator is used. The initial state of the solar atmosphere consists of the chromosphere and corona; thus the transition region from the photosphere is approximated. The purpose of this study is to understand the nature of propagation MHD waves in a two‐layer solar atmosphere. Thus a pressure pulse is added at the bottom of the chromosphere (i.e., r = 1 Rs). We compare the response of this case with one in which we introduce the pulse at the bottom of the corona (i.e., r = 1.018 Rs). We find that (1) where the disturbance is initiated at the bottom of the chromosphere, a pair of fast mode waves and a slow mode MHD wave are generated. The pair of fast mode MHD waves are expansion waves and their propagation speeds increase with increasing height, (2) if the disturbance is initiated at the bottom of the corona, (r = 1.018 Rs), we observed a pair of fast and slow mode MHD waves propagating upward to the corona and another pair propagating downward toward the photosphere, respectively. We suggest that the model has the potential to be used to show the correspondence between observed flare initiation Moreton waves of the chromosphere and Extreme Ultraviolet Imaging Telescope waves in the low corona.

On the Origins of Solar EIT Waves

Abstract : Approximately half of the large-scale coronal waves identified in images obtained by the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory from 1997 March to 1998 June were associated with small solar flares with soft X-ray intensities below C class. The probability of a given flare of this intensity having an associated EIT wave is low. For example, of ~8,000 B-class flares occurring during this 15 month period, only 1% were linked to EIT waves. These results indicate the need for a special condition that distinguishes flares with EIT waves from the vast majority of flares that lack wave association. Various lines of evidence, including the fact that EIT waves have recently been shown to be highly associated with coronal mass ejections (CMEs), suggest that this special condition is a CME. A CME is not a sufficient condition for a detectable EIT wave, however, because we calculate that 5 times as many front-side CMEs as EIT waves occurred during this period, after taking the various visibility factors for both phenomena into account. In general, EIT wave association increases with CME speed and width.

A Full View of EIT Waves

Early observations by the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory indicated that propagating diffuse fronts, now conventionally referred to as can often be seen on the solar disk with a propagation velocity several times smaller than that of Hα Moreton waves. They are almost always associated with coronal mass ejections. We have previously confirmed the existence of such a phenomenon with numerical simulations, which indicate that there does exist a slower moving much behind the coronal counterpart of the Hα Moreton wave. Further observations have disclosed many new features of the EIT waves: the waves stop near the separatrix between active regions, sometimes they experience acceleration from the active region to the quiet region, and so on. Here we report on MHD simulations performed to demonstrate how the typical features of EIT waves can all be accounted for within our theoretical model, in which the EIT waves are thought to be formed by successive stretching or opening of closed field lines driven by an erupting flux rope. The relationship between EIT waves, Hα Moreton waves, and type II radio bursts is discussed, with an emphasis on reconciling the discrepancies among different views of the wave phenomenon.

Coronal Shocks of November 1997 Revisited: The Cme?Type II Timing Problem

We re-examine observations bearing on the origin of metric type II bursts for six impulsive solar events in November 1997. Previous analyses of these events indicated that the metric type IIs were due to flares (either blast waves or ejecta). Our point of departure was the study of Zhang et al. (2001) based on the Large Angle and Spectrometric Coronagraph’s C1 instrument (occulting disk at 1.1 R0) that identified the rapid acceleration phase of coronal mass ejections (CMEs) with the rise phase of soft X-ray light curves of associated flares. We find that the inferred onset of rapid CME acceleration in each of the six cases occurred 1–3 min before the onset of metric type II emission, in contrast to the results of previous studies for certain of these events that obtained CME launch times ∼25–45 min earlier than type II onset. The removal of the CME-metric type II timing discrepancy in these events and, more generally, the identification of the onset of the rapid acceleration phase of CMEs with the flare impulsive phase undercuts a significant argument against CMEs as metric type II shock drivers. In general, the six events exhibited: (1) ample evidence of dynamic behavior [soft X-ray ejecta, extreme ultra-violet imaging telescope (EIT) dimming onsets, and wave initiation (observed variously in Hα, EUV, and soft X-rays)] during the inferred fast acceleration phases of the CMEs, consistent with the cataclysmic disruption of the low solar atmosphere one would expect to be associated with a CME; and (2) an organic relationship between EIT dimmings (generally taken to be source regions of CMEs) and EIT waves (which are highly associated with metric type II bursts) indicative of a CME-driver scenario. Our analysis indicates that the broad (∼90○ to halo) CMEs observed in the outer LASCO coronagraphs for these impulsive events began life as relatively small-scale structures, with angular spans of ∼15○ in the low corona. A review of on-going work bearing on other aspects (than timing) of the question of the origin of metric type II bursts (CME association; connectivity of metric and decametric-hectometric type II shocks; spatial relationship between CMEs and metric shocks) leads to the conclusion that CMEs remain a strong candidate to be the principal/sole driver of metric type II shocks vis-a-vis flare blast waves/ejecta.

Filament Oscillations and Moreton Waves Associated with EIT Waves

In this paper we compare EUV Imaging Telescope (EIT) waves with simultaneous phenomena seen in Hα in order to address the question of what an EIT wave is. We surveyed the events associated with solar flares larger than GOES M-class in 1999-2002. The Hα data are taken with the Flare-monitoring Telescope (FMT) at the Hida Observatory of Kyoto University. Among 14 simultaneous observations of EIT waves and Hα, 11 were found to have filament eruptions, three were associated with Moreton waves, and one was found to have only filament oscillations. This shows that we cannot see clear wave fronts in Hα even if EIT waves exist, but that it is possible to recognize invisible waves by means of filament oscillations. The nature of filament oscillations and Moreton waves associated with EIT waves is examined in detail, and it is found that the filament oscillations were caused by EIT waves.