Extreme Ultraviolet Imaging Telescope Research Articles

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.

A Comparison of CME‐Associated Atmospheric Waves Observed in Coronal (Fexii195 A) and Chromospheric (Hei10830 A) Lines

Although Moreton waves have historically been observed in Hα data, more recently waves have also been observed in chromospheric He I λ10830 images obtained at the Mauna Loa Solar Observatory. In an effort to better understand the nature of chromospheric waves and their relationship to coronal waves observed in EUV Imaging Telescope (EIT) data, we focus on two events in which waves are observed simultaneously in He I λ10830 and Fe XII λ195, lines that are formed in the chromosphere and the corona, respectively. Comparing the waves observed in these two lines allows the determination of the spatial relationship between coronal and chromospheric waves and thus aids in the understanding of the underlying physics of waves in the solar atmosphere. The main goal of this work is to begin an investigation into whether both coronal and chromospheric waves are mechanical (e.g., MHD waves) by looking at their spatial relationship. We find that the chromospheric waves in these two events are cospatial with their coronal counterparts, indicating that they are not mechanical in nature but are chromospheric imprints of mechanical waves propagating through the corona. This conclusion is based on the nature of the formation of the He I absorption line.

Signature of oscillations in coronal bright points

A detailed study of two consecutive bright points observed simultaneously with the Coronal Diagnostic Spectrometer (CDS), the Extreme ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) is presented. The analysis of the evolution of the photospheric magnetic fea- tures and their coronal counterpart shows that there is a linear dependence between the EIT Fe  195 A flux and the total magnetic flux of the photospheric bipolarity. The appearance of the coronal emission is associated with the emergence of new magnetic flux and the disappearance of coronal emission is associated with the cancellation of one of the polarities. In one of the cases the disappearance takes place ∼3-4 h before the full cancellation of the weakest polarity. The spectral data obtained with CDS show that one of the bright points experienced short time variations in the flux on a time scale of 420-650 s, correlated in the transition region lines (O  629.73 A and O  599.60 A) and also the He  584.34 A line. The coronal line (Mg  368.07 A) undergoes changes as well, but on a longer scale. The wavelet analysis of the temporal series reveals that many of these events appear in a random fashion and sometimes after periods of quietness. However, we have found two cases of an oscillatory behaviour. A sub-section of the O  temporal series of the second bright point shows a damped oscillation of five cycles peaking in the wavelet spectrum at 546 s, but showing in the latter few cycles a lengthening of that period. The period compares well with that detected in the S  933.40 A oscillations seen in another bright point observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer, which has a period of 491 s. The derived electron density in the transition region was 3 × 10 10 cm −3 with some small variability, while the coronal electron density was 5 × 10 8 cm −3 .

New insight into the blinker phenomenon and the dynamics of the solar transition region

We present, for the first time, blinker phenomena being associated with brightenings in pre-existing coronal loops registered by the Extreme-ultraviolet Imaging Telescope (EIT) in Fe xii 195 Å. The brightenings occur during the emergence of new magnetic flux as registered by the Big Bear Solar Observatory (BBSO) magnetograph. The blinkers were identified using simultaneous observations obtained with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrograph. In light of the new observational results, we present one possible theoretical interpretation of the blinker phenomenon. We suggest that the blinker activity we observe is triggered by interchange reconnection, serving to provide topological connectivity between newly emerging flux and pre-existing flux. The EIT images show the existence of loop structures prior to the onset of the blinker activity. Based on the available spatial resolution the blinker occurs within, or nearby, an existing coronal loop. The temperature interfaces created in the reconnection process between the cool plasma of the newly emerging loop and the hot plasma of the existing loop are what we suggest to causes the observed activity seen in both the SUMER and CDS data. As the temperature interfaces propagate with the characteristic speed of a conduction front, they heat up the cool chromospheric plasma to coronal temperatures, an increasing volume of which brightens at transition region temperatures. We believe this new interpretation gives further qualitative understanding about the evolution of newly emerging flux on the Sun. This also provides new insight into the dynamic nature of the solar transition region.

The Inability of Steady‐Flow Models to Explain the Extreme‐Ultraviolet Coronal Loops

Recent observations from the Transition Region and Coronal Explorer (TRACE) and the EUV Imaging Telescope (EIT) show that warm (T ≈ 1-1.5 MK) EUV coronal loops in active regions generally have enhanced densities, enhanced pressure scale heights, and flat filter ratio (temperature) profiles in comparison with the predictions of static-equilibrium theory. It has been suggested that mass flows may explain these discrepancies. We investigate this conjecture using one-dimensional hydrodynamic simulations of steady flows in coronal loops. The flows are driven by asymmetric heating that decreases exponentially along the loop from one footpoint to the other. We find that a sufficiently large heating asymmetry can produce density enhancements consistent with a sizable fraction of the observed loops, but that the pressure scale heights are smaller than the corresponding gravitational scale heights, and that the filter ratio profiles are highly structured, in stark contrast to the observations. We conclude that most warm EUV loops cannot be explained by steady flows. It is thus likely that the heating in these loops is time dependent.

Evidence for Gradual External Reconnection before Explosive Eruption of a Solar Filament

We observe a slowly-evolving quiet region solar eruption of 1999 April 18, using EUV images from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), and soft X-ray images from the Soft X-ray Telescope (SXT) on Yohkoh. Using difference images, where an early image is subtracted from later images, we examine dimmings and brightenings in the region for evidence of the eruption mechanism. A filament rose slowly at about 1 kilometer per second for six hours before being rapidly ejected at about 16 kilometers per second, leaving flare brightenings and post-flare loops in its wake. Magnetograms from the Michelson Doppler Imager (MDI) on SOHO show that the eruption occurred in a large quadrupo1ar magnetic region, with the filament located on the neutral line of the quadrupole's central inner lobe, between the inner two of the four polarity domains. In step with the slow rise, subtle EIT dimmings commence and gradually increase over the two polarity domains on one side of the filament, i.e. in some of the loops of one of the two side lobes of the quadrupole. Concurrently, soft X-ray brightenings gradually increase in both side lobes. Both of these effects suggest heating in the side-lobe magnetic arcades, which gradually increase over several hours before the fast eruption. Also during the slow pre-eruption phase, SXT dimmings gradually increase in the feet and legs of the central lobe, indicating expansion of the central-lobe magnetic arcade enveloping the filament. During the rapid ejection, these dimmings rapidly grow in darkness and in area, especially in the ends of the sigmoid field that erupts with the filament, and flare brightenings begin underneath the fast-moving but still low-altitude filament. We consider two models for explaining the eruption: breakout, which says that reconnection occurs high above the filament prior to eruption, and tether cutting, which says that the eruption is unleashed by reconnection beneath the filament. The pre-eruption evolution is consistent with gradual breakout that led to (and perhaps caused) the fast eruption. Tether-cutting reconnection below the filament begins early in the rapid ejection, but our data are not complete enough to determine whether this reconnection began early enough to be the cause of the fast-phase onset. Thus, our observations are consistent with gradual breakout reconnection causing the long slow rise of the filament, but allow the cause of the sudden onset of the explosive fast phase to be either a jump in the breakout reconnection rate or the onset of runaway tether-cutting reconnection, or both.

A Reexamination of the Evidence for Reconnection Inflow

In the flare event of 1999 March 18, a threadlike structure observed in EUV Imaging Telescope images was found to move inward and collapse to an X-shaped configuration below the ejecta, strongly suggestive of the occurrence of magnetic reconnection. On the basis of the numerical results of a coronal mass ejection (CME) flare model, a similar threadlike structure in the Fe XII 195 A image is reproduced in this Letter. It is found that, as in the observations, the thread experiences an outward motion in the preflare phase, which is followed by an inward motion. Our simulation suggests that its formation and outward motion in the preflare phase result from the CME expansion; after the onset of the flare, the threadlike structure is always located on the upstream side of the interface between the reconnection inflow and outflow. Its apparent inward motion, which is several times slower than the in situ reconnection inflow, is mainly attributed to the rising motion of the reconnection X-point.

Height correction in the measurement of solar differential rotation determined by coronal bright points

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse solar differential rotation by tracing coronal bright points for the period June 4, 1998 to May 22, 1999. A method for the simultaneous determination of the true solar synodic rotation velocity and the height of the tracers is applied to data sets analysed with interactive and automatic methods. The calculated height of coronal bright points is on average 8000–12000 km above the photosphere. Corrected rotation velocities are transformed into sidereal ones and compared with results from the literature, obtained with various methods and tracers. The differential rotation profile determined by coronal bright points with the interactive method corresponds roughly to the profile obtained by correlating photospheric magnetic fields and the profile obtained from the automatic method corresponds roughly to the rotation of sunspot groups. This result is interpreted in terms of the differences obtained in the latitudinal distribution of coronal bright points using the two methods.

Solar “EIT Waves” — What are They?

There has been substantial interest in the coronal wave phenomena since its discovery several years ago using the EUV Imaging Telescope onboard SOHO (Thompson et al. 1999). One of the first explanations was that finally the coronal part of the Moreton wave had been discovered. Since then, there have been a number of results giving different explanations. We will discuss these in this paper along with the latest results on coronal waves from a spectroscopic perspective.

Tether‐cutting Energetics of a Solar Quiet‐Region Prominence Eruption

We study the morphology and energetics of a slowly evolving quiet-region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fexv EUV 284 Angstrom data from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). After rising at approximately equal to l kilometer per second for about six hours, the prominence accelerates to a velocity of approximately equal to 10 kilometers per second, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominences rapid rise, flare-like brightenings occur beneath the rising prominence that might correspond to tether-cutting magnetic reconnection. These brightenings have heating requirements of up to approximately 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

H Dimmings Associated with the X1.6 Flare and Halo Coronal Mass Ejection on 2001 October 19

As the optical counterpart of coronal dimmings, we present the first direct evidence of Halpha associated with the X1.6 flare, filament eruption, and coronal mass ejection (CME) that occurred on 2001 October 19. As two main Halpha flare ribbons parallel to the erupting filament slowly separated, two other ribbons first quickly expanded near the feet of the filament, then their interiors substantially dimmed and became darker than the preflare brightness. This formed two dimming regions with sharp bright edges. The two Halpha were different from the flare nimbus and the Halpha darkenings found by Ellison, McKenna, & Reid and Neidig et al., respectively, but similar to the coronal double dimmings studied by Sterling & Hudson. The coronal of the event were also observed in 171 Angstrom images from the Transition Region and Coronal Explorer (TRACE). The 171 Angstrom and Halpha showed very similar evolutions and occurred at the same two sites with opposite magnetic polarities. They formed in the course of the filament eruption, and after the eruption, some 171 Angstrom loops of the postflare arcade developed to connect their inner edges while some loops with footpoints near their outer edges disappeared. Observations from the Solar and Heliospheric Observatory Extreme-ultraviolet Imaging Telescope (SOHO/EIT) showed more extended associated with the eruption. Only a small portion of EIT was cospatial with the Halpha dimmings, while the major other portion was associated with the disappeared 171 Angstrom loops. These joint observations are interpreted by using the flux rope model of CMEs, in which the Halpha represent the evacuated feet of the erupted flux rope. The mass supply of the halo CME could be coming from both the erupted flux rope itself and the lost material close to the flare footpoint regions as shown by Halpha dimmings.

Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High-Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (>25 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time, the underlying flare loops shrank at ~9 km s-1. The temperature of the underlying loops increased toward higher altitudes, while the temperature of the coronal source increased toward lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ~8 km s-1, and the coronal source moved outward at ~300 km s-1, indicating an upward expansion of the current sheet. About 30 minutes later, postflare loops seen in the Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope 195 A passband rose at ~10 km s-1. A large coronal looplike structure, observed by the SOHO Large Angle and Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ~300 km s-1. These observations are all consistent with the continued expansion of the current sheet.

Dynamical and Physical Properties of a Post–Coronal Mass Ejection Current Sheet

In the eruptive process of the Kopp-Pneuman type, the closed magnetic field is stretched by the eruption so much that it is usually believed to be open to infinity. Formation of the current sheet in such a configuration makes it possible for the energy in the coronal magnetic field to quickly convert into thermal and kinetic energies and cause significant observational consequences, such as growing postflare/CME loop system in the corona, separating bright flare ribbons in the chromosphere, and fast ejections of the plasma and the magnetic flux. An eruption on 2002 January 8 provides us a good opportunity to look into these observational signatures of and place constraints on the theories of eruptions. The event started with the expansion of a magnetic arcade over an active region, developed into a coronal mass ejection (CME), and left some thin streamer-like structures with successively growing loop systems beneath them. The plasma outflow and the highly ionized states of the plasma inside these streamer-like structures, as well as the growing loops beneath them, lead us to conclude that these structures are associated with a magnetic reconnection site, namely, the current sheet, of this eruptive process. We combine the data from the Ultraviolet Coronagraph Spectrometer, Large Angle and Spectrometric Coronagraph Experiment, EUV Imaging Telescope, and Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory, as well is from the Mauna Loa Solar Observatory Mark IV K-coronameter, to investigate the morphological and dynamical properties of this event, as well as the physical properties of the current sheet. The velocity and acceleration of the CME reached up to 1800 km/s and 1 km/sq s, respectively. The acceleration is found to occur mainly at the lower corona (<2.76 Solar Radius). The post-CME loop systems showed behaviors of both postflare loops (upward motion with decreasing speed) and soft X-ray giant arches (upward motion with constant speed, or acceleration) according to the definition of Svestka. In the current sheet, the presence of highly ionized ions, such as Fe(+17) and Ca(+13), suggests temperature as high as (3-4) x 10(exp 6) K, and the plasma outflows have speeds ranging from 300 to 650 km/s. Absolute elemental abundances in the current sheet show a strong first ionization potential effect and have values similar to those found in the active region streamers. The magnetic field strength in the vicinity of the current sheet is found to be of the order of 1 G.

Sizes of quiet Sun transition region structures

We studied the morphology of the transition region of the quiet Sun, with data obtained by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer (SUMER) and the Extreme-Ultraviolet Imaging Telescope (EIT), in September 1996. We analyzed lines emitted in the chromosphere, the low transition region and the low corona. We com- puted the mean autocorrelation function for the radiance images in order to estimate the characteristic size of the features present in the transition region. Moreover, we calculated autocorrelation functions for the dopplergrams and line width images deduced from the SUMER data. In addition to the line core of the C iv line, we investigated a broader tail component, whose origin is still unclear. We found that the size of the bright radiance features is always larger than the size of the structures of the dopplergrams and Doppler widths, at any altitude. The network features seem to diminish at a temperature around 10 5 K, due to the thermodynamic properties of the transition region. The mean size of the structures of the tail component radiance is smaller than the one of the core radiance.

Spatial and Temporal Properties of Hot and Cool Coronal Loops

A suite of images from the XUV Doppler Telescope (XDT), the Yohkoh Soft X-ray Telescope (SXT), and the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) allow us to see the whole (T > 1 MK) temperature evolution of coronal loops. The detailed morphological comparison of an active region shows that hot loops seen in SXT (T > 3 MK) and cool loops seen in the the EIT 195 A band (T ~ 1.5 MK) are located in almost alternating manner. The anticoincidence of the hot and the cool loops is conserved for a duration much longer than the estimated cooling timescale. However, both hot and cool loops have counterparts in the intermediate-temperature images. The cross-correlation coefficients are higher for neighboring temperature pairs and lower for pairs with larger temperature differences. These results suggest that loops are not isothermal but rather have a differential emission measure distribution of modest but finite width that peaks at different temperatures for different loops.

Properties of the solar velocity field indicated by motions of coronal bright points

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse properties of the solar velocity field by tracing coronal bright points from June 4, 1998 to May 22, 1999. Rotation velocity residuals, meridional motions and their relationship are investigated. Zones of slow and fast rotation found in motions of coronal bright points are consistent with the pattern of torsional oscillations, indicating that the statistical velocity pattern of bright point motions reflects the large-scale plasma flows. A complex pattern of meridional motion is deduced: The equatorward flows are found to dominate at low (B 40 ◦ ) latitudes, whereas at mid-latitudes (B ≈ 10 ◦ −40 ◦ ) a poleward flow is inferred. The complete data set shows no significant correlation between rotation residuals and meridional motions. However, when a subsample of coronal bright points including only the structures (predominantly young bright points) is considered, a statistically significant correlation is found. On average, faster tracers show equatorward motion and the slower ones show poleward motion. Such a segregation is reflected in a statistically significant covariance of the rotation residuals and meridional velocities in the order of −1000 m 2 s −2 , revealing an equatorward transport of angular momentum. The negative value of the covariance is provided by the high velocity tail in the velocity distribution of point-like structures, representing less than 15% of the population. The latitude dependence of the covariance can be expressed as Q = −62 B + 200 m 2 s −2 covering the range B = 0 ◦ -60 ◦ .

Sympathetic Coronal Mass Ejections

We address the question whether there exist sympathetic coronal mass ejections (CMEs), which take place almost simultaneously in different locations with a certain physical connection. For this study, the following three investigations are performed. First, we have examined the waiting-time distribution of the CMEs that were observed by the SOHO Large Angle and Spectrometric Coronagraph (LASCO) from 1999 February to 2001 December. The observed waiting-time distribution is found to be well approximated by a time-dependent Poisson distribution without any noticeable overabundance at short waiting times. Second, we have investigated the angular difference distribution of successive CME pairs to examine their spatial correlations. A remarkable overabundance relative to background levels is found within 10° of the position angle difference, which supports the existence of quasi-homologous CMEs that occur sequentially in the same active region. Both of the above results indicate that sympathetic (interdependent) CMEs are far less frequent than independent CMEs. Third, we have examined the EUV Imaging Telescope running difference images and the LASCO images of quasi-simultaneous CME pairs and found a candidate sympathetic CME pair, of which the second CME may be initiated by the eruption of the first CME. Possible mechanisms of the sympathetic CME triggering are discussed.

Spectroscopic characteristics of polar plumes

Extreme ultraviolet observations of plumes in polar coronal holes are presented and their spectroscopic signatures discussed. The study focuses on the base of plumes seen on the disk of the Sun with the Coronal DiagnosticSpectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) satellite. Spectroscopic diagnostic techniques are applied to characterise the plumes in terms of density, temperature, emission measure and element abundance. Attention is drawn to the particular limitations of some of the techniques when applied to plume structures. In particular, we revisit the Widing & Feldman (1992) findings of a plume having a large first ionization potential (FIP) effect of 10, showing that instead the Skylab data are consistent with no FIP effect. We present for the first time CDS-GIS (grazing incidence spectrometer) observations of a plume. These observations have been used to confirm the results obtained from normal incidence (NIS) observations. We find that polar plumes exhibit the same characteristics as the Elephant's Trunk equatorial plume. The most striking characteristic of the plume bases is that they are near-isothermal with a peak emission measure at transition region temperatures ≃8 x 10 5 K. At these temperatures, plumes have averaged densities N e ≃ 1.2 x 10 9 cm-3, about twice the value of the surrounding coronal hole region. Element abundances in the plumes are found to be close to photospheric, with the exception of neon which appears to be depleted by 0.2 dex relative to oxygen. The absence of a significant FIP effect in plumes is consistent with fast solar wind plasma, although it is not sufficient to prove a link between the two. Finally, we present a comparison between GIS spectra and the SOHO EIT (EUV Imaging Telescope) broad-band images, showing that temperatures derived from the EIT ratio technique are largely overestimated, for plumes and coronal holes. This is partly due to the fact that the so called Fe XII 195 A and Fe XV 284 A filters are not isothermal, and in coronal holes and plumes lower-temperature lines dominate the EIT signal.

Correlation between halo coronal mass ejections and solar surface activity

We survey all the coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) aboard the Solar & Heliospheric Observatory (SOHO) in the interval from 1997 to 2001, and select 197 frontside halo CMEs whose associated near-surface activity could be clearly identified from SOHO EUV Imaging Telescope (EIT) and other space-borne and ground-based observations. A statistical analysis has been made with the emphasis on the correlation between CMEs and solar surface activity. We have found in our sample that all the CMEs were accompanied by local brightening in the CME source regions in EIT and/or H α images. However, if we only classify the events with a brightening increase more than 50% above the quiet background as flares, as measured from EIT data, approximately 88% of the earth-directed CMEs are associated with flares and more than 94% are associated with eruptive filaments. With regard to the locations of CME source regions, there are about 79% CMEs initiating from active regions, while only 21% originate outside active regions. We evaluate the symmetry between CMEs and associated solar surface activity in spatial distributions. We find that in about half the events the surface activity is asymmetric with respect to the CME. For the flares having GOES X-ray records in this study, allowing for the uncertainties specified by the data cadence, 59% of the CME initiations seem to precede the flare onset in the CME source regions, while 41% are preceded by flare onsets. The statistical correlation seems to suggest that CMEs are intrinsically related to surface activity.

Soft X-ray observation of a large-scale coronal wave and its exciter

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfven speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3.