Solar Coronal Structures and Stray Light in TRACE

Using the 2004 Venus transit of the Sun to constrain a semi-empirical point-spread function for the TRACE EUV solar telescope, we have measured the effect of stray light in that telescope. We find that 43% of 171A EUV light that enters TRACE is scattered, either through diffraction off the entrance filter grid or through other nonspecular effects. We carry this result forward, via known-PSF deconvolution of TRACE images, to identify its effect on analysis of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the effect of visible haze in the TRACE 171A images, enhances bright features, and reveals that the smooth background component of the corona is considerably less bright (and hence much more rarefied) than commonly supposed. Deconvolution reveals that some prior conlclusions about the Sun appear to have been based on stray light in the images. In particular, the diffuse background "quiet corona" becomes consistent with hydrostatic support of the coronal plasma; feature contrast is greatly increased, possibly affecting derived parameters such as the form of the coronal heating function; and essentially all existing differential emission measure studies of small features appear to be affected by contamination from nearby features. We speculate on further implications of stray light for interpretation of EUV images from TRACE and similar instruments, and advocate deconvolution as a standard tool for image analysis with future instruments such as SDO/AIA.Comment: Accepted by APJ; v2 reformatted to single-column format for online readabilit

The Wave Properties of Coronal Bright Fronts Observed Using SDO/AIA

Coronal bright fronts (CBFs) are large scale wavefronts that propagate though the solar corona at hundreds of kilometers per second. While their kinematics have been studied in detail, many questions remain regarding the temporal evolution of their amplitude and pulse width. Here, contemporaneous high cadence, multi-thermal observations of the solar corona from the Solar Dynamic Observatory (SDO) and Solar TErrestrial RElations Observatory (STEREO) spacecraft are used to determine the kinematics and expansion rate of a CBF wavefront observed on 2010 August 14. The CBF was found to have a lower initial velocity with weaker deceleration in STEREO observations compared to SDO (~340 km/s and -72 m/s/s as opposed to ~410 km/s and -279 m/s/s). The CBF kinematics from SDO were found to be highly passband-dependent, with an initial velocity ranging from 379+/-12 km/s to 460+/-28 km/s and acceleration ranging from -128+/-28 m/s/s to -431+/-86 m/s/s in the 335A and 304A passbands respectively. These kinematics were used to estimate a quiet coronal magnetic field strength range of ~1-2 G. Significant pulse broadening was also observed, with expansion rates of ~130 km/s (STEREO) and ~220 km/s (SDO). By treating the CBF as a linear superposition of sinusoidal waves within a Gaussian envelope, the resulting dispersion rate of the pulse was found to be ~8-13 Mm^2 s^-1. These results are indicative of a fast-mode magnetoacoustic wave pulse propagating through an inhomogeneous medium.Comment: 14 pages, 2 figures. Accepted for publication in The Astrophysical Journal Letter

First SDO AIA Observations of a Global Coronal EUV "Wave": Multiple Components and "Ripples"

We present the first SDO AIA observations of a global coronal EUV disturbance (so-called "EIT wave") revealed in unprecedented detail. The disturbance observed on 2010 April 8 exhibits two components: one diffuse pulse superimposed on which are multiple sharp fronts that have slow and fast components. The disturbance originates in front of erupting coronal loops and some sharp fronts undergo accelerations, both effects implying that the disturbance is driven by a CME. The diffuse pulse, propagating at a uniform velocity of 204-238 km/s with very little angular dependence within its extent in the south, maintains its coherence and stable profile for ~30 minutes. Its arrival at increasing distances coincides with the onsets of loop expansions and the slow sharp front. The fast sharp front overtakes the slow front, producing multiple "ripples" and steepening the local pulse, and both fronts propagate independently afterwards. This behavior resembles the nature of real waves. Unexpectedly, the amplitude and FWHM of the diffuse pulse decrease linearly with distance. A hybrid model, combining both wave and non-wave components, can explain many, but not all, of the observations. Discoveries of the two-component fronts and multiple ripples were made possible for the first time thanks to AIA's high cadences (10-20 s) and high signal-to-noise ratio.Comment: 7 pages, 5 figure

Deceleration and Dispersion of Large-scale Coronal Bright Fronts

One of the most dramatic manifestations of solar activity are large-scale coronal bright fronts (CBFs) observed in extreme ultraviolet (EUV) images of the solar atmosphere. To date, the energetics and kinematics of CBFs remain poorly understood, due to the low image cadence and sensitivity of previous EUV imagers and the limited methods used to extract the features. In this paper, the trajectory and morphology of CBFs was determined in order to investigate the varying properties of a sample of CBFs, including their kinematics and pulse shape, dispersion, and dissipation. We have developed a semi-automatic intensity profiling technique to extract the morphology and accurate positions of CBFs in 2.5-10 min cadence images from STEREO/EUVI. The technique was applied to sequences of 171A and 195A images from STEREO/EUVI in order to measure the wave properties of four separate CBF events. Following launch at velocities of ~240-450kms^{-1} each of the four events studied showed significant negative acceleration ranging from ~ -290 to -60ms^{-2}. The CBF spatial and temporal widths were found to increase from ~50 Mm to ~200 Mm and ~100 s to ~1500 s respectively, suggesting that they are dispersive in nature. The variation in position-angle averaged pulse-integrated intensity with propagation shows no clear trend across the four events studied. These results are most consistent with CBFs being dispersive magnetoacoustic waves.Comment: 15 pages, 18 figure

Numerical Simulation of an EUV Coronal Wave Based on the February 13, 2009 CME Event Observed by STEREO

On 13 February 2009, a coronal wave -- CME -- dimming event was observed in quadrature by the STEREO spacecraft. We analyze this event using a three-dimensional, global magnetohydrodynamic (MHD) model for the solar corona. The numerical simulation is driven and constrained by the observations, and indicates where magnetic reconnection occurs between the expanding CME core and surrounding environment. We focus primarily on the lower corona, extending out to $3R_{\odot}$; this range allows simultaneous comparison with both EUVI and COR1 data. Our simulation produces a diffuse coronal bright front remarkably similar to that observed by STEREO/EUVI at 195 \AA. It is made up of \emph{two} components, and is the result of a combination of both wave and non-wave mechanisms. The CME becomes large-scale quite low ($<$ 200 Mm) in the corona. It is not, however, an inherently large-scale event; rather, the expansion is facilitated by magnetic reconnection between the expanding CME core and the surrounding magnetic environment. In support of this, we also find numerous secondary dimmings, many far from the initial CME source region. Relating such dimmings to reconnecting field lines within the simulation provides further evidence that CME expansion leads to the "opening" of coronal field lines on a global scale. Throughout the CME expansion, the coronal wave maps directly to the CME footprint. Our results suggest that the ongoing debate over the "true" nature of diffuse coronal waves may be mischaracterized. It appears that \emph{both} wave and non-wave models are required to explain the observations and understand the complex nature of these events

Case Study of Four Homologous Large-Scale Coronal Waves Observed on 2010 April 28 and 29

On 2010 April 28 and 29, the Solar TErrestrial Relations Observatory B/Extreme Ultraviolet Imager observed four homologous large-scale coronal waves, the so-called EIT-waves, within 8 hr. All waves emerged from the same source active region, were accompanied by weak flares and faint coronal mass ejections, and propagated into the same direction at constant velocities in the range of ~220-340 km s-1. The last of these four coronal wave events was the strongest and fastest, with a velocity of 337 +/- 31 km s-1 and a peak perturbation amplitude of ~1.24, corresponding to a magnetosonic Mach number of Mms ~ 1.09. The magnetosonic Mach numbers and velocities of the four waves are distinctly correlated, suggestive of the nonlinear fast-mode magnetosonic wave nature of the events. We also found a correlation between the magnetic energy buildup times and the velocity and magnetosonic Mach number

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.Comment: 12 pages, 4 figures, accepted for publication in ApJ Letter

Are "EIT Waves" Fast-Mode MHD Waves?

We examine the nature of large-scale, coronal, propagating wave fronts (``EIT waves'') and find they are incongruous with solutions using fast-mode MHD plane-wave theory. Specifically, we consider the following properties: non-dispersive single pulse manifestions, observed velocities below the local Alfven speed, and different pulses which travel at any number of constant velocities, rather than at the ``predicted'' fast-mode speed. We discuss the possibility of a soliton-like explanation for these phenomena, and show how it is consistent with the above-mentioned aspects.Comment: to be published in the Astrophysical Journa

First observations of a dome-shaped large-scale coronal EUV wave

We present first observations of a dome-shaped large-scale EUV coronal wave, recorded by the EUVI instrument onboard STEREO-B on January 17, 2010. The main arguments that the observed structure is the wave dome (and not the CME) are: a) the spherical form and sharpness of the dome's outer edge and the erupting CME loops observed inside the dome; b) the low-coronal wave signatures above the limb perfectly connecting to the on-disk signatures of the wave; c) the lateral extent of the expanding dome which is much larger than that of the coronal dimming; d) the associated high-frequency type II burst indicating shock formation low in the corona. The velocity of the upward expansion of the wave dome ($v \sim 650$ km s$^{-1}$) is larger than that of the lateral expansion of the wave ($v \sim 280$ km s$^{-1}$), indicating that the upward dome expansion is driven all the time, and thus depends on the CME speed, whereas in the lateral direction it is freely propagating after the CME lateral expansion stops. We also examine the evolution of the perturbation characteristics: First the perturbation profile steepens and the amplitude increases. Thereafter, the amplitude decreases with r$^{-2.5 \pm 0.3}$, the width broadens, and the integral below the perturbation remains constant. Our findings are consistent with the spherical expansion and decay of a weakly shocked fast-mode MHD wave.Comment: Astrophysical Journal Letters, in pres

Coronal loop seismology using multiple transverse loop oscillation harmonics

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