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

First Evidence of Coexisting EIT Wave and Coronal Moreton Wave from SDO/AIA Observations

"EIT waves" are a globally propagating wavelike phenomenon. They were often interpreted as a fast-mode magnetoacoustic wave in the corona, despite various discrepancies between the fast-mode wave model and observations. To reconcile these discrepancies, we once proposed that "EIT waves" are apparent propagation of the plasma compression due to successive stretching of the magnetic field lines pushed by the erupting flux rope. According to this model, an "EIT wave" should be preceded by a fast-mode wave, which however was rarely observed. With the unprecedented high cadence and sensitivity of the {\em Solar Dynamics Observatory} ({\em SDO}) observations, we discern a fast-moving wave front with a speed of 560 km s$^{-1}$, ahead of an "EIT wave", which had a velocity of $\sim 190$ km s$^{-1}$, in the "EIT wave" event on 2010 July 27. The results, suggesting that "EIT waves" are not fast-mode waves, confirm the prediction of our fieldline stretching model for "EIT wave". In particular, it is found that the coronal Moreton wave was $\sim 3$ times faster than the "EIT wave" as predicted.Comment: 13 pages, 4 figures, submitted for publication in ApJ Letter

Ubiquitous rotating network magnetic fields and EUV cyclones in the quiet Sun

We present the {\it Solar Dynamics Observatory} (SDO) Atmospheric Imaging Assembly (AIA) observations of EUV cyclones in the quiet Sun. These cyclones are rooted in the Rotating Network magnetic Fields (RNFs). Such cyclones can last several to more than ten hours, and, at the later phase, they are found to be associated with EUV brightenings (microflares) and even EUV waves. SDO Helioseismic and Magnetic Imager (HMI) observations show an ubiquitous presence of the RNFs. Using HMI line-of-sight magnetograms on 2010 July 8, we find 388 RNFs in an area of 800$\times$980 square arcseconds near the disk center where no active region is present. The sense of rotation shows a weak hemisphere preference. The unsigned magnetic flux of the RNFs is about 4.0$\times10^{21}$ Mx, or 78% of the total network flux. These observational phenomena at small scale reported in this letter are consistent with those at large scale in active regions. The ubiquitous RNFs and EUV cyclones over the quiet Sun may suggest an effective way to heat the corona.Comment: 13 pages, 5 figures; accepted for publication in ApJ

'EUV Waves' are Waves: First Quadrature Observations of an EUV Wave from STEREO

The nature of CME-associated low corona propagating disturbances, 'EUV waves', has been controversial since their discovery by EIT on \textit{SOHO}. The low cadence, single viewpoint EUV images and the lack of simultaneous inner corona white light observations has hindered the resolution of the debate on whether they are true waves or just projections of the expanding CME. The operation of the twin EUV imagers and inner corona coronagraphs aboard \textsl{STEREO} has improved the situation dramatically. During early 2009, the \textsl{STEREO} Ahead (STA) and Behind (STB) spacecraft observed the Sun in quadrature having an $\approx 90^\circ$ angular separation. An EUV wave and CME erupted from active region 11012, on February 13, when the region was exactly at the limb for STA and hence at disk center for STB. The \textit{STEREO} observations capture the development of a CME and its accompanying EUV wave not only with high cadence but also in quadrature. The resulting unprecentented dataset allowed us to separate the CME structures from the EUV wave signatures and to determine without doubt the true nature of the wave. It is a fast-mode MHD wave after all!Comment: ApJL, 2009, submitte

High cadence observations of a global coronal wave by EUVI/STEREO

We report a large-scale coronal wave (so-called "EIT wave") observed with high cadence by EUVI onboard STEREO in association with the GOES B9.5 flare and double CME event on 19 May 2007. The EUVI instruments provide us with the unprecedented opportunity to study the {\it dynamics} of flare/CME associated coronal waves imaged in the extreme ultraviolet. The coronal wave under study reveals deceleration, indicative of a freely propagating MHD wave. Complementary analysis of the associated flare and erupting filament/CME hint at wave initiation by the CME expanding flanks, which drive the wave only over a limited distance. The associated flare is very weak and occurs too late to account for the wave initiation.Comment: Astrophysical Journal Letters (in press

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

Spectroscopic Analysis of an EIT Wave/Dimming Observed by Hinode/EIS

EIT waves are a wavelike phenomenon propagating outward from the coronal mass ejection (CME) source region, with expanding dimmings following behind. We present a spectroscopic study of an EIT wave/dimming event observed by Hinode/EIS. Although the identification of the wave front is somewhat affected by the pre-existing loop structures, the expanding dimming is well defined. We investigate the line intensity, width, and Doppler velocity for 4 EUV lines. In addition to the significant blue shift implying plasma outflows in the dimming region as revealed in previous studies, we find that the widths of all the 4 spectral lines increase at the outer edge of the dimmings. We illustrate that this feature can be well explained by the field line stretching model, which claims that EIT waves are apparently moving brightenings that are generated by the successive stretching of the closed field lines.Comment: 11 pages, 7 figure

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.Comment: 34 pages, 19 figure

Global Coronal Waves

After the {\em Solar and Heliospheric Observatory} ({\em SOHO}) was launched in 1996, the aboard Extreme Ultraviolet Imaging Telescope (EIT) observed a global coronal wave phenomenon, which was initially named "EIT wave" after the telescope. The bright fronts are immediately followed by expanding dimmings. It has been shown that the brightenings and dimmings are mainly due to plasma density increase and depletion, respectively. Such a spectacular phenomenon sparked long-lasting interest and debates. The debates were concentrated on two topics, one is about the driving source, and the other is about the nature of this wavelike phenomenon. The controversies are most probably because there may exist two types of large-scale coronal waves that were not well resolved before the {\em Solar Dynamics Observatory} ({\em SDO}) was launched: one is a piston-driven shock wave straddling over the erupting coronal mass ejection (CME), and the other is an apparently propagating front, which may correspond to the CME frontal loop. Such a two-wave paradigm was proposed more than 13 years ago, and now is being recognized by more and more colleagues. In this paper, we review how various controversies can be resolved in the two-wave framework and how important it is to have two different names for the two types of coronal waves.Comment: 14 pages, 6 figures, a review pape