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

Gemma Attrill   Submitted: 2009-09-17 12:44

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⊙; 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 A. It is made up of 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 both wave and non-wave models are required to explain the observations and understand the complex nature of these events.

Authors: Ofer Cohen, Gemma D. R. Attrill, Ward B. Manchester IV and Meredith J. Wills-Davey

Publication Status: ApJ (accepted)
Last Modified: 2009-09-18 10:31
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Hinode/XRT and STEREO observations of a diffuse coronal ''wave''-CME-dimming event  

Gemma Attrill   Submitted: 2009-09-17 12:39

We report on observations of the first diffuse coronal wave detected by Hinode/XRT. The event occurred near the West solar limb on 23 May 2007, originating from active region (AR) 10956 and was associated with a coronal mass ejection (CME) and coronal dimmings. The bright emission forming the coronal wave expanded both to the East and South of the AR. We use XRT and STEREO Behind (B) data combined with a potential magnetic field extrapolation to derive an understanding of the global magnetic field connectivity. The brightenings observed East and South of the source AR are generated by two different physical processes, due to the distinct magnetic environments in these regions. We attribute the brightening to the East of the AR to compression and channelling of the plasma along large-scale loops. The brightening to the South of the AR expands across the quiet Sun, making the southern component a likely candidate for identification as a diffuse coronal wave. We analyse the bright front in STEREO/EUVI (B) 171, 195 and 284 A images, as well as in XRT data, finding the strongest components to be largely co-spatial in all bandpasses. We also exploit the near-limb location of this event by combining STEREO/COR1 and EUVI data. Using all the data we derive a full picture of the low-coronal development of the eruption. The COR1 data show that the southern-most outer edge of the CME is progressively displaced southward during the expansion. EUVI data below the COR1 occulting disk show that the CME is significantly distorted in the low corona as a result of the associated filament eruption. The core coronal dimmings map to the core of the CME; the secondary coronal dimmings map to the CME cavity; and the diffuse coronal wave maps to the outermost edge of the expanding CME shell. The analysis of this near-limb event has important implications for understanding earlier eruptions originating from the same AR on 16, 19 and 20 May 2007.

Authors: Gemma D. R. Attrill, Alexander J. Engell, Meredith J. Wills-Davey, Paolo Grigis and Paola Testa
Projects: Hinode/XRT,STEREO

Publication Status: ApJ (in press)
Last Modified: 2009-09-18 10:37
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Automatic Detection and Extraction of Coronal Dimmings from SDO/AIA Data  

Gemma Attrill   Submitted: 2009-08-18 07:40

The volume of data anticipated from the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) highlights the necessity for the development of automatic-detection methods for various types of solar activity. Initially recognised in the 1970s it is now well established that coronal dimmings are closely associated with coronal mass ejections (CMEs), and are particularly noted as a reliable indicator of front-side (halo) CMEs, which can be difficult to detect in white-light coronagraph data. Existing work clearly demonstrates that several properties derived from analysis of coronal dimmings can give useful information about the associated CME. The development and implementation of an automated coronal dimming region detection and extraction algorithm removes visual observer bias, however unintentional, from determination of physical quantities such as spatial location, area, and volume. This allows for reproducible, quantifiable results to be mined from very large datasets. The information derived may facilitate more reliable early space weather detection, as well as offering the potential for conducting large-sample studies focused on determining the geoeffectiveness of CMEs, coupled with analysis of their associated coronal dimming signatures. In this paper we present examples of both simple and complex dimming events extracted using our algorithm, which will be run as a module for the SDO/Computer Vision Centre. Contrasting and well-studied events at both the minimum and maximum of solar cycle 23 are identified in Solar and Heliospheric Observatory/Extreme ultra-violet Imaging Telescope (SOHO/EIT) data. A more recent example extracted from Solar and Terrestrial Relations Observatory/Extreme Ultra-Violet Imager (STEREO/EUVI) data is also presented, demonstrating the potential for the anticipated application to SDO/AIA data. The detection part of our algorithm is based largely on the principle of operation of the NEMO software, namely detection of significant variation in the statistics of the EUV image pixels (Podladchikova & Berghmans, 2005). As well as running on historic datasets, the presented algorithm is capable of detecting and extracting coronal dimmings in near real-time.

Authors: G. D. R. Attrill and M. J. Wills-Davey

Publication Status: Solar Physics (in press)
Last Modified: 2009-08-18 08:39
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The Recovery of CME-Related Dimmings and the ICME's Enduring Magnetic Connection to the Sun  

Gemma Attrill   Submitted: 2008-07-22 05:07

It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal uni-directional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings. We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J. 560, 425, 2001) of interchange reconnections between "open" magnetic field and small coronal loops, is a strong candidate for the mechanism facilitating the recovery of the dimmings. This process disperses the concentration of "open" magnetic field (forming the dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic connectivity to the Sun.

Authors: G.D.R. Attrill, L. van Driel-Gesztelyi, P. Demoulin, A.N. Zhukov, K. Steed, L.K. Harra, C.H. Mandrini, J. Linker
Projects: SoHO-EIT,SoHO-MDI

Publication Status: Solar Physics (accepted).
Last Modified: 2008-09-23 20:53
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Coronal ''Wave'': a signature of the mechanism making CMEs large-scale in the low corona?  

Gemma Attrill   Submitted: 2007-04-27 10:22

We analyse one of the first coronal waves observed by STEREO/EUVI associated with a source region just behind the limb, NOAA 10940. We apply the coronal "wave" model proposed by Attrill et al. (2007) to explain the evolution of the observed bright fronts, thereby arguing that the bright fronts and dimmings are due to magnetic reconnections between the expanding CME core and surrounding magnetic structures. We offer a discussion showing that this model provides a mechanism via which CMEs, expanding from a small source region can naturally become large-scale in the low corona.

Authors: G.D.R. Attrill, L.K. Harra, L. van Driel-Gesztelyi, P. Demoulin & J.P. Wulser

Publication Status: Astronomische Nachrichten (in press)
Last Modified: 2007-04-27 10:26
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Coronal ''Wave'': Magnetic Footprint of a CME?  

Gemma Attrill   Submitted: 2007-01-26 06:03

We investigate the properties of two ''classical'' EIT coronal waves. The two source regions of the associated 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 behaviour 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.

Authors: Gemma Attrill, Louise Harra, Lidia van Driel-Gesztelyi & Pascal Demoulin
Projects: SoHO-EIT,SoHO-MDI

Publication Status: ApJL (In press)
Last Modified: 2007-01-26 10:40
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Abstracts by Author
Numerical Simulation of an EUV Coronal Wave Based on the February 13, 2009 CME Event Observed by STEREO
Hinode/XRT and STEREO observations of a diffuse coronal ''wave''-CME-dimming event
Automatic Detection and Extraction of Coronal Dimmings from SDO/AIA Data
The Recovery of CME-Related Dimmings and the ICME's Enduring Magnetic Connection to the Sun
Coronal ''Wave'': a signature of the mechanism making CMEs large-scale in the low corona?
Coronal ''Wave'': Magnetic Footprint of a CME?

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