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IRIS Si IV line profiles: An indication for the plasmoid instability during small-scale magnetic reconnection on the Sun  

Davina Innes   Submitted: 2015-10-10 04:55

Our understanding of the process of fast reconnection has undergone a dramatic change in the last 10 years driven, in part, by the availability of high-resolution numerical simulations that have consistently demonstrated the break-up of current sheets into magnetic islands, with reconnection rates that become independent of Lundquist number, challenging the belief that fast magnetic reconnection in flares proceeds via the Petschek mechanism that invokes pairs of slow-mode shocks connected to a compact diffusion region. The reconnection sites are too small to be resolved with images but these reconnection mechanisms, Petschek and the plasmoid instability, have reconnection sites with very different density and velocity structures and so can be distinguished by high-resolution line-profiles observations. Using IRIS spectroscopic observations we obtain a survey of typical line profiles produced by small-scale events thought to be reconnection sites on the Sun. Slit-jaw images are used to investigate the plasma heating and re-configuration at the sites. A sample of 15 events from two active regions is presented. The line profiles are complex with bright cores and broad wings extending to over 300 km s-1. The profiles can be reproduced with the multiple magnetic islands and acceleration sites that characterise the plasmoid instability but not by bi-directional jets that characterise the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.

Authors: Davina Innes, LiJia Guo, YiMin Huang, Amitava Bhattacharjee
Projects: None

Publication Status: ApJ -accepted
Last Modified: 2015-10-15 07:27
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Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities  

Davina Innes   Submitted: 2012-02-24 09:09

Context. A prominence eruption on 7 June 2011 produced spectacularcurtains of plasma falling through the lower corona. At thesolar surface they created an incredible display of extremeultraviolet brightenings.
Aims. To identify and analyze some of the local instabilities whichproduce structure in the falling plasma.
Methods. The structures were investigated using SDO/AIA 171A and193A images in which the falling plasma appeared dark againstthe bright coronal emission.
Results. Several instances of the Rayleigh-Taylor instability wereinvestigated. In two cases the Alfvén velocity associated withthe dense plasma could be estimated from the separation of theRayleigh-Taylor fingers. A second type of feature, which has theappearance of self-similar branching horns was discussed.

Authors: D.E. Innes, R. H. Cameron, L. Fletcher, B. Inhester, S.K. Solanki
Projects: SDO-AIA

Publication Status: submitted A&A
Last Modified: 2012-02-24 12:12
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EUV jets, type III radio bursts and sunspot waves investigated using SDO/AIA observations  

Davina Innes   Submitted: 2011-06-17 03:23

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: D.E. Innes, R.H. Cameron, S.K. Solanki
Projects: None

Publication Status: Accepted by A&A Letters
Last Modified: 2011-06-17 11:40
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STEREO quadrature observations of coronal dimming at the onset of mini-CMEs  

Davina Innes   Submitted: 2010-05-12 04:28

Context: Using unique quadrature observations with the two STEREO spacecraft, we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs they are believed to indicate the opening up of the coronal magnetic fields at the start of the eruption. Aims: It is to determine whether coronal dimming seen in small-scale eruptions starts before or after chromospheric plasma ejection. Methods: One STEREO spacecraft obtained high cadence, 75 s, images in the He II 304A channel, and the other simultaneous images in the Fe IX/FeX 171A channel. We concentrate on two well-positioned chromospheric eruptions that occurred at disk center in the 171A images, and on the limb in 304A. One was in the quiet Sun and the other was in an equatorial coronal hole. We compare the timing of chromospheric eruption seen in the 304A limb images with the brightenings and dimmings seen on disk in the 171A images. Further we use off-limb images of the low frequency 171A power to infer the coronal structure near the eruptions. Results: In both the quiet Sun and the coronal hole eruption, on disk 171A dimming was seen before the chromospheric eruption, and in both cases it extends beyond the site of the chromospheric eruption. The quiet Sun eruption occurred on the outer edge of the enclosing magnetic field of a prominence and may be related to a small disruption of the prominence just before the 171A dimming. Conclusions: These small-scale chromospheric eruptions started with a dimming in coronal emission just like their larger counterparts. We therefore suggest that a fundamental step in triggering them was the removal of overlying coronal field.

Authors: Davina Innes, Scott McIntosh, Anna Pietarila
Projects: None

Publication Status: To appear in A&A Letters
Last Modified: 2010-05-12 10:37
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Quiet Sun mini-CMEs activated by supergranular flows  

Davina Innes   Submitted: 2008-11-17 09:27

Context: The atmosphere of the quiet Sun is controlled by photospheric flows sweeping up concentrations of mixed polarity magnetic field. Along supergranule boundaries and junctions, there is a strong correlation between magnetic flux and bright chromospheric and transition region emission. Aim: The aim is to investigate the relationship between photospheric flows and small flare-like brightenings seen in Extreme Ultraviolet images. Method: We describe observations of small eruptions seen in quiet Sun images taken with the Extreme UltraViolet Imager (EUVI) on STEREO. The photospheric flows during the eruption build-up phase are investigated by tracking granules in high resolution MDI continuum images. Results: Eruptions with characteristics of small coronal mass ejections (CMEs) occur at the junctions of supergranular cells. The eruptions produce brightening at the onset site, dark cloud or small filament ejections, and faint waves moving with plane-of-sky speeds up to 150 km s-1. In the two examples studied, they appear to be activated by converging and rotating supergranular flows, twisting small concentrations of opposite polarity magnetic field. An estimate of the occurrence rate is about 1400 events per day over the whole Sun. One third of these events seem to be associated with waves. Typically, the waves last for about 30 min and travel a distance of 100 arcsec, so at any one time they cover 1/30th of the lower corona.

Authors: Innes, D.E., Genetelli, A., Attie, R., Potts, H.E.
Projects: None

Publication Status: A&A in press
Last Modified: 2008-11-17 17:32
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Abstracts by Author
IRIS Si IV line profiles: An indication for the plasmoid instability during small-scale magnetic reconnection on the Sun
Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities
EUV jets, type III radio bursts and sunspot waves investigated using SDO/AIA observations
STEREO quadrature observations of coronal dimming at the onset of mini-CMEs
Quiet Sun mini-CMEs activated by supergranular flows

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