E-Print Archive

There are 3783 abstracts currently viewable.


Search:

Advanced Search
Options
Main Page Add New E-Print Submitter
Information
Feedback
News Help/FAQ About Preferences
Manage Key Phrase
Notification
Large-scale Bright Fronts in the Solar Corona: A Review of ``EIT waves''  

Peter Gallagher   Submitted: 2010-06-01 04:01

``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_sleq200 km s-1) and comparable to the local Alfvén speed (v_Aleq1000 km s-1), they were initially interpreted as fast-mode magnetoacoustic waves (vf=(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.

Authors: Peter T. Gallagher, David M Long
Projects: None

Publication Status: Space Science Reviews, BUKS Special Issue, (In review)
Last Modified: 2010-06-01 07:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Investigating the driving mechanisms of coronal mass ejections  

Peter Gallagher   Submitted: 2010-04-07 03:20

Aims: The objective of this study was to examine the kinematics of coronal mass ejections (CMEs) using EUV and coronagraph images, and to make a quantitative comparison with a number of theoretical models. One particular aim was to investigate the acceleration profile of CMEs in the low corona. Methods: We selected two CME events for this study, which occurred on 2006 December 17 (CME06) and 2007 December 31 (CME07). CME06 was observed using the EIT and LASCO instruments on-board SOHO, while CME07 was observed using the SECCHI imaging suite on STEREO. The first step of the analysis was to track the motion of each CME front and derive its velocity and acceleration. We then compared the observational kinematics, along with the information of the associated X-ray emissions from GOES and RHESSI, with the kinematics proposed by three CME models (catastrophe, breakout and toroidal instability). Results: We found that CME06 lasted over eight hours while CME07 released its energy in less than three hours. After the eruption, both CMEs were briefly slowed down before being accelerated again. The peak accelerations during the re-acceleration phase coincided with the peak soft X-ray emissions for both CMEs. Their values were ∼60 m/s/s for CME06 and ∼600 m/s/s for CME07. CME07 reached a maximum speed of over 1000 km s-1 before being slowed down to propagate away at a constant, final speed of ∼700 km s-1. CME06 did not reach a constant speed but was moving at a small acceleration by the end of the observation. Our comparison with the theories suggested that CME06 can be best described by a hybrid of the catastrophe model and breakout model while the characteristics of CME07 were most consistent with the breakout model. Based on the catastrophe model, we deduced that the reconnection rate in the current sheet for CME06 was intermediate, the onset of its eruption occurred at a height of ∼200 Mm, and the Alfvén speed and the magnetic field strength at this height were approximately 130?250 km s-1 and 7 Gauss, respectively.

Authors: Chia-Hsien Lin, Peter T. Gallagher, and Claire L. Raftery
Projects: RHESSI,SoHO-EIT,SoHO-LASCO,STEREO

Publication Status: A&A (accepted)
Last Modified: 2010-04-07 13:37
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Constraining 3D Magnetic Field Extrapolations Using The Twin Perspectives of STEREO  

Peter Gallagher   Submitted: 2010-04-07 03:12

The 3D magnetic topology of a solar active region (NOAA 10956) was reconstructed using a linear force-free field extrapolation constrained using the twin perspectives of STEREO. A set of coronal field configurations was initially generated from extrapolations of the photospheric magnetic field observed by the Michelson Doppler Imager (MDI) on SOHO. Using an EUV intensity-based cost function, the extrapolated field lines that were most consistent with 171 Å passband images from the Extreme UltraViolet Imager (EUVI) on STEREO were identified. This facilitated quantitative constraints to be placed on the twist α of the extrapolated field lines, where curl(B) = α B. Using the constrained values of α the evolution in time of twist, connectivity, and magnetic energy were then studied. A flux emergence event was found to result in significant changes in the magnetic topology and total magnetic energy of the region.

Authors: Paul A. Conlon, Peter T. Gallagher
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2010-04-07 13:37
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Coronal Mass Ejection Detection using Wavelets, Curvelets and Ridgelets: Applications for Space Weather Monitoring  

Peter Gallagher   Submitted: 2010-04-07 03:07

Coronal mass ejections (CMEs) are large-scale eruptions of plasma and magnetic field that can produce adverse space weather at Earth and other locations in the Heliosphere. Due to the intrinsic multiscale nature of features in coronagraph images, wavelet and multiscale image processing techniques are well suited to enhancing the visibility of CMEs and supressing noise. However, wavelets are better suited to identifiying point-like features, such as noise or background stars, than to enhancing the visibility of the curved form of a typical CME front. Higher order multiscale techniques, such as ridgelets and curvelets, were therefore explored to characterise the morphology (width, curvature) and kinematics (position, velocity, acceleration) of CMEs. Curvelets in particular were found to be well suited to characterising CME properties in a self-consistent manner. Curvelets are thus likely to be of benefit to autonomous monitoring of CME properties for space weather applications.

Authors: P.T. Gallagher, C.A. Young, J.P. Byrne, R.T.J. McAteer
Projects: SoHO-LASCO

Publication Status: Advances in Space Research (accepted)
Last Modified: 2010-04-07 13:37
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Rapid Acceleration of a Coronal Mass Ejection in the Low Corona and Implications for Propagation  

Peter Gallagher   Submitted: 2003-03-25 12:24

A high-velocity Coronal Mass Ejection (CME) associated with the 2002 April 21 X1.5 flare is studied using a unique set of observations from the Transition Region and Coronal Explorer (TRACE), the Ultraviolet Coronagraph Spectrometer (UVCS), and the Large-Angle Spectrometric Coronagraph (LASCO). The event is first observed as a rapid rise in GOES X-rays, followed by two simultaneous brightenings which appear to be connected by an ascending loop-like feature. While expanding, the appearance of the feature remains remarkably constant as it passes through the TRACE 195-A passband and LASCO fields-of-view, allowing its height-time behaviour to be acurately determined. The acceleration is consistent with an exponential rise with an e-folding time of ~138-s and peaks at ~1500 m s^-2 when the leading-edge is at ~1.7 R_sun from Sun center. The acceleration subsequently falls off with an e-folding time of over 1000-s. At distances beyond ~3.4 R_sun, the height-time profile is approximately linear with a constant velocity of ~2500 km s-1. These results are briefly discussed in light of recent kinematic models of CMEs.

Authors: Gallagher, P. T., Lawrence, G. R., Dennis, B. R.
Projects:

Publication Status: ApJL (accepted)
Last Modified: 2003-03-25 12:25
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

RHESSI and TRACE observations of the 21 April 2002 X1.5 flare  

Peter Gallagher   Submitted: 2002-10-10 09:07

Observations of the X1.5 flare on 21 April 2002 are reviewed using the {it Ramaty High Energy Solar Spectroscopic Imager} (RHESSI) and the {it Transition Region and Coronal Explorer} (TRACE). The major findings are as follows: 1. The 3-25 keV X-rays started lsim4~mins before the EUV (195~AA) emission suggesting that the initial energy release heated plasma directly to gsim20 MK, well above the 1.6 MK needed to produce the Fe~{sc xii} (195~AA) line. 2. Using coaligned 12-25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15-20 MK) plasma in the 195~AA passband. This hot, diffuse emission is attributed to the presence of the Fe~{sc xxiv} (192~AA) line within the TRACE 195~AA passband. 3. The 12-25 keV source centroid moves away from the limb with an apparent velocity of sim9.9 km s-1, slowing to sim1.7 km s-1 after 3 hours, its final altitude being sim120~Mm after sim12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. 4. The 50-100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50-100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced thethick-target hard X-ray emission also heat the plasma seen in soft X-rays. 5. X-ray footpoint emission has an E-3 spectrum down to sim10 keV suggesting a lower electron cutoff energy than previously thought. 6. The hard X-ray (25-200 keV) peaks have FWHM durations of sim1~min suggesting a more gradual energy release process than expected. 7. The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of geq120 km s-1. This may be associated with fast CME observed several minutes later by LASCO. 8. Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ``fan of spines" becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.

Authors: Peter T. Gallagher, Brian R. Dennis, S"{a}m Krucker, Richard A. Schwartz, A. Kimberley Tolbert
Projects:

Publication Status: Solar Physics 2002 (in press)
Last Modified: 2002-10-10 09:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 


Key
Go to main E-Print pageGo to main E-Print page.
Download PreprintDownload Preprint.
Submitter's HomepageSubmitters Homepage.
Edit EntryEdit Entry.
Delete AbstractDelete abstract.

Abstracts by Author
Large-scale Bright Fronts in the Solar Corona: A Review of ``EIT waves''
Investigating the driving mechanisms of coronal mass ejections
Constraining 3D Magnetic Field Extrapolations Using The Twin Perspectives of STEREO
Coronal Mass Ejection Detection using Wavelets, Curvelets and Ridgelets: Applications for Space Weather Monitoring
Rapid Acceleration of a Coronal Mass Ejection in the Low Corona and Implications for Propagation
RHESSI and TRACE observations of the 21 April 2002 X1.5 flare

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2003 Solar Physics Group - Montana State University