Observing the early stages of CME
development in the low corona
Characterizing the link between
coronal mass ejections (CMEs) and coronal waves
STEREO A/COR 1, 19:25 UT on 25
March 2008. Running Difference.
STEREO A/EUVI, 19:05 UT on 25
March 2008. Base Difference
from 18:45 UT.
Presented by: Joel Leja
Solar Corona
 Part of solar atmosphere; Hot, tenuous (~109 cm-3) plasma
superheated to ~106 K.
 Plasma ~0.1 (plasma motion is dominated by magnetic field)
 Most information is derived from emission lines of highly ionized atoms
 The 195 Å line, where coronal waves are best viewed, is emitted by Fe
XII and represents a temperature of ~1.6 x 106 K
Image courtesy of NASA
STEREO spacecraft
 Two identical satellites
 One trails behind and one pulls ahead of
the Earth’s orbit
 Allow a “stereoscopic” view of solar
eruptions
Instruments
EUVI (195 Å)
-Extreme Ultraviolet Imager
-FOV: 0-1.7 R
-Images the low corona
-~5-10 minute cadence
COR 1
-occulting white-light
coronagraph
-FOV: 1.3-4 R
-primarily for CME detection
-~5-10 minute cadence
Image courtesy of NASA
Coronal Mass Ejections
 Violent expulsions of plasma and magnetic flux from the Sun
-Typically release 1015 grams of plasma/1030 ergs of kinetic energy,
according to the LASCO/CDAW catalogue.
-Frequency depends on stage in solar cycle; once every other day
during minimum, 5-6 times a day during maximum.
•Observations indicate CMEs can
originate in an erupting magnetic flux
rope.
-Magnetic flux present on the order
of 1020 Mx.
Running difference STEREO B/COR 1 movie of the
25 March 2008 eruption.
Coronal Waves
 First observed in 1997 by EIT aboard
SOHO in 195 Å (Dere et al., 1997;
Thompson et al., 1998)
 Expanding bright front from some
source, usually an active region.
 Appears to cause persistent,
stationary dimmings and brightenings.
(Delannèe, 2000; Attrill et al., 2007)
Observational Characteristics
-Semi-isotropic expansion
-Speeds of 25-438 km/s (Thompson
& Meyers, 2009; Wills-Davey et al.,
2007)
-Primarily observed in the quiet Sun
corona; blocked by active regions
-Observed at height of ~1.1 R
(Patsourakos et al., 2009)
Movies: Base difference, STEREO A/B (top/bottom)
EUVI, eruption of 26 April 2008, 13:35-15:15 UT
Image Processing
Raw Image
Base Difference
-Data directly from the
CCD
-A pre-event image is
subtracted from each frame
-No higher-level
processing necessary
-Highlights real brightenings
and dimmings
STEREO B/EUVI data taken on
13 February 2009, 15:05-16:35 UT
Running Difference
-The previous frame is
subtracted from the current
frame
-Highlights changes
between frames
Coronal Wave Models
Wave models (CME or flare-driven)
Fast-mode MHD wave (e.g. Thompson et al., 1999; Cliver et al., 1999;
Wang, 2000; Wu et al., 2001; Vršnak et al., 2002; Warmuth et al., 2004)
-can propagate perpendicular to magnetic field
Slow-mode MHD wave (e.g. Krasnoselskikh & Podladchikova, 2007;
Wang et al., 2009)
-must travel at an oblique angle to magnetic field
Solitary slow-mode MHD wave (Wills-Davey et al., 2007)
Non-wave models (CME driven)
Plasma compression due to stretching of the overlying magnetic
field (Delanée & Aulanier, 1999; Chen et al., 2002, 2005; Chen, 2009)
A series of current-shells form around the erupting flux rope
(Delanée et al., 2008)
Magnetic reconnection occuring between favorably oriented quietSun magnetic field and the expanding CME flanks (Attrill et al., 2007)
A link between CMEs and
coronal waves?
Biesecker et al. (2002) performed a large-scale statistical study evaluating the
link between coronal waves and (I) CMEs; (II) flares; and (III) type-II radio bursts
Result: For every coronal wave, there is
an associated CME
(not every CME has a coronal wave, however!)
The exact nature of this CME/coronal wave relationship is pivotal to the success of
coronal wave models, yet currently, it remains elusive.
Where does the intern come in?
Project Goal
Correlation of CME flank expansion (which is not well-studied, in general) to
the propagation of the coronal wave and the spatial extent of associated
coronal dimmings
Project Outline
CACTus (Robbrecht, 2007), an automated CME tracker, was used to search
between March 2008 and May 2009 for large-scale CMEs in STEREO COR1
data.
STEREO EUVI data was then examined for a corresponding coronal wave.
A list of 23 large-scale events was compiled.
4 representative events were chosen for a case study. Only 3 events,
however, are included in this presentation.
Composites
A series of COR1/EUVI composite movies are
formed for each event
-The blocked area of the coronagraph is
“filled in” with images of the disc.
-Allows for study of the link between lower
and upper corona
-FESTIVAL software is generally used for
this task; however, it is incompatible with
base difference images
Upper left: STEREO A, eruption
of 05 April 2008. Running
difference.
Lower left: STEREO A, eruption
of 25 March 2008. Running
difference.
Lower right: STEREO A, eruption
of 13 February 2009. Running
difference.
Angular Diameter
Quantization of the findings in the composite
movies. The maximum angular extent of the
coronal wave is compared to the maximum lateral
extent of the CME.
-Error bars represent the “width” of the diffuse
bright front.
-Data is taken until (I) the CME flanks become
too ill-defined or (II) both the CME and coronal
wave stop showing appreciable expansion
Upper left: 05 April 2008. CME
and coronal wave appear to be
decoupled.
Lower left: 25 March 2008. CME
and coronal wave appear to be
strongly coupled.
Lower right: 13 February 2009.
CME and coronal wave appear
to be coupled until 6:25, then
a decoupling occurs.
Coronal Dimmings
Defined in detection algorithm to be a decrease in emission by more
than one sigma below the mean value of the entire pre-event base
difference image.
Physical Causes
-Decrease in number density (via
plasma outflow)
-Change in temperature (taking it
out of the range of bandpass
sensitivity)
“Secondary” dimming
Base difference STEREO A/EUVI
stills from 13 February 2009 eruption.
05:15 UT on left, 08:35 UT on right.
“Core” dimming
Coronal dimmings associated with CMEs
have been proven, in case studies, to be
associated with plasma outflow (Harra
and Sterling, 2001; Harra et al., 2007).
Dimmings Analysis
An algorithm (Attrill & Wills-Davey, 2009) identified
regions of coronal dimming and a composite was formed
with the “maximum” extent of the CME in each event.
-In theory, magnetic reconnection -----> “open” field
lines -----> plasma outflow -----> coronal dimmings.
-To explore this, the correlation of the secondary
dimmings to the extent of the CME was examined.
All images: data from STEREO A. COR 1 is
running difference; solar disc is location of
dimmings marked by algorithm.
Upper left: 05 April 2008
Lower left: 25 March 2008
Lower right: 13 February 2009
Conclusions
 One case study shows a strong link to expanding CME flanks; another seems
consistent with interpretation as a fast-mode MHD wave; a third shows signs of
links to both. Why?
Perhaps more than one physical process occurs in any
given event (Zhukov & Auchére, 2004; Cohen et al., 2009)
An avenue for future
research? Two
frames from
the 25 March 2008
eruption
showing two distinct
types of coronal
“wave”.
Importance
-Understand and, hopefully, someday predict the magnetic
orientation of CMEs to prepare for geomagnetic storms and
space weather.
Image courtesy of National Geographic
Thank You!
Gemma (for everything)
Kelly and Trae (for making this all
possible!)
Everyone else for their patience in
enduring ~6-7 hours of presentations!