Sigmoidal Active Regions on the Sun:
Statistical and Detailed Studies
Lily Hanson
Advisors: Ed DeLuca and Antonia Savcheva
2012.08.09
Overview
• Introduction to sigmoids
– What they are
– What makes them interesting
– Our catalog
• Model of NOAA region 11474 (sigmoid S66)
– Model creation and selection
– Properties of the modeled region
– Current sheets and QSL maps
Introduction to Sigmoids
• Solar Active Region (AR) with forward or
backward S-shape in soft x-rays. (Orientation
is somewhat correlated to hemisphere.)
• Long-lasting sigmoid lifetime: days or weeks
• Transient sigmoid lifetime: hours
• May be near a coronal hole, other active
region(s), or sunspots.
S55
XRT image
S56
XRT
image
S64
XRT image
S52a,
S52b
AIA 335Å
S56
XRT image
S65, S66
AIA 335Å
S46, S47, S48
XRT image
Relevance of Sigmoids
• Shown to be a good predictor for flares and
coronal mass ejections (CMEs).
[Canfield et al. 1999, 2007]
• First step to understanding general behavior is
to gather as much information as possible:
this is the value of creating a catalog.
The Sigmoid Catalog
• Full catalog ranges from Feb 2007 – May 2012
and contains 66 sigmoidal ARs, found by
inspecting XRT synoptic images.
• Sigmoids are subjectively rated based on the
clarity of their S-shape. We focused on the
higher-rated sigmoids.
• Priority was given to regions for which we
have high-resolution data from AIA.
• Catalog is most complete for sigmoids during
Aug 2010 – May 2012. (15 sigmoids)
Data Collected for the Catalog
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Spreadsheet with the following information:
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Sigmoid ID (which we assigned)
Rating of sigmoid clarity
NOAA region number
AR start/end date/time
Position on solar disk
Size (arcsec) of longest axis
Aspect ratio (long axis / short axis)
S-shape start/end date/time
Date/time of strongest S-shape
Orientation (straight or inverted)
Hemisphere where sigmoid occurs
EUV or Hα filament visible
Sunspots associated with AR
Coronal hole(s) nearby
Other AR(s) nearby
GOES flares (date/time and class)
Flare-associated phenomena: filament eruptions, flare ribbons, transient coronal holes, post-flare loops
Videos of each AR in 335Å and 171Å (full disk and zoomed-in)
High-cadence videos of each GOES flare in 335Å, 304Å, and 171Å (zoomed-in only)
Daily screenshots of filaments (Hα) and sunspots (4500Å) from SolarMonitor.org (full disk only)
Screenshot of “Best S” time for each sigmoid in XRT and in AIA 335Å (full disk only)
Plots of positive and negative magnetic flux changing with time, created from magnetogram data
The Sigmoid Catalog
The Sigmoid Catalog
The catalog lets us combine data: flare
events, S-shape, disk center crossing, and
flux can be plotted together against time.
Some flux plots show a parabolic trend,
which may require geometric corrections.
Green shading
represents S-shape.
Time [hours]
Solid vertical line shows when
center of solar disk is crossed.
Dashed vertical lines represent
flare events.
Time [hours]
The Sigmoid Catalog
• Flare phenomena (relatively rare using GOES
classification):
– Filament eruptions (failed or successful)
– Flare ribbons
– Transient coronal holes
– Post-flare loops
The Sigmoid Catalog
• Flare phenomena
S39, C4.4 flare: successful filament eruption,
flare ribbons, and post-flare loops
AIA 171Å
S46, C6.7 flare: failed filament eruption
AIA 171Å
The Sigmoid Catalog
• Flare phenomena
S66, flare not GOES classified:
flare ribbons
AIA 304Å
S66, flare not GOES classified:
transient coronal holes
AIA 335Å
Model of S66 / AR 11474
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2012 May 3 – 14 (11.7 days)
S-shape visible from May 4 – 11 (6.8 days)
Filament seen in EUV and Hα
Small sunspot present for part of lifetime
Modeled at 05:38 on 2012 May 08, shortly
before eruption at 09:26 (shown in videos
from the previous slide).
Creating a Model with CMS2
• Line-of-sight (LoS) magnetogram data is used
to calculate the potential field of the entire
sun. Assumptions:
– The magnetic field at the photosphere is radial
and is specified by the magnetograms.
– All magnetic field lines at the outer surface of the
computational volume are radial.
Creating a Model with CMS2
• A flux rope is inserted into the calculated
potential field along the filament path,
connecting positive and negative flux
elements. The rope’s poloidal and axial flux
values are different for each model.
High resolution magnetogram contours superimposed on AIA 304Å image
Best Fit Model
• After a relaxation process, the best model’s
field lines lie closest to the observed coronal
loops (red). Best fit: Model 8.
Model 1
Model 8
High resolution magnetogram contours on XRT image
Basic Properties of Model 8
• Poloidal flux: -1 × 1010 Mx cm-1
• Axial flux: 7 × 1020 Mx
• Fit parameter: 0.0064 Rsun
(from comparison with five coronal loops)
• Potential energy: 1.06 × 1032 ergs
• Free energy: 2.96 × 1031 ergs
• Relative helicity: -2.41 × 1042 Mx2
Current Structures in Model 8
• A cross-sectional plot of the modeled currents
shows a unique topology with four zones.
Field lines in each zone create the S-shape.
Yellow line
becomes s-axis
s-axis
x-axis
x-axis
s-axis
Further Study
of the Model
60k iterations
s-axis
• Cross-sectional
currents at 30k and
60k iterations show
the flux rope is rising.
We ran the model for
another 80k iterations
to mimic the region’s
time evolution.
80k iterations
s-axis
120k
iterations
s-axis
140k iterations
s-axis
Conclusions
• We have created a catalog of strong sigmoids
from Aug 2010 – May 2012.
– This lets us identify variations and unifying
characteristics.
– Analyzing typical behavior and evolution will improve
space weather forecasting capabilities.
• S66 is modeled by a flux rope on the filament,
with poloidal and axial flux given by model 8.
– Unique topological features are apparent in current
cross-section.
– Slight instability of model can be used to imitate and
study changes with time.
Future Work
• The slightly unstable model of S66 will be used
to compare topological structures (QSLs) with
observations of energetic particle
precipitation (flare ribbons) to investigate the
magnetic field configuration around
reconnection sites.
References and Bibliography
•
“Field Topology Analysis of a Long-Lasting Coronal Sigmoid”, A. Savcheva, A. van
Ballegooijen, E. DeLuca, 2012, ApJ, 744, 78
•
“Nonlinear Force-Free Modeling of a Long-Lasting Coronal Sigmoid”, A. Savcheva
and A. van Ballegooijen, 2009, ApJ, 703, 1766
•
“YOHKOH SXT Full-Resolution Observations of Sigmoids: Structure, Formation, and
Eruption”, R. Canfield et al., 2007, ApJ, 671, L81
•
“Sigmoidal Morphology and Eruptive Solar Activity”, R. Canfield et al., 1999, GRL,
Vol 26, No 6, 627
Acknowledgements and Special Thanks
• Dr. Edward DeLuca and Antonia Savcheva for their extensive
knowledge and valuable help
• Dr. Adriaan van Ballegooijen for assistance with CMS2
• CfA researchers and support staff for organizing the REU programs,
answering questions, and welcoming us to the CfA
• NSF Grant ATM-0851866 for making it all possible
• CfA grad students, postdocs, and other well-wishers for exquisite
baked goods and bloodthirsty games of Mafia
• The Solar and Astro REU crews for teaming up to find ice cream,
the beach, and creative solutions to the Two Refrigerator Problem
• Boston for its many Wonders and Marvels
Thank you!
… Questions?
AIA 335Å
Formation of Sigmoids
• Long-lasting sigmoids are theorized to form
from shearing in a potential arcade. Generally
we assume that a twisted flux rope is present,
which is held in place by the overlying arcade.
Cartoon showing formation of a flux rope in the presence of shearing,
from A.A. van Ballegooijen and P.C.H. Martens, "Formation and eruption of solar prominences," ApJ 343, 971 (1989)
Copied from Hudson’s Solar Cartoon Archive.
Relaxation of Flux Rope Models
• Non-Linear Force-Free Fields (NLFFF)
• Models are relaxed by magnetofrictional
relaxation with hyperdiffusion for 60k
iterations.
• The models’ fit quality is inspected at 30k
iterations.
Comparison of Selected Models
Coronal Loops
(selected from XRT
image)
Model 1
Poloidal flux: -5e9
Axial flux: 1e20
Model 18
Poloidal flux: -5e10
Axial flux: 1e21
Model 8
Poloidal flux: -1e10
Axial flux: 7e20
High-resolution magnetogram contours superimposed on XRT image
High-resolution magnetogram contours with model field lines
QSL Maps and Flare Ribbons
• A Quasi-Separatrix Layer
(QSL) marks the boundary
between regions of strong
magnetic field divergence.
These are possible sites for
reconnection and energy
release.
• Flare ribbons are a sign of
energy release that may be
occurring at a QSL. We can
compare QSL maps at
different heights to flare
ribbon shape.
QSL Map superimposed on AIA 304 Å
image with flare ribbons.