automated detection and characterization of solar filaments and sigmoids k. wagstaff, d. m. rust, b....
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Automated Detection and Characterization of Solar Filaments and Sigmoids
K. Wagstaff, D. M. Rust,
B. J. LaBonte and P. N. BernasconiJohns Hopkins University Applied Physics Laboratory
Laurel, Maryland USA
Solar Image Recognition Workshop
Brussels
October 23-24, 2003
Objectives of Solar Filament Detection and Classification
• Report automatically on filament disappearances
• Provide warning of geomagnetic storms• Characterize magnetic flux rope chirality and
orientation of principal axis• Forecast pattern of Bz in magnetic clouds
Filament Detection Method
• Identify filament pixels– Apply darkness threshold– Group dark pixels into contiguous regions– Prune out small dark regions and artifacts– Draw contours around filament boundary
• Find spines (filament centerlines)– Use simplified Kegl’s algorithm for finding the
principal curve defined by a set of points
Find Barbs (protrusions from filament)
• Identify points farthest from the spine• Follow boundary in each direction to find
bays, i.e. local minimum distances from spine
• Establish each barb centerline by connecting the farthest point to the midpoint of left and right bays
Chirality (handedness) Classification
• Calculate angle between barb centerline and spine
• Classify barbs by obtuse and acute angles• Assign filament chirality based on majority
classification: right-handed for acute angles; left-handed for obtuse angles
The solar disk observed in H on 30 June 2002 at 1540 UTC (BBSO image). Ten filaments identified, five filaments classified.
Southern hemisphere filament rests in a right-handed flux rope.
Solar disk in H on 22 August 2002 at 1603 UTC (BBSO image)
Northern hemisphere filament rests in a left-handed flux rope.
Mirror image would be associated with right-handed flux rope.
Future Developments
• Make detection algorithm more robust
• Test against man-made lists
• Compare filament positions on successive images after correcting for solar rotation
• Set alarm bit if filament can’t be found
• Estimate geoeffectiveness from filament position on the disk and magnetic indices
Sigmoid Detection
• Sigmoid = elongate structure, S or inverse-S shape = signal of enhanced CME probability
• Present method: observers watching 24 hr/day
• Improved space weather forecasts require automatic, accurate sigmoid detection
Sigmoid Detection Problems
• Structure and intensity not well correlated
• Intensity dynamic range as high as 1000
• Internal structure makes detection dependent on spatial resolution
• Visibility varies with temperature. Visibility is best at 2 - 4 x 106 K, but often only 106 K images are available
Chart: Outline of a Sigmoid Recognition Program
contour the imageoptional: smooth/sharpen
trace out individual contoursstore contour data in a structure
compute k-curvature from data
extract curvature stats
compare curve stats to model stats
Sigmoid Decision
Curvature Stats and Measures
Curvature vs. Arc-length Plots
Structured Contour Data
Contour Map of Image
Solar Imagetaken from Yohkoh
Image Contouring
• Threshold image at different intensity levels• Lines of equal intensity create closed contours• Closed contours have distinct shapes
Characterizing a Shape with Curvature
• Curvature is change in tangent angle per change in arc length
• Counter-clockwise curving lines have positive curvature.
κ p =dθds
≈φ
q−p + p−o
κ p ≈σ ⋅
φv u 2+
v v 2
Interpreting the curvature-arclength plot
• Unique features: position of extrema and zeros; number of zeros; area under the curve; length of perimeter
κ ds=s2
s1
∫ dθ =s2
s1
∫ θ2 −θ1
Sample Case 1: Non-Sigmoid
• Number of Regions between zeros: 6• Extrema at: s = 0.05, 0.18, 0.35,
0.60, 0.70, 0.93• Area under the curve in each region:
-2.88, 0.09, -2.89, 0.08, -2.60, 1.03
Sample Case 2: Sigmoid
• Number of Regions between zeros: 4• Extrema at: s = 0.36, 0.50, 0.83, 0.94• Area under the curve in each region:
-4.36, 0.57, -4.53, 0.61
Successes and Problems
• 8 out of 10 Sigmoids Correctly Identified– 6 false detections in 4 different images
• Reasons for False Detections– Sigmoids are not yet precisely defined
– Sigmoids are often superposed on complicated background
• Recent Developments:– Algorithm refined and tested on SXI and EIT images
– Web-based implementation operates on real-time images