Small Events Detection. Fludra 1 , D. Haigh 1,2 , D. Bewsher 1 , V. Graffagnino 1 , P.R. Young 1 1 Rutherford Appleton Laboratory, UK 2 Birmingham University, UK. eSDO : Virtual Observatory Access to SDO Data
Fludra1, D. Haigh1,2, D. Bewsher1, V. Graffagnino1, P.R. Young1
1 Rutherford Appleton Laboratory, UK
2 Birmingham University, UK
eSDO: Virtual Observatory Access to SDO Data
UK’s Particle Physics and Astronomy Research Council (PPARC) has funded the eSDO project to prepare algorithms for the analysis of SDO data and make them available to the solar community using the virtual observatory (poster 26 - Culhane et al. More details in a talk by Elizabeth Auden in session S6 on Friday).
‘Small Events Detection’ will be one of ten eSDO algorithm packages.
Aschwanden et al. 2000, ApJ, 535, 1047
Length: 4’’ – 30’’
Area: 4 – 500 arcsec2
Any event smaller than 20’’ - 30’’, down to AIA pixel size.
What are Small Events? - Energy Range
Benz & Krucker, 2002, ApJ,
Aschwanden et al. 2000, ApJ, 535, 1047
Nanoflares: 1023 – 1026 erg
Microflares: 1026 – 1029 erg
Is the quiet Sun corona heated by nanoflares?
If γ>2, small-scale events provide the dominant contribution to the heating of the corona.
Some recent analyses give γ<2, suggesting nanoflares cannot heat the quiet sun corona.
Krucker and Benz, 1998, ApJ, 501, L213
Berghmans, Clette and Moses, 1998, A&A, 336, 1039
Benz and Krucker, 1999, A&A, 341, 286
Parnell and Jupp, 2000, ApJ, 529, 554
*Aschwanden et al. (I) 2000, ApJ, 535, 1027
Aschwanden et al. (II) 2000, ApJ, 535, 1047
Harra, Gallagher and Phillips, 2000, A&A, 362, 371
Aschwanden and Parnell, 2002, ApJ, 572, 1048
Benz and Krucker, 2002, ApJ, 568, 413
*Bewsher, Parnell and Harrison, 2002, Sol. Phys., 206, 21
* marks algorithms selected for testing. This talk presents first results from Bewsher et al. method.
CDS NIS in O V 630 A line
Sit & stare time series
5 arc min
Movie duration: 2 hours
‘Quiet’ Sun areas show thousands of short-lived intensity enhancements
CDS time series in one of 71 pixels, with 2500 exposures and 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
Peak detection 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
Algorithm from Bewsher et al. 2000, Sol. Phys. 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
I. Peak detection:
Stage 1. Identify maximum peak (*) in time series
Stage 2. Identify minimum troughs (□) on either side of peak.
Intensity jump between peak and troughs is greater than nεε, so peak is kept.
Stage 3. Identify largest peaks (*) on either side of original peak, and minimum troughs (□) on either side. If intensity jumps are larger than nεε, so those peaks are kept.
Stage 4. Identify next largest peaks (*)on either side of (*) peaks and their minimum troughs (□). If none of the intensity jumps from peaks to troughs are greater than nεε, peaks are neglected and time series is not investigated further.
II. Pixel Grouping
Adjacent pixels that peak within +/- nΔt (n=0, 1, 2,…) are grouped together to form an event. All 8 neighbour pixels are examined around each pixel already classified as part of the event. The lightcurves of the pixels are integrated over all the pixels in an event to produce an
III. Check! 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
A check is made to see if each ‘event' still meets the same criteria as for individual pixels, i.e. its peaks and troughs are identified and compared in the same way as discussed in Section I.
If a peak is identified in the summed `event' intensity, and this peak is still greater than above the troughs, then the event is counted.
IV. Picking Troughs
The nearest minimum (NM) trough on either side of the peak is identified. Look
at all other troughs between the NM trough and the peak and calculate
increase in intensity between troughs and small peaks in between. If change
in intensity is less than 2ε, then the NM trough is correct. If the change in
intensity is greater than 2ε, then the nearer trough is correct. Finally, the intensity difference between the peak and the corrected trough is checked to ensure that they meet the criteria for an ‘event’, i.e. that the intensity increase is still greater than nεε.
Example results from CDS sit & stare time series 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
71 spatial pixels (3.4’’)
15 s cadence
10 hour run
Threshold: high, large peaks only
Min event size = 1 pixel
time coincidence of +/- 3 steps
Result: 52 events
Example 2: 15 s cadence. Note that previous TRACE & EIT nanoflare studies dealt with 13-25 frames with 80-120 s cadence.
Threshold: 3x lower, 255 events
60 rasters, 10 min cadence. Time coincidence requirement: Δt=0
DISCUSSION (1) series
How many nanoflares can we expect:
Krucker & Benz, 1998, ApJ Let.: 7’x7’ field of view, 42 minutes.
Energy range 8x1024 – 2.8x1026 erg
>3 sigma = 11,150 events. > 5sigma = 2600 events
Aschwanden et al. 2000:
173 A, >3 sigma = 3130 events
195 A: >3 sigma = 900 events
Parnell and Jupp, 2000 (TRACE 7’x7’ fov):
>3sigma = 11,700 pixels = 4500 events in 80 minutes
CDS: ~1,500 events/hour in a 4’x4’ fov.
Expect 50,000 – 100,000 events/hour on the entire disk
DISCUSSION (2) series
Should we catalogue all small events? Down to what energy and size?
Should we catalogue events in each AIA channel separately, or attempt to identify the same events in different channels?
Data products: What event parameters should be stored for each AIA channel: start/peak/end time, coordinates of all pixels, area, duration, peak intensity, total intensity
How can individual event parameters be used? Time evolution of individual events – heating phase, decay phase
Are statistical frequency distributions of these parameters sufficient?
DISCUSSION (3) series
Analyse full disk or a central part of disk? (can events near the limb merge and spoil statistics?)
Analyse 24-hours/day or shorter samples (1-hour/day, 1-hour/week etc.)?
Frequency distributions of event energies:
Conversion of signal (DN/s) to event energy, through Te and emission measure (and Ne).
What statistical significance level should we adopt for event detection? 3σ ?
Cosmic ray removal, solar rotation correction, other issues?
DISCUSSION (4) series
What constitutes an ‘event’ – every act of energy release even if occurring within a bigger event?