Small Events Detection

A. 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

What are Small Events? - Size

Any event smaller than 20’’ - 30’’, down to AIA pixel size.

Length: 4’’ – 30’’ Area: 4 – 500 arcsec2

Benz & Krucker, 2002, ApJ, Aschwanden et al. 2000, ApJ, 535, 1047

What are Small Events? - Energy Range

Nanoflares: 1023 – 1026 erg

Microflares: 1026 – 1029 erg

Why study Small Events?

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.

EfEf 0)(

Past research on small event detection:

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 Quiet Sun Brightenings

‘Quiet’ Sun areas show thousands of short-lived intensity enhancements

CDS NIS in O V 630 A line

5 arc min

Movie duration: 2 hours

Advantage of using O V observations for testing SED algorithms:

-15 s time cadence and long series

- high variability in the transition region

Sit & stare time series

Time

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

Algorithm from Bewsher et al. 2000, Sol. Phys.

I. Peak detection:Stage 1. Identify maximum peak (*) in time seriesStage 2. Identify minimum troughs (□) on either side of peak.Intensity jump between peak and troughs is greater than nε ε, 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 nε ε, 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 nε ε, peaks are neglected and time series is

not investigated further.

II. Pixel GroupingAdjacent 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`event' lightcurve.

III. Check!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 TroughsThe nearest minimum (NM) trough on either side of the peak is identified. Lookat all other troughs between the NM trough and the peak and calculateincrease in intensity between troughs and small peaks in between. If changein intensity is less than 2ε, then the NM trough is correct. If the change inintensity 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 nε ε.

Example results from CDS sit & stare time series

4’’x240’’ slit71 spatial pixels (3.4’’)15 s cadence10 hour run2500 exposures

Threshold: high, large peaks onlyMin event size = 1 pixeltime coincidence of +/- 3 stepsResult: 52 events

distance

Tim

e

distance

Tim

e

Threshold: 3x lower, 255 events

Example 2:

Example of individual events in the CDS sit & stare time series

Example of individual events in the CDS sit & stare time series

Example of detected events in CDS rastered images

60 rasters, 10 min cadence. Time coincidence requirement: Δt=0

240’’

24

0’’

Example of a detected event in the CDS rastered image

DISCUSSION (1)

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 events195 A: >3 sigma = 900 events

Parnell and Jupp, 2000 (TRACE 7’x7’ fov):>3sigma = 11,700 pixels = 4500 events in 80 minutes

Our estimate:CDS: ~1,500 events/hour in a 4’x4’ fov.

Expect 50,000 – 100,000 events/hour on the entire disk

DISCUSSION (2)

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)

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)

What constitutes an ‘event’ – every act of energy release even if occurring within a bigger event?

Example: