Using Feature Tracking to Quantify Flux Cancellation Rates

Evidence suggests that flux cancellation might play a central role in both formation and

eruption of prominences.

Can feature tracking techniques help us

understand the process of cancellation?

B. Welsch, with G.Fisher & Y. Li,Space Sciences Lab, UC-Berkeley

Case Study: AR 8038, 10-13 May 1997

• Sub-region of MDI full-disk magnetograms– 1433 km pixels, approx. 2”– 96 minute cadence

• Geoeffective CME occurred on 12 May, c. 5:00UT, when the active region was at N21 W08.

• This decaying AR was the only one on the disk!– Papers, e.g.: Thompson et al., 1998; Webb et al., 2000– SHINE/MURI/CISM groups studying this event

BLOS in AR 8038, 10-13 May 1997

What is flux cancellation?

Livi, Wang, & Martin (1985), in studies of photospheric magnetograms, defined it phenomenologically:

“mutual apparent loss of magnetic flux in closely-spaced features of opposite polarity”

What is flux cancellation, physically?

Evidence suggests feature cancellation is the photospheric manifestation of (Zwaan):

1. Emergence of a U-loop. (*)

2. Submergence of an inverted U-loop. (*)

3. Reconnection-associated emergence and/or submergence – depending on height. (*)

What are these magnetogram “features”?

• My feature tracking algorithm groups contiguous, unipolar pixels into features.

• Optionally, a feature may contain:1. All pixels of a |Bz| > Bthr , in which case they are isolated:

Bz = 0, or |Bz|< Bthr , obtains on their boundaries. (*)

2. All pixels that are convex-up in |Bz| & contiguous with a local maximum in |Bz|. (*)

“mutual apparent loss of magnetic flux in closely-spaced features of opposite polarity”

Upon partitioning:• Sizes are compared to a min. size – 4 pix

• The existence of an “interior point” – but not an interior grid cell – is confirmed.

• Flux-weighted quantities are calculated & stored:

(velocities from LCT on magnetograms)

etc.,vv,v,v,v,v

,,,,,

22

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yxyxyx

),(),(),( yxBdayxByxQdaQ zz

Feature Tracking via Overlap

Feature Tracking via Overlap , p.2

Do this over & over again…

And over & over again…

Cancellation: find ‘contacts’

Cancellation: find ‘contacts’, p.2

If two fluxes in contact also…1. both decrease in flux across time step, and

2. approach each other across time step

…then it’s a cancellation!

These criteria are probably overly restricitive:

1. Fluxes fluctuate in time…

2. Elements’ positions exhibit jitter…

3. Fluxes can prob’ly be “in contact” without meeting my contact criteria!

Hence, my results are lower limits on cancelled flux!

Cancelled Flux vs. Time

Flux Cancellation 10-13 May:

• Peak rate of ~ 3 x 1019 Mx/hr

• Total of 4.4 x 1020 Mx cancelled

• Flux in a filament is ~ 2 x 1020 Mx– |B| ~ 100 G

– A = H x W ~ (30 Mm)(6 Mm) ~ 2 x 1018 cm2

…and a filament was observed to form at AR 8038’s SE corner in this period.

‘Specific Rate’ of Cancellation

• Chae has defined an interface length,

• So one can determine cancellation/unit length!– Average value: 2.9 x 106 G cm / sec– Maximal value:16.3 x 106 G cm / sec

222222 /)())((2)( sxyyxyxyxL

Conclusions

• Used overlap in feature tracking algorithm,

can apply to AR or QS magnetograms.

• Used contact in preliminary cancellation detection algorithm.

• Can calculate cancellation rates and cumulative values, and (hopefully) relate these to filament formation & eruption.

• Routines available – just ask!

Emergence of a U-loop

• L. van Driel-Gesztelyi, J.-M. Malherbe, & Demoulin, P., 2000: lack of coronal activity over cancellation site in MDI

(BACK)

t1 t2 t3

Submergence of an inverted U-loop

• J. Chae, Y.- J. Moon, A. A. Pevtsov, 2004: Doppler shifts of ~1 km/s at cancellation site

(BACK)

t1 t2 t3

Reconnection-associated submergence

t1 t2 t3

• K. Harvey et al., 1999: TRACE; chromospheric,photospheric BLOS

• J. Chae et al., 1998: SUMER, BBSO magnetograms

(BACK)

Contiguous-Pixel Feature Identification

(*)

Gradient-Based Feature Identification

(*)

Normal Comp. of Induction Equation Applied to Flux Cancellation

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