Active Region Flux Dispersal (SH13A-1518)B.T. Welsch & Y.Li

Space Sciences Lab, UC-BerkeleyThe ultimate fate of the magnetic flux introduced into the solar photosphere by active region (AR) emergence is unknown, but some process(es) must remove it from the photosphere: over each 11-year sunspot cycle, about 3000 ARs emerge, many introducing on the order of 1021 Mx of flux into the photosphere, and most (if not all) of this flux disappears over the cycle. Does AR flux submerge, a process that might underlie observed "flux cancellation" events? Does diffusion destroy AR flux in-place? Is photospheric flux from ARs somehow removed by ejection into the heliosphere? We expect some of these processes to affect the evolution of the spatial distribution of magnetic flux in individual ARs. Accordingly, we investigate the evolution of AR flux in MDI full-disk magnetograms of several active regions from 1996-1998 that reappear on the disk (which is relatively empty at this phase of the cycle) at least once. In addition, we characterize changes in the distribution functions of flux concentrations within each active region from one appearance to the next. Log- normal distributions, indicative of fragmentation and merging, have been found in snapshots of ARs fields, but the evolution of these distributions within individual ARs has yet to be characterized. This work is supported by the NSF, under grant ATM-051438.

Un-recalibrated MDI full-disk magnetograms of several AR complexes were analyzed.

• Over at least two disk passages, we analyzed one near- disk center magnetogram for each complex.

• Pixels are ~ 2” ~ 1400 km

• Radial fields were estimated by cosine correction.

• The Mercator projection was used to remap irregular (, ) data onto an (x,y) plane.

• Fluxes were corrected for Mercator distortion.

Flux-weighted 1st & 2nd spatial moments were computed for each magnetogram, and are shown on each image.

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For each sequence, we computed the change in center-of-flux separation (in yellow) ... W

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…and the change in total unsigned flux from one disk passage to the next (in yellow).

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We also computed the distributions of magnetic “features” in each magnetogram.

Contiguous, same- sign pixels above 20 G were grouped into “clumps” (yellow).

Same-sign pixels above 20 G on the same “hill” in field strength were grouped into “downhill” features (blue).

Features were also required to have at least 4 pixels.

We then separately grouped features from each AR’s 1st and 2nd disk passages.

Perhaps not surprisingly, the low-flux end of the clump distribution was higher for the second pass (yellow).

Similarly, the low-flux end of the downhill distribution was higher for the second pass (yellow).

We also computed the distributions of field

strengths in pixels between disk passages.

Most notably, the high-flux end of the distribution is significantly diminished from one passage to the next.

Over a single solar rotation, AR evolution is not well-described by modeled diffusive evolution.

We diffused the first AR for 27 days, using D = 250 km2/s.

The resulting model AR does not resemble the actual mag-netogram 27 days later.

Conclusions (no real surprises)• AR centers of flux move apart

– typical rates are ~ 1.5 1 Mm/day

• ARs tend to lose flux;– typical rates are ~ 2 3 x 1020 Mx/day– this value will ~double with recalibrated MDI data

• The low-flux ends of the distributions of features & pixels increase, while the high-flux ends decrease.

• This work is still preliminary!