On the Origin of Strong Gradients in Photospheric Magnetic Fields

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On the Origin of Strong Gradients in Photospheric
Magnetic Fields

• Brian Welsch and Yan Li
• Space Sciences Lab, UC-Berkeley,
• 7 Gauss Way, Berkeley, CA 94720-7450, USA

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Abstract

• Several studies correlated observations of
  impulsive solar activity flares and coronal
  mass ejections (CMEs) with the amount of
  magnetic flux near strong angular gradients in
  active regions radial magnetic field, as
  measured in photospheric magnetograms.
• Practically, this empirical correlation holds
  promise as a space weather forecasting tool.
• Scientifically, however, the mechanisms that
  generate strong gradients in photospheric
  magnetic fields remain unknown.
• Hypotheses include the (1) emergence of highly
  twisted or kinked flux ropes, and (2) flux
  cancellation driven by photospheric flows acting
  fields that have already emerged.
• If such concentrations of flux near strong
  gradients are formed by emergence, then increases
  in unsigned flux near strong gradients should be
  correlated with increases in total unsigned
  magnetic flux a signature of emergence.
• Here, we analyze time series of MDI
  line-of-sight (LOS) magnetograms from several
  dozen active regions, and conclude that
• Increases in unsigned flux near strong gradients
  tend to occur during emergence, though strong
  gradients can arise without flux emergence.

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Studies have correlated gradients in photospheric
LOS magnetograms with flares CMEs.

• Falconer et al., 2003, JGR, v. 108, A10, 1380
• Falconer et al., 2006, ApJ, v. 644,1258
• Schrijver, 2007, ApJ, v. 655, 117
• But how do these gradients arise?
• From convergence of flux, and cancellation?
• From flux emergence?
• OUR GOAL Correlate changes in gradients with
  changes in flux, to see if the occurrence of
  gradients is correlated with increases in total
  unsigned flux

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Active Region (AR) Selection

• MDI full-disk, 96-minute cadence magnetograms
  from 1996-98 were used.
• NAR 64 active regions were selected.
• ARs were selected for an LCT tracking study.
• Each had a single, well-defined neutral line.
• Hence, most were bipolar.
• ARs both with without CMEs were selected.
• Several ARs were followed over multiple
  rotations some lacked NOAA AR designation.
• Here, we analyze Nmag 4062 AR magnetograms.

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Data Handling

• Pixels more that 45º from heliographic origin
  were ignored.
• To estimate the radial field, cosine corrections
  were used, BR BLOS/cos(T)
• Mercator projections were used to conformally map
  the irregularly gridded BR(?,f) to a regularly
  gridded BR(x,y).
• (While this projection preserves shapes, it
  distorts spatial scales but this distortion can
  be corrected.)

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Fig. 1 A typical, deprojected AR magnetogram.
Each AR was tracked over 3 - 5 days, and
cropped with a moving window. A list of tracked
ARs, as well as mpegs of the ARs, are online.1

• 1http//sprg.ssl.berkeley.edu/yanli/lct/

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Finding Strong-Gradients Near PILs

• We used the gradient identification technique of
  Schrijver (2007).
• Positive/negative maps M where BR gt 150 G
  BR lt -150 G, resp. were found (Fig. 2), then
  dilated by a (3x3) kernel.
• Regions of overlap, where MOL MM- ? 0, were
  identified as sites of strong-field gradients
  near PILs.2

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Fig. 2 Using positive negative masks (black
white contours, resp.) that were dilated (red
blue contours, resp.), strong-field gradients
near PILs2 were identified as points of overlap
(white arrow).

• 2Polarity Inversion Lines

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Quantifying Flux Near Strong Gradients

• MOL was convolved with a normalized3 Gaussian, G
  G0-1 exp(-x2y2/2s2), with s 12.6 in pixel
  units (15 Mm at the equator).
• Fig. 3 shows a map of BR x CMG, where
• CMG convol( MOL, G).
• Following Schrijver (2007), we summed the
  unsigned magnetic field in BR x CMG, to get a
  measure, R, of the flux near strong-field PILs.
• 3with G0 ? ? dx dy exp(-x2y2/2s2)

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Fig. 3 A map of the product of BR with CMG ,
the convolution of the overlap map MOL and a
normalized Gaussian, G. Schrijver (2007)
showed that the integral R of unsigned magnetic
field BR over such maps is correlated with
large flares.
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Changes in R vs. Total Unsigned Field, SBR

• For the NR 1621 magnetograms with R ? 0, we used
  the product of the previous BR with same CMG to
  compute the backwards-difference ?R.
• (When the overlap map MOL is identically zero, R
  is also zero, and no ?R is computed.)
• We also computed the difference in summed,
  unsigned BR between the current and previous
  magnetograms.

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What factors can cause changes in R? And/or in
the total unsigned field, SBR?

• Flux can emerge or submerge, which only happens
  at PILs. Either process could increase or
  decrease R.
• Horizontal flows could compress or disperse
  field, which could increase or decrease R.
• Flux emergence can only increase SBR, and flux
  cancellation can only decrease SBR.
• Flux could cross into or out of the field of
  view, thereby increasing or decreasing SBR.

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With these ambiguities in mind, how are changes
in R and SBR related?
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• Fig. 4 Change in R vs. change in summed,
  unsigned BR.

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• Fig. 5 A slight zoom of Fig. 4.

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Conclusions

• Increases in R, the measure of unsigned flux near
  strong-field PILs, defined by Schrijver (1997),
  are associated with increases in total unsigned
  flux.
• With caveats, this supports Schrijvers
  contention that flux emergence creates the strong
  field gradients that he found to be correlated
  with impulsive energy release.
• Our active region sample was not unbiased with
  respect to active region morphology and age.
  Hence, this bears further study, with a larger
  sample of active regions.

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Comment Is Space Weather Forecasting simply a
matter of tracking R, or emerging flux?
CME

• Simply put, NO!

Fig. 6 A geomagnetic storm occurred for the May
12, 1997 CME, though it occurred without flux
emergence!