1. Ron Moore and Alphonse Sterling
Origin of the Sheared Magnetic Fields that Erupt in Flares and Coronal Mass Ejections
Ron Moore and Alphonse Sterling
NASA/MSFC/NSSTC

2. Abstract
From a search of the Yohkoh/SXT whole-Sun movie in the years 2000 and 2001, we found 37 flare-arcade events for which there were full-disk magnetograms from SOHO/MDI, coronagraph movies from SOHO/LASCO, and full-disk chromospheric images from SOHO/EIT and/or from ground-based observatories. Each of these events was apparently produced by the ejective eruption of sheared core magnetic field (as a flux rope) from along the neutral line inside a mature bipolar magnetic arcade. Two thirds (25) of these bipoles had the normal leading-trailing magnetic polarity arrangement of active regions in the hemisphere of the bipole, but the other third (12) had reversed polarity, their leading flux being the trailing-polarity remnant of one or more old active regions and their trailing flux being the leading-polarity remnant of one or more other old active regions. From these observations, we conclude: (1) The sheared core field in a reversed-polarity bipole must be formed by processes in and above the photosphere, not by the emergence of a flux rope bodily from below the photosphere. (2) The sheared core fields in the normal-polarity bipoles were essentially the same as those in the reversed-polarity bipoles. (3) Hence, the sheared core fields in normal-polarity mature bipoles are likely formed mainly by the same processes as in reversed-polarity bipoles. (4) A prime objective of Solar-B should be to discover and elucidate these processes.

3. Main Points 
Nearly every long-duration (> 6 hr) flare arcade is produced by
the re-closing of a mature, sheared-core magnetic arcade that
has “spit out” a CME by exploding open. 
The sheared core field in a reversed-polarity mature arcade must
be formed by processes in and above the photosphere, not by
bodily emergence of a flux rope from below the photosphere. 
The core fields in reversed-polarity mature arcades look and act
like those in normal-polarity mature arcades.
The sheared core fields in normal-polarity mature arcades
form in the same way as in reversed-polarity mature arcades. 
A high priority for Solar-B should be to discover the evolutionary
processes that build the sigmoidal sheared fields along mature
neutral lines.

4. Onset of a Typical Fast CME
2002
January 4
(Sterling & Moore, 2004,
ApJ, 613, 1221)

5. Formation Concept

6. Sigmoidal
Sheared Core Field
in the
Reversed-Polarity
Arcade

7. Standard
Hirayama Picture
for a
CME Explosion
from a
Sigmoidal
Sheared-Core
Arcade

8. Normal Polarity Eruption-Onset Sigmoid Post-Eruption Flare Arcade
Without
Sunspots
2000
May 10

9. Reversed Polarity Without Sunspots 2000 May 14 Early-Phase
Flare Arcade
Late-Phase
Flare Arcade

10. Normal Polarity Pre-Eruption Post-Eruption Sigmoid Flare Arcade With
Sunspots
2001
November 4

11. Reversed Polarity With Sunspots 2001 January 10 Pre-Eruption Sigmoid
Post-Eruption
Flare Arcade

12. Aspects of 25 Normal-Polarity Incidence (Number of Events)
Flare-Arcade Events
Feature
Incidence (Number of Events)
Present
Feature Ambiguous
Absent
CME 24 1
Sigmoid  Arcade 17 8
Coronal Dimming 13 12
Filament
Disappearance 9 4
Filament Channel 25
Sunspots

13. Aspects of 12 Reversed-Polarity Incidence (Number of Events)
Flare-Arcade Events
Feature
Incidence (Number of Events)
Present
Feature Ambiguous
Absent
CME 10 2
Sigmoid  Arcade 7 5
Coronal Dimming 8 4
Filament
Disappearance 9 1
Filament Channel 11
Sunspots 3

14. Summary of Results from our 37 Yohkoh SXT Flare-Arcade Events
Normal-polarity events and reversed-polarity events have these
basic similarities:
- For both, the flare arcade nearly always straddles a filament channel,
a mark of strongly sheared core field.
- For both, events having a long-duration (>6 hr) flare arcade nearly always
produce a CME.
- They both show high incidences (>~50%) of sigmoidal form, coronal
dimming, and filament disappearance. 
Normal-polarity events outnumber reversed-polarity events
roughly 2 to 1 [compatible with the formation concept]. 
Reversed-polarity events tend to occur in older, weaker magnetic
arcades than normal-polarity events do [compatible with the formation
concept, and with reversed-polarity events showing lower incidence of
sigmoidal form, higher incidence of coronal dimming, and higher incidence of
filament disappearance, than normal-polarity events show].

15. Conclusion    The sheared core field is basically the same in both
normal-polarity and reversed-polarity events, in terms
of its pre-eruption form and in the form and consequences
of its eruption. 
Therefore, the sheared core field in a mature magnetic
arcade of either polarity does not come from the bodily
emergence of a flux rope along the neutral line, but
must be formed by processes in and above the photosphere. 
The shear-buildup processes should be observable by Solar-B.

16. Normal Polarity Without Sunspots 2001 August 14 Pre-Eruption Sigmoid
Post-Eruption
Flare Arcade

17. Reversed Polarity Without Sunspots 2000 May 31 Pre-Eruption
Partial Sigmoid
Post-Eruption
Flare Arcade