Origin of the Sheared Magnetic Fields that Erupt in Flares and Coronal Mass Ejections

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Origin of the Sheared Magnetic Fields that Erupt
in Flares and Coronal Mass Ejections
Ron Moore and Alphonse Sterling
NASA/MSFC/NSSTC
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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.
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Main Points
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Nearly every long-duration (gt 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.
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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.
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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.
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A high priority for Solar-B should be to discover
the evolutionary processes that build the
sigmoidal sheared fields along mature neutral
lines.
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Onset of a Typical Fast CME
2002 January 4 (Sterling Moore, 2004, ApJ,
613, 1221)
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Formation Concept
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Sigmoidal Sheared Core Field in
the Reversed-Polarity Arcade
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Standard Hirayama Picture for a CME
Explosion from a Sigmoidal Sheared-Core Arcade
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Eruption-Onset Sigmoid
Post-Eruption Flare Arcade
Normal Polarity Without Sunspots 2000 May 10
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Reversed Polarity Without Sunspots 2000 May 14
Early-Phase Flare Arcade
Late-Phase Flare Arcade
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Post-Eruption Flare Arcade
Pre-Eruption Sigmoid
Normal Polarity With Sunspots 2001 November 4
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Reversed Polarity With Sunspots 2001 January 10
Pre-Eruption Sigmoid
Post-Eruption Flare Arcade
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Aspects of 25 Normal-Polarity Flare-Arcade Events Aspects of 25 Normal-Polarity Flare-Arcade Events Aspects of 25 Normal-Polarity Flare-Arcade Events Aspects of 25 Normal-Polarity Flare-Arcade Events
Feature Incidence (Number of Events) Incidence (Number of Events) Incidence (Number of Events)
Feature Feature Present Feature Ambiguous Feature Absent
CME 24 1 0
Sigmoid ? Arcade 17 0 8
Coronal Dimming 13 0 12
Filament Disappearance 9 4 12
Filament Channel 25 0 0
Sunspots 12 0 13
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Aspects of 12 Reversed-Polarity Flare-Arcade Events Aspects of 12 Reversed-Polarity Flare-Arcade Events Aspects of 12 Reversed-Polarity Flare-Arcade Events Aspects of 12 Reversed-Polarity Flare-Arcade Events
Feature Incidence (Number of Events) Incidence (Number of Events) Incidence (Number of Events)
Feature Feature Present Feature Ambiguous Feature Absent
CME 10 0 2
Sigmoid ? Arcade 7 0 5
Coronal Dimming 8 0 4
Filament Disappearance 9 1 2
Filament Channel 11 1 0
Sunspots 3 0 9
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Summary of Results from our 37 Yohkoh SXT
Flare-Arcade Events
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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 (gt6 hr) flare arcade nearly always
produce a CME. - They both show high incidences
(gt50) of sigmoidal form, coronal dimming,
and filament disappearance.
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Normal-polarity events outnumber
reversed-polarity events roughly 2 to 1
compatible with the formation concept.
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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.
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Conclusion
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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.
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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.
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The shear-buildup processes should be observable
by Solar-B.
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Normal Polarity Without Sunspots 2001 August 14
Pre-Eruption Sigmoid
Post-Eruption Flare Arcade
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Reversed Polarity Without Sunspots 2000 May 31
Pre-Eruption Partial Sigmoid
Post-Eruption Flare Arcade