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

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.

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.

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

Formation Concept

Sigmoidal Sheared Core Field in the Reversed-Polarity Arcade

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

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

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

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

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

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

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

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].

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.

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

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