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

This research study analyzes 37 flare-arcade events to determine the origin of sheared magnetic fields in solar flares and coronal mass ejections (CMEs). The study finds that these sheared magnetic fields are formed by processes in and above the photosphere, rather than by the emergence of a flux rope from below the photosphere. The findings highlight the importance of understanding these processes for future solar observations.

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

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

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

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

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

  12. 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].

  13. 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 fieldin 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.

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

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

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