Average motion of emerging solar active region polarities I. Two phases of emergence

H. Schunker, A. C. Birch, R. H. Cameron, D. C. Braun, Laurent Gizon, R. B. Burston

Research output: Contribution to journalArticle

Abstract

Aims. Our goal is to constrain models of active region formation by tracking the average motion of active region polarity pairs as they emerge onto the surface. Methods. We measured the motion of the two main opposite polarities in 153 emerging active regions using line-of-sight magnetic field observations from the Solar Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR) survey. We first measured the position of each of the polarities eight hours after emergence, when they could be clearly identified, using a feature recognition method. We then tracked their location forwards and backwards in time. Results. We find that, on average, the polarities emerge with an east-west orientation and the separation speed between the polarities increases. At about 0.1 days after emergence, the average separation speed reaches a peak value of 229 ± 11 ms-1, and then starts to decrease. About 2.5 days after emergence the polarities stop separating. We also find that the separation and the separation speed in the east-west direction are systematically larger for active regions that have higher flux. The scatter in the location of the polarities increases from about 5Mm at the time of emergence to about 15Mm at two days after emergence. Conclusions. Our results reveal two phases of the emergence process defined by the rate of change of the separation speed as the polarities move apart. Phase 1 begins when the opposite polarity pairs first appear at the surface, with an east-west alignment and an increasing separation speed. We define Phase 2 to begin when the separation speed starts to decrease, and ends when the polarities have stopped separating. This is consistent with a previous study: The peak of a flux tube breaks through the surface during Phase 1. During Phase 2 the magnetic field lines are straightened by magnetic tension, so that the polarities continue to move apart, until they eventually lie directly above their anchored subsurface footpoints. The scatter in the location of the polarities is consistent with the length and timescales of supergranulation, supporting the idea that convection buffets the polarities as they separate.

Original languageEnglish (US)
Article number201834627
JournalAstronomy and Astrophysics
Volume625
DOIs
StatePublished - Jan 1 2019

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emerging
polarity
magnetic field
speed
observatory
convection
timescale
helioseismology
magnetic fields
line of sight
observatories
alignment
tubes

Keywords

  • Sun: Activity
  • Sun: Helioseismology
  • Sun: Magnetic fields

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Average motion of emerging solar active region polarities I. Two phases of emergence. / Schunker, H.; Birch, A. C.; Cameron, R. H.; Braun, D. C.; Gizon, Laurent; Burston, R. B.

In: Astronomy and Astrophysics, Vol. 625, 201834627, 01.01.2019.

Research output: Contribution to journalArticle

Schunker, H. ; Birch, A. C. ; Cameron, R. H. ; Braun, D. C. ; Gizon, Laurent ; Burston, R. B. / Average motion of emerging solar active region polarities I. Two phases of emergence. In: Astronomy and Astrophysics. 2019 ; Vol. 625.
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abstract = "Aims. Our goal is to constrain models of active region formation by tracking the average motion of active region polarity pairs as they emerge onto the surface. Methods. We measured the motion of the two main opposite polarities in 153 emerging active regions using line-of-sight magnetic field observations from the Solar Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR) survey. We first measured the position of each of the polarities eight hours after emergence, when they could be clearly identified, using a feature recognition method. We then tracked their location forwards and backwards in time. Results. We find that, on average, the polarities emerge with an east-west orientation and the separation speed between the polarities increases. At about 0.1 days after emergence, the average separation speed reaches a peak value of 229 ± 11 ms-1, and then starts to decrease. About 2.5 days after emergence the polarities stop separating. We also find that the separation and the separation speed in the east-west direction are systematically larger for active regions that have higher flux. The scatter in the location of the polarities increases from about 5Mm at the time of emergence to about 15Mm at two days after emergence. Conclusions. Our results reveal two phases of the emergence process defined by the rate of change of the separation speed as the polarities move apart. Phase 1 begins when the opposite polarity pairs first appear at the surface, with an east-west alignment and an increasing separation speed. We define Phase 2 to begin when the separation speed starts to decrease, and ends when the polarities have stopped separating. This is consistent with a previous study: The peak of a flux tube breaks through the surface during Phase 1. During Phase 2 the magnetic field lines are straightened by magnetic tension, so that the polarities continue to move apart, until they eventually lie directly above their anchored subsurface footpoints. The scatter in the location of the polarities is consistent with the length and timescales of supergranulation, supporting the idea that convection buffets the polarities as they separate.",
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KW - Sun: Activity

KW - Sun: Helioseismology

KW - Sun: Magnetic fields

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