MHD wave refraction and the acoustic halo effect around solar active regions: a 3D study

Carlos Rijs, Hamed Moradi, Damien Przybylski, Paul Stuart Cally

Research output: Contribution to journalArticleResearchpeer-review

9 Citations (Scopus)

Abstract

An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform three-dimensional linear forward wave modeling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfven speed a matches the sound speed c, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross a = c. In simulations where we have capped a such that waves are not permitted to refract after reaching the a = c height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.
Original languageEnglish
Pages (from-to)1-8
Number of pages8
JournalThe Astrophysical Journal
Volume801
Issue number1
DOIs
Publication statusPublished - 2015

Keywords

  • magnetohydrodynamics (MHD)
  • Sun: helioseismology
  • Sun: magnetic fields
  • Sun: oscillations
  • waves

Cite this

Rijs, Carlos ; Moradi, Hamed ; Przybylski, Damien ; Cally, Paul Stuart. / MHD wave refraction and the acoustic halo effect around solar active regions: a 3D study. In: The Astrophysical Journal. 2015 ; Vol. 801, No. 1. pp. 1-8.
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abstract = "An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform three-dimensional linear forward wave modeling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfven speed a matches the sound speed c, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross a = c. In simulations where we have capped a such that waves are not permitted to refract after reaching the a = c height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.",
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MHD wave refraction and the acoustic halo effect around solar active regions: a 3D study. / Rijs, Carlos; Moradi, Hamed; Przybylski, Damien; Cally, Paul Stuart.

In: The Astrophysical Journal, Vol. 801, No. 1, 2015, p. 1-8.

Research output: Contribution to journalArticleResearchpeer-review

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Y1 - 2015

N2 - An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform three-dimensional linear forward wave modeling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfven speed a matches the sound speed c, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross a = c. In simulations where we have capped a such that waves are not permitted to refract after reaching the a = c height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.

AB - An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform three-dimensional linear forward wave modeling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfven speed a matches the sound speed c, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross a = c. In simulations where we have capped a such that waves are not permitted to refract after reaching the a = c height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.

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