On mode conversion, reflection, and transmission of magnetoacoustic waves from above in an isothermal stratified atmosphere

Shelley C Hansen, Paul S Cally, Alina-C Donea

Research output: Contribution to journalArticleResearchpeer-review

4 Citations (Scopus)

Abstract

We use the exact solutions for magnetoacoustic waves in a two-dimensional (2D) isothermal atmosphere with uniform inclined magnetic field to calculate thewave reflection, transmission, and conversion of slow and fast waves incident from above (z = ∞). This is relevant to the question of whether waves excited by flares in the solar atmosphere can penetrate the Alfvén/acoustic equipartition layer (which we identify as the canopy) to reach the photosphere with sufficient energy to create sunquakes. It is found that slow waves above the acoustic cutoff frequency efficiently penetrate (transmit) as acoustic (fast) waves if directed at a small attack angle to the magnetic field, with the rest converting to magnetic (slow) waves, in accord with Generalized Ray Theory. This may help explain the compact nature of seismic sources of sunquakes identified using seismic holography. The incident slow waves can also efficiently transmit at low frequency in inclined field due to the reduction in acoustic cutoff frequency (ramp effect). Incident fast (magnetic) 'waves' from infinity with specified non-zero horizontal wavenumber are necessarily evanescent, but can carry energy to the equipartition level by tunnelling. It is found that this can then efficiently convert to acoustic (fast) energy that can again reach the photosphere as a travelling wave. Overall, there appear to be ample avenues for substantial compressive wave energy to penetrate the canopy and impact the photosphere.
Original languageEnglish
Pages (from-to)1826-1836
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume456
Issue number2
DOIs
Publication statusPublished - 2016

Keywords

  • MHD
  • Sun: flares
  • Sun: helioseismology
  • Sun: oscillations

Cite this

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abstract = "We use the exact solutions for magnetoacoustic waves in a two-dimensional (2D) isothermal atmosphere with uniform inclined magnetic field to calculate thewave reflection, transmission, and conversion of slow and fast waves incident from above (z = ∞). This is relevant to the question of whether waves excited by flares in the solar atmosphere can penetrate the Alfv{\'e}n/acoustic equipartition layer (which we identify as the canopy) to reach the photosphere with sufficient energy to create sunquakes. It is found that slow waves above the acoustic cutoff frequency efficiently penetrate (transmit) as acoustic (fast) waves if directed at a small attack angle to the magnetic field, with the rest converting to magnetic (slow) waves, in accord with Generalized Ray Theory. This may help explain the compact nature of seismic sources of sunquakes identified using seismic holography. The incident slow waves can also efficiently transmit at low frequency in inclined field due to the reduction in acoustic cutoff frequency (ramp effect). Incident fast (magnetic) 'waves' from infinity with specified non-zero horizontal wavenumber are necessarily evanescent, but can carry energy to the equipartition level by tunnelling. It is found that this can then efficiently convert to acoustic (fast) energy that can again reach the photosphere as a travelling wave. Overall, there appear to be ample avenues for substantial compressive wave energy to penetrate the canopy and impact the photosphere.",
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On mode conversion, reflection, and transmission of magnetoacoustic waves from above in an isothermal stratified atmosphere. / Hansen, Shelley C; Cally, Paul S; Donea, Alina-C.

In: Monthly Notices of the Royal Astronomical Society, Vol. 456, No. 2, 2016, p. 1826-1836.

Research output: Contribution to journalArticleResearchpeer-review

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AU - Cally, Paul S

AU - Donea, Alina-C

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N2 - We use the exact solutions for magnetoacoustic waves in a two-dimensional (2D) isothermal atmosphere with uniform inclined magnetic field to calculate thewave reflection, transmission, and conversion of slow and fast waves incident from above (z = ∞). This is relevant to the question of whether waves excited by flares in the solar atmosphere can penetrate the Alfvén/acoustic equipartition layer (which we identify as the canopy) to reach the photosphere with sufficient energy to create sunquakes. It is found that slow waves above the acoustic cutoff frequency efficiently penetrate (transmit) as acoustic (fast) waves if directed at a small attack angle to the magnetic field, with the rest converting to magnetic (slow) waves, in accord with Generalized Ray Theory. This may help explain the compact nature of seismic sources of sunquakes identified using seismic holography. The incident slow waves can also efficiently transmit at low frequency in inclined field due to the reduction in acoustic cutoff frequency (ramp effect). Incident fast (magnetic) 'waves' from infinity with specified non-zero horizontal wavenumber are necessarily evanescent, but can carry energy to the equipartition level by tunnelling. It is found that this can then efficiently convert to acoustic (fast) energy that can again reach the photosphere as a travelling wave. Overall, there appear to be ample avenues for substantial compressive wave energy to penetrate the canopy and impact the photosphere.

AB - We use the exact solutions for magnetoacoustic waves in a two-dimensional (2D) isothermal atmosphere with uniform inclined magnetic field to calculate thewave reflection, transmission, and conversion of slow and fast waves incident from above (z = ∞). This is relevant to the question of whether waves excited by flares in the solar atmosphere can penetrate the Alfvén/acoustic equipartition layer (which we identify as the canopy) to reach the photosphere with sufficient energy to create sunquakes. It is found that slow waves above the acoustic cutoff frequency efficiently penetrate (transmit) as acoustic (fast) waves if directed at a small attack angle to the magnetic field, with the rest converting to magnetic (slow) waves, in accord with Generalized Ray Theory. This may help explain the compact nature of seismic sources of sunquakes identified using seismic holography. The incident slow waves can also efficiently transmit at low frequency in inclined field due to the reduction in acoustic cutoff frequency (ramp effect). Incident fast (magnetic) 'waves' from infinity with specified non-zero horizontal wavenumber are necessarily evanescent, but can carry energy to the equipartition level by tunnelling. It is found that this can then efficiently convert to acoustic (fast) energy that can again reach the photosphere as a travelling wave. Overall, there appear to be ample avenues for substantial compressive wave energy to penetrate the canopy and impact the photosphere.

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