Benchmarking fast-to-alfven mode conversion in a cold magnetohydrodynamic plasma

Paul Stuart Cally, Shelley Hansen

Research output: Contribution to journalArticleResearchpeer-review

43 Citations (Scopus)

Abstract

Alfven waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. In active regions this reflection typically occurs high enough that the Alfven speed a greatly exceeds the sound speed c, well above the a = c level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfven conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold magnetohydrodynamic model c -> 0. This provides a benchmark for fast-to-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber kappa = kh, the magnetic field inclination from the stratification direction theta, and the polarization angle phi of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfven waves typically occurs for small theta and large phi (80 degrees-90 degrees), though it is absent entirely when theta is exactly zero ( vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.
Original languageEnglish
Pages (from-to)1 - 12
Number of pages12
JournalThe Astrophysical Journal
Volume738
Issue number2
DOIs
Publication statusPublished - 2011

Cite this

@article{139e8e734f4547588b649521e84dbae4,
title = "Benchmarking fast-to-alfven mode conversion in a cold magnetohydrodynamic plasma",
abstract = "Alfven waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. In active regions this reflection typically occurs high enough that the Alfven speed a greatly exceeds the sound speed c, well above the a = c level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfven conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold magnetohydrodynamic model c -> 0. This provides a benchmark for fast-to-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber kappa = kh, the magnetic field inclination from the stratification direction theta, and the polarization angle phi of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfven waves typically occurs for small theta and large phi (80 degrees-90 degrees), though it is absent entirely when theta is exactly zero ( vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.",
author = "Cally, {Paul Stuart} and Shelley Hansen",
year = "2011",
doi = "10.1088/0004-637X/738/2/119",
language = "English",
volume = "738",
pages = "1 -- 12",
journal = "The Astrophysical Journal",
issn = "0004-637X",
publisher = "American Astronomical Society",
number = "2",

}

Benchmarking fast-to-alfven mode conversion in a cold magnetohydrodynamic plasma. / Cally, Paul Stuart; Hansen, Shelley.

In: The Astrophysical Journal, Vol. 738, No. 2, 2011, p. 1 - 12.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Benchmarking fast-to-alfven mode conversion in a cold magnetohydrodynamic plasma

AU - Cally, Paul Stuart

AU - Hansen, Shelley

PY - 2011

Y1 - 2011

N2 - Alfven waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. In active regions this reflection typically occurs high enough that the Alfven speed a greatly exceeds the sound speed c, well above the a = c level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfven conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold magnetohydrodynamic model c -> 0. This provides a benchmark for fast-to-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber kappa = kh, the magnetic field inclination from the stratification direction theta, and the polarization angle phi of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfven waves typically occurs for small theta and large phi (80 degrees-90 degrees), though it is absent entirely when theta is exactly zero ( vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.

AB - Alfven waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. In active regions this reflection typically occurs high enough that the Alfven speed a greatly exceeds the sound speed c, well above the a = c level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfven conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold magnetohydrodynamic model c -> 0. This provides a benchmark for fast-to-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber kappa = kh, the magnetic field inclination from the stratification direction theta, and the polarization angle phi of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfven waves typically occurs for small theta and large phi (80 degrees-90 degrees), though it is absent entirely when theta is exactly zero ( vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.

UR - http://iopscience.iop.org/0004-637X/738/2/119/pdf/0004-637X_738_2_119.pdf

U2 - 10.1088/0004-637X/738/2/119

DO - 10.1088/0004-637X/738/2/119

M3 - Article

VL - 738

SP - 1

EP - 12

JO - The Astrophysical Journal

JF - The Astrophysical Journal

SN - 0004-637X

IS - 2

ER -