Umbral oscillations in sunspots

Absorption of p-modes and active region heating by mode conversion

P. S. Cally, T. J. Bogdan, E. G. Zweibel

Research output: Contribution to journalArticleResearchpeer-review

80 Citations (Scopus)

Abstract

The linear adiabatic oscillations of an infinite, stratified, perfectly conducting atmosphere pervaded by a uniform vertical magnetic field are computed. The stratification is chosen to approximate the conditions appropriate for a sunspot umbra where strong reflection of the fast magnetoacoustic-gravity waves from the rapid increase of the Alfvén speed with height and the sound speed with depth is anticipated. Since the umbral oscillations are presumably excited by external p-mode forcing, the angular frequency ω is prescribed - being set by the p-modes - and it is required to solve for the allowed eigenvalues assumed by the horizontal wavenumber k and the corresponding displacement eigenfunctions. We term these allowed solutions π-modes in recognition of the crucial influence of the imposed magnetic field and to distinguish them from their p-mode cousins present in the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are complex and are consistent with the spatial decay of the π-modes inward toward the center of the sunspot from the umbral boundary. This spatial attenuation is a consequence of the slow magnetoacoustic-gravity waves that propagate along the magnetic field lines and extract energy from the trapped fast waves through mode coupling in the layers where the sound and Alfvén speeds are comparable. Through the consideration of several ancillary computations we argue that this salient attribute of the π-modes may be essential both in explaining the observed absorption of (the forcing) p-modes by sunspots and in providing a source of mechanical energy for the overlying active regions. However, more realistic calculations are clearly called for before these notions may be confirmed.

Original languageEnglish
Pages (from-to)505-521
Number of pages17
JournalAstrophysical Journal
Volume437
Issue number1
Publication statusPublished - 10 Dec 1994

Keywords

  • MHD
  • Sun: magnetic fields
  • Sun: oscillations
  • Sunspots

Cite this

@article{d65c662e38524b5e8a5679d25c767d0c,
title = "Umbral oscillations in sunspots: Absorption of p-modes and active region heating by mode conversion",
abstract = "The linear adiabatic oscillations of an infinite, stratified, perfectly conducting atmosphere pervaded by a uniform vertical magnetic field are computed. The stratification is chosen to approximate the conditions appropriate for a sunspot umbra where strong reflection of the fast magnetoacoustic-gravity waves from the rapid increase of the Alfv{\'e}n speed with height and the sound speed with depth is anticipated. Since the umbral oscillations are presumably excited by external p-mode forcing, the angular frequency ω is prescribed - being set by the p-modes - and it is required to solve for the allowed eigenvalues assumed by the horizontal wavenumber k and the corresponding displacement eigenfunctions. We term these allowed solutions π-modes in recognition of the crucial influence of the imposed magnetic field and to distinguish them from their p-mode cousins present in the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are complex and are consistent with the spatial decay of the π-modes inward toward the center of the sunspot from the umbral boundary. This spatial attenuation is a consequence of the slow magnetoacoustic-gravity waves that propagate along the magnetic field lines and extract energy from the trapped fast waves through mode coupling in the layers where the sound and Alfv{\'e}n speeds are comparable. Through the consideration of several ancillary computations we argue that this salient attribute of the π-modes may be essential both in explaining the observed absorption of (the forcing) p-modes by sunspots and in providing a source of mechanical energy for the overlying active regions. However, more realistic calculations are clearly called for before these notions may be confirmed.",
keywords = "MHD, Sun: magnetic fields, Sun: oscillations, Sunspots",
author = "Cally, {P. S.} and Bogdan, {T. J.} and Zweibel, {E. G.}",
year = "1994",
month = "12",
day = "10",
language = "English",
volume = "437",
pages = "505--521",
journal = "The Astrophysical Journal",
issn = "0004-637X",
publisher = "American Astronomical Society",
number = "1",

}

Umbral oscillations in sunspots : Absorption of p-modes and active region heating by mode conversion. / Cally, P. S.; Bogdan, T. J.; Zweibel, E. G.

In: Astrophysical Journal, Vol. 437, No. 1, 10.12.1994, p. 505-521.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Umbral oscillations in sunspots

T2 - Absorption of p-modes and active region heating by mode conversion

AU - Cally, P. S.

AU - Bogdan, T. J.

AU - Zweibel, E. G.

PY - 1994/12/10

Y1 - 1994/12/10

N2 - The linear adiabatic oscillations of an infinite, stratified, perfectly conducting atmosphere pervaded by a uniform vertical magnetic field are computed. The stratification is chosen to approximate the conditions appropriate for a sunspot umbra where strong reflection of the fast magnetoacoustic-gravity waves from the rapid increase of the Alfvén speed with height and the sound speed with depth is anticipated. Since the umbral oscillations are presumably excited by external p-mode forcing, the angular frequency ω is prescribed - being set by the p-modes - and it is required to solve for the allowed eigenvalues assumed by the horizontal wavenumber k and the corresponding displacement eigenfunctions. We term these allowed solutions π-modes in recognition of the crucial influence of the imposed magnetic field and to distinguish them from their p-mode cousins present in the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are complex and are consistent with the spatial decay of the π-modes inward toward the center of the sunspot from the umbral boundary. This spatial attenuation is a consequence of the slow magnetoacoustic-gravity waves that propagate along the magnetic field lines and extract energy from the trapped fast waves through mode coupling in the layers where the sound and Alfvén speeds are comparable. Through the consideration of several ancillary computations we argue that this salient attribute of the π-modes may be essential both in explaining the observed absorption of (the forcing) p-modes by sunspots and in providing a source of mechanical energy for the overlying active regions. However, more realistic calculations are clearly called for before these notions may be confirmed.

AB - The linear adiabatic oscillations of an infinite, stratified, perfectly conducting atmosphere pervaded by a uniform vertical magnetic field are computed. The stratification is chosen to approximate the conditions appropriate for a sunspot umbra where strong reflection of the fast magnetoacoustic-gravity waves from the rapid increase of the Alfvén speed with height and the sound speed with depth is anticipated. Since the umbral oscillations are presumably excited by external p-mode forcing, the angular frequency ω is prescribed - being set by the p-modes - and it is required to solve for the allowed eigenvalues assumed by the horizontal wavenumber k and the corresponding displacement eigenfunctions. We term these allowed solutions π-modes in recognition of the crucial influence of the imposed magnetic field and to distinguish them from their p-mode cousins present in the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are complex and are consistent with the spatial decay of the π-modes inward toward the center of the sunspot from the umbral boundary. This spatial attenuation is a consequence of the slow magnetoacoustic-gravity waves that propagate along the magnetic field lines and extract energy from the trapped fast waves through mode coupling in the layers where the sound and Alfvén speeds are comparable. Through the consideration of several ancillary computations we argue that this salient attribute of the π-modes may be essential both in explaining the observed absorption of (the forcing) p-modes by sunspots and in providing a source of mechanical energy for the overlying active regions. However, more realistic calculations are clearly called for before these notions may be confirmed.

KW - MHD

KW - Sun: magnetic fields

KW - Sun: oscillations

KW - Sunspots

UR - http://www.scopus.com/inward/record.url?scp=12044259388&partnerID=8YFLogxK

M3 - Article

VL - 437

SP - 505

EP - 521

JO - The Astrophysical Journal

JF - The Astrophysical Journal

SN - 0004-637X

IS - 1

ER -