Acoustic absorption by sunspots

Braun, D. C.; Duvall, T. L.; Labonte, B. J.

doi:10.1086/184949

Cited by 224 publications

(160 citation statements)

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“…To compare our results with observations of sunspots and plages (e.g., Braun 1995;Braun et al 1987), we have to estimate the response of global mode energy and width to individual features. For this purpose, we take the original synoptic maps that correspond to each 108 day time series, define a threshold of magnetic activity, and then count the number of bins with a flux value greater than this threshold for each point in latitude.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…Since it is well known from previous observations that sunspots absorb p-modes (Braun et al 1998;Braun 1995;Bogdan et al 1993;Braun, Duvall, & LaBonte 1987 and references therein), we expect that energy and lifetime of global modes are influenced by the local presence of active regions. We use GONG and Michelson Doppler Imager (MDI) 108 day time series analyzed with the GONG peak-fitting algorithm to measure mode width, C, and amplitude, A, of individual modes.…”

Section: Introductionmentioning

confidence: 95%

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Localizing Width and Energy of Solar Globalp‐Modes

Komm

Howe

Hill

2002

ApJ

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We present the first attempt at localizing in latitude the temporal variation of mode energy, energy supply rate, and lifetime of global acoustic modes. We use Global Oscillation Network Group (GONG) and Michelson Doppler Imager data analyzed with the GONG peak-fitting algorithm to measure mode width and amplitude of individual (l, n, m) modes. While measured amplitude and width values are inherently noisier than frequency measurements, it is possible to use the ðm=lÞ dependence of these mode parameters to extract their variation in latitude. With the currently analyzed data sets, we construct maps in time and latitude of acoustic mode energy, lifetime (inverse of mode width), and energy supply rate covering the rising phase of the current solar cycle from the previous minimum to the current maximum. We find that the energy and width of global modes vary in latitude as well as in time and that the variation is clearly related to the distribution of magnetic flux. After removing the average quantity, the residual mode width shows a linear correlation with magnetic activity with a correlation coefficient of 0.88, while the corresponding residual mode energy is anticorrelated with magnetic activity with a correlation coefficient of À0.90. These mode parameters derived from global p-modes respond to the local distribution of surface magnetic activity. The energy supply rate shows no correlation with the latitudinal distribution of magnetic activity within the limits of the current measurements. We estimate the variation of global mode energy in response to an individual magnetic feature, such as a plage, and find that the global mode energy and the mode lifetime are reduced by about 40% by an active region compared to the quiet Sun.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

Localizing Width and Energy of Solar Globalp‐Modes

Komm

Howe

Hill

2002

ApJ

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“…The underlying reason for such a strong frequency dependence of travel times at large distances is not entirely clear, however. The interaction of helioseismic waves with sunspots can be influenced by a number of effects: the vertical extent of the sunspot and the Wilson depression, p-mode absorption (e.g., Braun, Duvall, and Labonte, 1987;Cally, Crouch, and Braun, 2003), the contribution from subsurface flows, radiative-transfer effects (Rajaguru et al, 2006), and the impact of power reduction and source suppression in sunspots (Rajaguru et al, 2007;Hanasoge et al, 2008;Chou et al, 2009). More detailed analyses and modeling of the sunspot in AR 9787 (e.g.…”

Section: Frequency Dependence Of One-way Travel Timesmentioning

confidence: 99%

Modeling the Subsurface Structure of Sunspots

et al. 2010

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While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009aGizon et al. ( , 2009b. We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.

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“…Since different p-modes sample different depths in the Sun, they reasoned that it should be possible to infer subsurface details such as flux-tube width dependence on depth from observations of umbral Doppler shifts. A complementary approach was developed by Braun, Duvall, & LaBonte (1987, 1988, who observed ingoing and outgoing waves in annuli outside of spots. Their unanticipated finding was that sunspots absorb up to half of the incident p-mode power at favored frequencies and horizontal wavenumbers.…”

Section: Introductionmentioning

confidence: 99%

Simulation of [CLC][ITAL]f[/ITAL][/CLC]- and [CLC][ITAL]p[/ITAL][/CLC]-Mode Interactions with a Stratified Magnetic Field Concentration

Cally

Bogdan

1997

The Astrophysical Journal

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The interaction of f-and p-modes with a slab of vertical magnetic field of sunspot strength is simulated numerically in two spatial dimensions. Both f-modes and p-modes are partially converted to slow magnetoatmospheric gravity (MAG) waves within the magnetic slab because of the strong gravitational stratification of the plasma along the magnetic lines of force. The slow MAG waves propagate away from the conversion layer guided by the magnetic field lines, and the energy they extract from the incident f-and p-modes results in a reduced amplitude for these modes as they exit from the back side of the slab. In addition, the incident p-modes are partially mixed into f-modes of comparable frequency, and therefore larger spherical harmonic degree, when they exit the magnetic flux concentration. These findings have important implications for the interpretation of observations of p-mode absorption by sunspots, both in terms of the successes and failures of this simple numerical simulation viewed in the sunspot seismology context.

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Acoustic absorption by sunspots

Cited by 224 publications

References 0 publications

Localizing Width and Energy of Solar Globalp‐Modes

Localizing Width and Energy of Solar Globalp‐Modes

Modeling the Subsurface Structure of Sunspots

Simulation of [CLC][ITAL]f[/ITAL][/CLC]- and [CLC][ITAL]p[/ITAL][/CLC]-Mode Interactions with a Stratified Magnetic Field Concentration

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