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Brüggen, M.; Spruit, H. C.

doi:10.1023/a:1005234409055

Cited by 12 publications

(8 citation statements)

References 6 publications

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“…Numerous authors have demonstrated that effects other than changes in the local sound speed introduce travel-time perturbations around sunspots. For example : Bruggen & Spruit (2000) described the role of changes in the upper boundary condition in sunspots due to the Wilson depression; Lindsey & Braun (2005) argued that the effect of photospheric magnetic field on observed oscillation velocities, i.e., the ''showerglass effect,'' can be important; Woodard (1997) and Gizon & Birch (2002) demonstrated that increased wave damping in sunspots can introduce shifts in travel times; S. P. Rajaguru et al (2006, in preparation) show that the reduced visibility of p-mode oscillations in sunspots can, in some cases, introduce substantial artifacts into time-distance measurements; finally, Cally et al (2003) showed that models that include the effects of nonvertical magnetic field on wave propagation can essentially reproduce the observed Hankel analysis phase shifts (Braun 1997) around sunspots. A critical parameter that determines the role of magnetic field effects in the wave propagation is the plasma parameter ¼ 8P/B 2 , where P is the gas pressure and B is the magnetic field strength.…”

Section: Introductionmentioning

confidence: 99%

Three‐dimensional Inversion of Sound Speed below a Sunspot in the Born Approximation

Couvidat¹,

Birch²,

Kosovichev³

2006

ApJ

115

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We revise the inversion of acoustic travel times for the three-dimensional sound-speed structure below the solar NOAA Active Region 8243 of 1998 June. We benefit from recent progress in time-distance helioseismology that provides us with more reliable tools to infer subsurface solar properties. Among the improvements we implement here are the use of Born approximation-based travel-time sensitivity kernels that take into account finite-wavelength effects and thus are more accurate than the previously employed ray-path kernels, the inclusion of solar noise statistical properties in the inversion procedure through the noise covariance matrix, and the use of the actual variance of the noise in the temporal cross-covariances in the travel-time fitting procedure. Of these three improvements, the most significant is the application of the Born approximation to time-distance helioseismology. This puts the results of this discipline at the same level of confidence as those of global helioseismology based on inversion of normal-mode frequencies. Also, we compare inversion results based on ray-path and Born approximation kernels. We show that both approximations return a similar two-region structure for sunspots. However, the depth of inverted structures may be offset by 1 or 2 Mm, and the spatial resolution of the results is more accurately estimated with the more realistic Born sensitivity kernels. Finally, using artificial realizations of Doppler velocities of the quiet Sun, we are now able to estimate the statistical uncertainties of these inversion results.

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Section: Introductionmentioning

confidence: 99%

Three‐dimensional Inversion of Sound Speed below a Sunspot in the Born Approximation

Couvidat¹,

Birch²,

Kosovichev³

2006

ApJ

115

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“…Clearly it is important to determine whether these inferences from the data are robust. Perturbations to the travel times in the vicinity of sunspots do not necessarily arise wholly from perturbations to the speed at which the waves propagate: a shift in the height of the upper boundary of the acoustic cavity of the waves can also play a role, e.g., Brüggen & Spruit (2000) and Braun & Lindsey (2000).…”

Section: Introductionmentioning

confidence: 99%

Comparison of GONG and MDI: Sound‐Speed Anomalies beneath Two Active Regions

Hughes

Rajaguru

Thompson

2005

ApJ

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Travel times of acoustic waves are calculated from Dopplergrams of solar oscillations obtained using the Global Oscillation Network Group (GONG) ground-based network and the Michelson Doppler Imager ( MDI) instrument on board the SOHO satellite. These travel times are inverted using a standard ray approximation to ascertain the sound-speed anomalies below two active regions. Some simple methods for ignoring the possibly corrupted measurements from within a sunspot are considered, as are diagnostics for optimizing the inversion. Results are then presented for two different spot regions, and the results of the instruments are compared: both regions behave in similar ways, and the agreement between the two instruments is good. First-skip and second-skip data are found to produce similar results for deeper layers of the model, but the significance of the shallower results from second-skip data is questionable. We conclude that GONG data are appropriate for time-distance analysis.

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“…Helioseismology in some of its more sophisticated forms, e.g. time-distance analysis, holds promise of detecting and mapping subsurface flows, and perhaps even subsurface magnetic fields (D'Silva & Duvall 1995;Brüggen & Spruit 2000;Kosovichev & Duvall 2000). Combining this with the observational opportunities for a future ground based 4-m telescope with an adaptive optics system, capable of resolving features down to perhaps 50 km, and with space observations across the entire electromagnetic spectrum, from IR to X-rays, we can expect substantial progress in the coming decades.…”

Section: Magnetic Activitymentioning

confidence: 99%

Solar Activity and Classical Physics

Parker

2001

Chin. J. Astron. Astrophys.

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This review of solar physics emphasizes several of the more conspicuous scientific puzzles posed by contemporary observational knowledge of the magnetic activity of the Sun. The puzzles emphasize how much classical physics we have yet to learn from the Sun. The physics of solar activity is based on the principles of Newton, Maxwell, Lorentz, Boltzmann, et. al., along with the principles of radiative transfer. In the large, these principles are expressed by magnetohydrodynamics. A brief derivation of the magnetohydrodynamic induction and momentum equations is provided, with a discussion of popular misconceptions.

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Cited by 12 publications

References 6 publications

Three‐dimensional Inversion of Sound Speed below a Sunspot in the Born Approximation

Three‐dimensional Inversion of Sound Speed below a Sunspot in the Born Approximation

Comparison of GONG and MDI: Sound‐Speed Anomalies beneath Two Active Regions

Solar Activity and Classical Physics

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