Magnetic field inclination and atmospheric oscillations above solar active regions

Schunker, Hannah; Cally, Paul Stuart

doi:10.1111/j.1365-2966.2006.10855.x

Cited by 193 publications

(247 citation statements)

References 48 publications

(84 reference statements)

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“…The 5 min oscillations form a ring shape at the umbra-penumbra boundary. The localisation of the strong power of 5 min oscillations may indicate a strong interaction between the acoustic waves with the magnetic field, where the local condition favoured the absorption of global solar p-modes (Cally et al 2003;Schunker & Cally 2006).…”

Section: Discussionmentioning

confidence: 99%

“…The 5 min oscillation is found to be strongest in the 1700 Å bandpass at a region where the magnetic field was inclined by 20−50 • . Cally et al (2003) and Schunker & Cally (2006) demonstrate that efficient p-mode absorption occurs at the height where the Alfvén speed reaches the local sound speed and at an attack angle of about 30 • . Therefore, the 5 min oscillation observed near the umbra-penumbra boundary may originate at or lower than the photosphere.…”

Section: Spatial and Height Distribution Of Dominant Oscillationsmentioning

confidence: 99%

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Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Yuan

Сыч

Reznikova

et al. 2013

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Context. The cut-off frequency of magnetoacoustic gravity (MAG) waves could be decreased by the inclined magnetic field, and therefore, low-frequency waves could penetrate into the upper atmosphere. Aims. We observe the distribution of the cut-off frequency of compressive waves at various heights and reconstruct the magnetic field inclination, according to the MAG wave theory in a stratified atmosphere permeated by a uniform magnetic field. Methods. We analysed the emission intensity oscillations of sunspot AR11131 (08 Dec. 2010) observed at the 1700 Å, 1600 Å, and 304 Å bandpasses of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), and computed the narrow-band power maps with the pixelised wavelet filtering method. The distribution of the cut-off frequency was defined as the median contour in the azimuthally-averaged oscillation power. The magnetic field inclination was estimated with the local cut-off frequency according to the MAG wave theory in the low-β limit and was compared to the potential field extrapolation. Results. Shorter period oscillations dominate in the sunspot umbra, while longer period oscillations form an annular shape approximately concentric with the sunspot. Oscillations with longer periods are distributed further away from the sunspot centre. The 5 min oscillations appear to originate at or lower than the photosphere. The magnetic field inclinations determined with the cut-off frequency theory are about 30−40% larger than the values obtained by the potential field extrapolation. Conclusions. The oscillation power distribution in a sunspot atmosphere reflects its magnetic and thermal structure. The cut-off frequency could be used to probe the magnetic field inclination, however, other factors have to be included to fully understand this phenomenon. The existence of return magnetic flux at the outer penumbra was evidenced by the cut-off frequency distribution.

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Section: Discussionmentioning

confidence: 99%

Section: Spatial and Height Distribution Of Dominant Oscillationsmentioning

confidence: 99%

Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Yuan

Сыч

Reznikova

et al. 2013

A&A

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“…The surface field is therefore quite realistic, which is important because there is evidence (e.g. Schunker and Cally, 2006) that magnetic effects in helioseismology are dominated by the top few hundred kilometers. The fits of B r and B z are then used to derive an analytical form for the flux function.…”

Section: Martensmentioning

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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“…This aspect of the problem was followed up by Cally, Crouch, and Braun (2003) and Crouch et al (2005), who placed constraints on the strength of the sunspot's magnetic field. More recently, attention has been placed on upward propagating magnetoacoustic waves and their possible observational signatures (Schunker and Cally, 2006;Khomenko and Collados, 2006;Cally and Goossens, 2008). The use of direct numerical simulations in helioseismology, with or without magnetic fields, has also undergone rapid development.…”

Section: Introductionmentioning

confidence: 99%

Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation

2008

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The propagation of solar waves through the sunspot of AR 9787 is observed by using temporal cross-correlations of SOHO/MDI Dopplergrams. We then use threedimensional MHD numerical simulations to compute the propagation of wave packets through self-similar magnetohydrostatic sunspot models. The simulations are set up in such a way as to allow a comparison with observed cross-covariances (except in the immediate vicinity of the sunspot). We find that the simulation and the f -mode observations are in good agreement when the model sunspot has a peak field strength of 3 kG at the photosphere and less so for lower field strengths. Constraining the sunspot model with helioseismology is only possible because the direct effect of the magnetic field on the waves has been fully taken into account. Our work shows that the full-waveform modeling of sunspots is feasible.

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Magnetic field inclination and atmospheric oscillations above solar active regions

Cited by 193 publications

References 48 publications

Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Modeling the Subsurface Structure of Sunspots

Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation

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