Observational signatures of the interaction between acoustic waves and the solar magnetic canopy

Moretti, P. F.; Jefferies, S. M.; Armstrong, J. D.; Intosh, S. W. Mc

doi:10.1051/0004-6361:20077247

Cited by 34 publications

(43 citation statements)

References 18 publications

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“…Of course, it is also equally likely that the substantial differences in the wave-propagation physics in active regions could also be leading to these enhancements, a possibility noted by a number of authors (e.g. Hindman and Brown, 1998;Donea, Braun, and Lindsey, 1999;Moretti et al, 2007).…”

Section: Theoriesmentioning

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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Section: Theoriesmentioning

confidence: 99%

Modeling the Subsurface Structure of Sunspots

et al. 2010

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“…By choosing Sodium and Potassium lines that form 500 and 250 km above the photosphere, e.g. Moretti et al (2007) have observed the appearance of haloes at frequencies as high as 7 mHz.…”

Section: Theorymentioning

confidence: 99%

“…Observations of acoustic power in regions of strong magnetic field are numerous: Lites et al (1982); Brown et al (1992); Braun et al (1992); Bogdan et al (1993); Balthasar et al (1998); Hindman & Brown (1998); Donea et al (2000); Jain & Haber (2002); Moretti et al (2007); Nagashima et al (2007). In general, there is agreement that trapped acoustic wave power (with frequencies below the acoustic cut-off) measured using Doppler velocities is strongly suppressed in the strongest field regions, and that in the weak field surrounding area, there is an enhancement of wave power at frequencies comparable to and above the acoustic cut-off.…”

Section: Introductionmentioning

confidence: 99%

A wave scattering theory of solar seismic power haloes

Hanasoge¹

2009

A&A

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Spatial maps of the high-pass frequency filtered time-averaged root-mean-squared (RMS) Doppler velocities tend to show substantial decrements within regions of strong field and curiously, randomly distributed patches of enhancement in the vicinity. We propose that these haloes or enhancements are a consequence of magnetic-field-induced mode mixing (scattering), resulting in the preferential powering of waves that possess strong surface velocity signatures (i.e. scattering from low to high wavenumbers). Evidently, this process can occur in the reverse, and therefore in order to determine if the haloes are indeed caused by mode mixing, we must answer the question: how are acoustic waves scattered by magnetic fields? Through simulations of the interactions between waves and sunspots and models of plage, we demonstrate that the high to low modal order scattering channels are favoured. With increasing frequency and consequently, decreasing wavelength, a growing number of modes are scattered by the sunspot, thereby rendering the enhancements most visible around the high-frequency parts of the spectrum. The haloes obtained from the simulations are on the same order of magnitude but weaker than those observed. We also present observational evidence to support this theory: observations of active region AR9787 are firstly frequency filtered to isolate the 5-6 mHz signals and secondly, decomposed into three wavenumber bandpasses, l−[0, 400], [400, 800], [800, 2222]. With increasing wavenumber, the extent of the halo effect is seen increase dramatically, in line with theoretical expectation.

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“…This deficit, which they termed "magnetic shadow", had already been reported in previous studies (Kneer & von Uexküll 1986;Cauzzi et al 2000), but it has been further exploited in two-dimensional power maps in the past few years (Krijger et al 2001;Muglach 2003;Muglach et al 2005;Vecchio et al 2007;Reardon et al 2009). It is now generally accepted that the interaction of acoustic oscillations with the inclined magnetic field lines together with the location of the magnetic canopy are responsible for the formation of the power halos and magnetic shadows and may reflect common physical processes (McIntosh & Judge 2001;McIntosh et al 2003;Muglagh et al 2005;Moretti et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Power halo and magnetic shadow in a solar quiet region observed in the Hαline

Kontogiannis

Tsiropoula

Tziotziou

2010

A&A

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Context. We investigate the oscillatory behavior of the quiet solar chromosphere and its discrete components in terms of oscillation properties, i.e. network and internetwork. For this purpose, we use a time series of high resolution filtergrams at five wavelengths along the Hα profile, obtained by the Dutch Open Telescope. Aims. We aim to gain insight on the distribution of power in different period bands and its variation between network and internetwork. Our spectral resolution provides information on the vertical distribution of power, since the Hα line has both photospheric and chromospheric components. We investigate the effect of Hα mottles on chromospheric oscillations, since they are the most prominent feature of the Hα chromosphere and outline inclined magnetic fields. Methods. We use wavelet and phase difference analyses of Hα intensities and Doppler signals. Two-dimensional power maps in the 3, 5 and 7 min period bands as well as coherence and phase difference maps were constructed. Results. At photospheric heights, where the Hα ± 0.7 Å wing is formed, the 3 and 5 min power is enhanced around the network, and forms power halos. Higher in the chromosphere these areas are replaced by magnetic shadows, i.e. places of power suppression. Interestingly, the power maps show a filamentary structure in the network which correlates very well with mottles. These areas show positive phase differences at the 3 min period band. At the 5 min and 7 min period bands both positive and negative phase differences are obtained with an increased number of pixels with high coherence, indicating the existence of both upward and downward propagating waves. Conclusions. We attribute our findings to the interaction between acoustic oscillations and the magnetic fields that constitute the magnetic network. The network flux tubes diverge at chromospheric levels and obtain a significant horizontal component, which is betrayed by the presence of mottles. The variation of power reveals the discrete role of the magnetic field at different heights, which guides or suppresses the oscillations, depending on its inclination. Spectral resolution in Hα provides useful information on the coupling between the acoustic sub-canopy atmosphere and the magnetized chromosphere.

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Observational signatures of the interaction between acoustic waves and the solar magnetic canopy

Cited by 34 publications

References 18 publications

Modeling the Subsurface Structure of Sunspots

Modeling the Subsurface Structure of Sunspots

A wave scattering theory of solar seismic power haloes

Power halo and magnetic shadow in a solar quiet region observed in the Hαline

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