Directional time–distance probing of model sunspot atmospheres

Moradi, Hamed; Cally, Paul Stuart; Przybylski, Damien; Shelyag, Sergiy

doi:10.1093/mnras/stv506

Cited by 10 publications

(13 citation statements)

References 55 publications

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“…This approach makes direct comparison with most other studies on atmospheric waves difficult, as they typically rely on time averaged properties of long lasting wave trains or stochastic fluctuations (Fedun et al 2011a;Santamaria et al 2015;Rijs et al 2016). Some more recent efforts have included either short or instantaneous pulses Nutto et al 2012;Santamaria et al 2015;Rijs et al 2015;Moradi et al 2015), but were analyzed by Fourier transform of, say, the velocity signal, which is still a time-averaged approach. An exception is Shelyag et al (2016), whose primary focus was dissipation due to ambipolar diffusion in a small, 3D flux tube.…”

Section: Comparison With Simulations Of Stratified Atmospheresmentioning

confidence: 99%

Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

Tarr¹,

Linton²,

Leake³

2017

ApJ

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We perform nonlinear MHD simulations to study the propagation of magnetoacoustic waves from the photosphere to the low corona. We focus on a 2D system with a gravitationally stratified atmosphere and three photospheric concentrations of magnetic flux that produce a magnetic null point with a magnetic dome topology. We find that a single wavepacket introduced at the lower boundary splits into multiple secondary wavepackets. A portion of the packet refracts towards the null due to the varying Alfvén speed. Waves incident on the equipartition contour surrounding the null, where the sound and Alfvén speeds coincide, partially transmit, reflect, and mode convert between branches of the local dispersion relation. Approximately 15.5% of the wavepacket's initial energy (E input ) converges on the null, mostly as a fast magnetoacoustic wave. Conversion is very efficient: 70% of the energy incident on the null is converted to slow modes propagating away from the null, 7% leaves as a fast wave, and the remaining 23% (0.036E input ) is locally dissipated. The acoustic energy leaving the null is strongly concentrated along field lines near each of the null's four separatrices. The portion of the wavepacket that refracts towards the null, and the amount of current accumulation, depends on the vertical and horizontal wavenumbers and the centroid position of the wavepacket as it crosses the photosphere. Regions that refract towards or away from the null do not simply coincide with regions of open versus closed magnetic field or regions of particular field orientation. We also model wavepacket propagation using a WKB method and find that it agrees qualitatively, though not quantitatively, with the results of the numerical simulation.

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Section: Comparison With Simulations Of Stratified Atmospheresmentioning

confidence: 99%

Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

Tarr¹,

Linton²,

Leake³

2017

ApJ

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“…The VALIIIC chromosphere (Vernazza et al 1981) is smoothly joined onto these distributions to complete the full model, yielding a sunspot-like magnetic field configuration embedded into the atmosphere. The sunspots we use in this instance are similar to those used in Rijs et al (2015) and Moradi et al (2015), except for some parameters, such as the peak field strength, the inclination at the surface, and the simulation box size, which have been modified. The sunspot model not only provides the freedom to choose the peak field strength at the surface of the photosphere but also the depth of the Wilson depression (the height at which the atmosphere becomes optically thin is depressed in high field regions such as the umbra).…”

Section: The Simulationmentioning

confidence: 99%

“…As in our previous work, we use the SPARC code for forward modelling (Hanasoge 2007;Hanasoge et al 2007). The code has been used several times for wave-sunspot interaction studies , 2014Moradi et al 2015). The code solves the ideal linearised MHD equations in cartesian geometry.…”

Section: Forward Modellingmentioning

confidence: 99%

3d Simulations of Realistic Power Halos in Magnetohydrostatic Sunspot Atmospheres: Linking Theory and Observation

Rijs

Rajaguru

Przybylski

et al. 2016

ApJ

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The well-observed acoustic halo is an enhancement in time-averaged Doppler velocity and intensity power with respect to quiet-sun values which is prominent for weak and highly inclined field around the penumbra of sunspots and active regions. We perform 3D linear wave modelling with realistic distributed acoustic sources in a MHS sunspot atmosphere and compare the resultant simulation enhancements with multi-height SDO observations of the phenomenon. We find that simulated halos are in good qualitative agreement with observations. We also provide further proof that the underlying process responsible for the halo is the refraction and return of fast magnetic waves which have undergone mode conversion at the critical a = c atmospheric layer. In addition, we also find strong evidence that fast-Alfvén mode conversion plays a significant role in the structure of the halo, taking energy away from photospheric and chromospheric heights in the form of field-aligned Alfvén waves. This conversion process may explain the observed "dual-ring" halo structure at higher (> 8 mHz) frequencies.

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“…This strategy allows us to determine the independent contributions of the thermal and magnetic perturbations to the travel-time shifts. Moradi et al (2009Moradi et al ( , 2015 have successfully employed a similar approach for suppressing the direct magnetic effect on the waves (thermal sunspot) by means of linear numerical simulations.…”

Section: Numerical Simulationsmentioning

confidence: 99%

Helioseismic Holography of Simulated Sunspots: Magnetic and Thermal Contributions to Travel Times

Felipe

Braun

Crouch

et al. 2016

ApJ

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Wave propagation through sunspots involves conversion between waves of acoustic and magnetic character. In addition, the thermal structure of sunspots is very different than that of the quiet Sun. As a consequence, the interpretation of local helioseismic measurements of sunspots has long been a challenge. With the aim of understanding these measurements, we carry out numerical simulations of wave propagation through sunspots. Helioseismic holography measurements made from the resulting simulated wavefields show qualitative agreement with observations of real sunspots. We use additional numerical experiments to determine, separately, the influence of the thermal structure of the sunspot and the direct effect of the sunspot magnetic field. We use the ray approximation to show that the travel-time shifts in the thermal (non-magnetic) sunspot model are primarily produced by changes in the wave path due to the Wilson depression rather than variations in the wave speed. This shows that inversions for the subsurface structure of sunspots must account for local changes in the density. In some ranges of horizontal phase speed and frequency there is agreement (within the noise level in the simulations) between the travel times measured in the full magnetic sunspot model and the thermal model. If this conclusion proves to be robust for a wide range of models, it would suggest a path towards inversions for sunspot structure.

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Directional time–distance probing of model sunspot atmospheres

Cited by 10 publications

References 55 publications

Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

3d Simulations of Realistic Power Halos in Magnetohydrostatic Sunspot Atmospheres: Linking Theory and Observation

Helioseismic Holography of Simulated Sunspots: Magnetic and Thermal Contributions to Travel Times

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