Felipe T, Braun D C, Crouch A D, Birch A C
Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain.
Instituto de Astrofísica de Canarias, c/vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain.
Astrophys J. 2016 Oct 1;829(No 2). doi: 10.3847/0004-637X/829/2/67. Epub 2016 Sep 23.
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 toward inversions for sunspot structure.
波在太阳黑子中的传播涉及声学和磁学性质的波之间的转换。此外，太阳黑子的热结构与宁静太阳的热结构有很大不同。因此，对太阳黑子进行局部日震测量的解释长期以来一直是一项挑战。为了理解这些测量结果，我们对波在太阳黑子中的传播进行了数值模拟。从由此产生的模拟波场进行的日震全息测量显示出与真实太阳黑子观测结果的定性一致。我们使用额外的数值实验分别确定太阳黑子热结构的影响和太阳黑子磁场的直接效应。我们使用射线近似来表明，热（非磁性）太阳黑子模型中的传播时间偏移主要是由威尔逊凹陷导致的波路径变化而非波速变化产生的。这表明对太阳黑子地下结构的反演必须考虑密度的局部变化。在水平相速度和频率的某些范围内，全磁太阳黑子模型和热模型中测量的传播时间之间存在（在模拟的噪声水平内）一致。如果这一结论在广泛的模型范围内都成立，那么它将为太阳黑子结构的反演指明一条道路。
