Helioseismology of sunspots: Defocusing, folding, and healing of wavefronts
Helioseismology of sunspots: Defocusing, folding, and healing of wavefronts
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太阳黑子的日震学：波前的散焦、折叠和修复
DOI：
10.1051/0004-6361/201321483
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发表时间：
2013
影响因子：
6.5
通讯作者：
Philippe
中科院分区：
文献类型：
--
作者：
Schunker;Philippe
我们观察和表征的声波包的散射由太阳黑子在政权的波长是可比的太阳黑子的大小。利用SOHO/MDI观测到的活动区NOAO 9787黑子周围随机波场的互协方差函数，测量了波的走时和振幅的空间分布图。我们分别考虑传入的平面波包组成的f模式和p模式的径向订单高达4。观测表明，旅行时间扰动随着远离太阳黑子而减小--这是一种在散射理论中被称为波前愈合的有限波长现象。观测还表明，波的振幅在穿过太阳黑子后会减小。我们认为，这种振幅减少的一个显着的部分是由于散焦的波能量的快速波速度扰动引入的太阳黑子。除了太阳黑子对波的物理吸收之外，这种“几何衰减”将有助于波振幅的降低。我们还观察到远离中心路径的波振幅的增强：衍射射线与未扰动射线（焦散线）相交，波前折叠并重复三次。日震学中的波振幅测量提供了独立的信息，可以与走时测量一起使用。
We observe and characterize the scattering of acoustic wave packets by a sunspot in a regime where the wavelength is comparable to the size of the sunspot. Spatial maps of wave travel times and amplitudes are measured from the cross-covariance function of the random wave field observed by SOHO/MDI around the sunspot in active region NOAO 9787. We consider separately incoming plane wave packets consisting of f modes and p modes with radial orders up to four. Observations show that the travel-time perturbations diminish with distance far away from the sunspot – a finite-wavelength phenomenon known as wavefront healing in scattering theory. Observations also show a reduction of the amplitude of the waves after their passage through the sunspot. We suggest that a significant fraction of this amplitude reduction is due to the defocusing of wave energy by the fast wave-speed perturbation introduced by the sunspot. This “geometrical attenuation” will contribute to the wave amplitude reduction in addition to the physical absorption of waves by sunspots. We also observe an enhancement of wave amplitude away from the central path: diffracted rays intersect with unperturbed rays (caustics) and wavefronts fold and triplicate. Wave amplitude measurements in time-distance helioseismology provide independent information that can be used in concert with travel-time measurements.