标题： 太阳He I 1083纳米多重图像的空间结构成因 显示英文标题

标题： The cause of spatial structure in solar He I 1083 nm multiplet images

摘要： 背景。 He i 1083纳米线是对推断太阳色球层上部性质的强大诊断工具，特别是磁场的诊断。 在一维模型中该线的基本形成过程已经被很好地理解，但是色球层和日冕复杂三维结构的影响却从未被研究过。 这种结构一定起着重要作用，因为用He i 1083纳米拍摄的图像显示了宽度小至100公里的结构。 目的。 理解太阳大气中三维温度和密度结构对He i 1083纳米线形成的影响。 方法。 我们通过假设一个简单的9级氦原子的统计平衡来求解非局部热平衡（non-LTE）辐射传递问题，尽管这个模型捕捉到了所有关键物理现象。 作为大气模型，我们使用了一个由Bifrost代码计算的三维辐射磁流体力学模拟的快照。 来自日冕的电离辐射被自洽地考虑。 结果。 He i 1083纳米线的发射强度由源函数和色球层上部的吸收率决定。 前者主要由光球辐射的散射主导，并且不随空间位置变化太多。 后者由He i基态连续区的光子电离率以及色球层中的电子密度决定。 电离辐射通量的空间变化是由过渡区中T=100千开物质的空间分布的电离光子发射率引起的。 较热的日冕物质产生更多的电离光子，但由此产生的辐射场是平滑的，不会导致UV通量的小尺度变化。 过渡区的波纹进一步增加了色球层中UV辐射量的空间变化。 显示英文摘要

摘要： Context. The He i 1083 nm is a powerful diagnostic for inferring properties of the upper solar chromosphere, in particular for the magnetic field. The basic formation of the line in one-dimensional models is well understood, but the influence of the complex 3D structure of the chromosphere and corona has however never been investigated. This structure must play an essential role because images taken in He i 1083 nm show structures with widths down to 100 km. Aims. To understand the effect of the three-dimensional temperature and density structure in the solar atmosphere on the formation of the He i 1083 nm line. Methods. We solve the non-LTE radiative transfer problem assuming statistical equilibrium for a simple 9-level helium atom that nevertheless captures all essential physics. As a model atmosphere we use a snapshot from a 3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation from the corona is self-consistently taken into account. Results. The emergent intensity in the He i 1083 nm is set by the source function and the opacity in the upper chromosphere. The former is dominated by scattering of photospheric radiation and does not vary much with spatial location. The latter is determined by the photonionisation rate in the He i ground state continuum, as well as the electron density in the chromosphere. The spatial variation of the flux of ionising radiation is caused by the spatially-structured emissivity of the ionising photons from material at T = 100 kK in the transition region. The hotter coronal material produces more ionising photons, but the resulting radiation field is smooth and does not lead to small-scale variation of the UV flux. The corrugation of the transition region further increases the spatial variation of the amount of UV radiation in the chromosphere.