太阳大气中氢原子的赖曼谱线

氢是太阳大气中最主要的元素。氢原子的赖曼(Lyman)谱线,尤其是赖曼阿尔法(Ly-α)谱线的辐射,是太阳色球和低过渡区能量损失的主要形式。在太阳的赖曼α像中,网络组织的辐射比较强,而辐射最强的地方是活动区。由于存在辐射转移效应,在宁静区,低阶赖曼谱线的谱形中央一般会形成一个凹陷,而在中央两侧则形成两个峰,两峰往往呈现出一定的不对称性。数值模拟和观测研究表明,赖曼谱线双峰的不对称性与高层大气中各种系统性流动有关。在太阳活动区,赖曼谱形在谱斑区与在宁静区类似;而在黑子区,赖曼谱形几乎没有中央凹陷。赖曼谱形也可用于诊断日珥、耀斑和日冕物质抛射等结构和现象的等离子体特性。该文回顾了赖曼谱线的观测历史,阐明了观测与模拟结果所揭示的物理过程,并结合笔者的认识进行了相应的评论。

Hydrogen Lyman Lines Emitted from the Solar Atmosphere

Hydrogen is the most abundant element in the solar atmosphere and its resonance lines, especially the Lyman-alpha (Ly-α) line, plays an important role in the overall radiative energy transport of the Sun. In this paper we review the observational and modeling results on the solar hydrogen Lyman line emissionsThe Ly-α line was used to observe the Sun since 1950s, when the era of space just began. It was found that the Ly-α emission is enhanced at lanes of the chromospheric network, and the brightest features in Ly-α images are active regions. However, so far no continuous Ly-α observation with both high spatial and temporal resolutions has been done.In the normal quiet Sun region, due to the effect of radiative transfer, the average lower-order Lyman line (Ly-β through Ly-ε) profiles are found to be self-reversed at the centers and stronger in the red-horns. While the higher H Lyman series line (beyond Ly-ζ) profiles are flat-topped or even Gaussian-shaped. Recently, the Ly-α profiles were also obtained with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument in high spectral, temporal, and spatial resolutions. It turns out that most Ly-α profiles are strongly reversed and stronger in the blue horns. It is believed that the opposite asymmetries in the average profiles of Ly-α and higher Lyman lines are probably caused by the combined effect of flows in the different layers of the solar atmosphere and opacity differences of the lines. A mechanism for line formation can not be simply imagined but must be thoroughly devised and further investigated with the help of models.In active regions, Lyman line profiles, especially Ly-α and Ly-β profiles, were obtained from early rocket observations and found to be similar to those in the quiet Sun. However, profiles obtained in these early observations suffered from geocoronal absorption. SUMER observations avoided this problem and revealed that the Lyman line profiles in sunspots show properties different from the average profiles. The self-reversals of the hydrogen Lyman line profiles are almost absent in umbra regions. In the sunspot plume, the Lyman lines are also not reversed. While the lower-order Lyman line profiles observed in the plage region are obviously reversed, a phenomenon also found in the normal quiet Sun. This result indicates that the opacity of the hydrogen lines is much smaller above sunspots, as compared to plage regions. The ability of diagnosing plasma properties in solar flares and CMEs by analyzing the Lyman line profiles is also demonstrated.Hydrogen Lyman lines were also frequently used to reveal information on the fine structures and physical properties of quiescent solar prominences. Observations and modelings show that the profiles are more reversed when seen across than along the magnetic field lines.