Investigating the excitation mechanism of the Na D-line nightglow emission

Na d线发射是地球大气中最突出的夜光发射特征之一。1929年维斯托·斯里弗发现了它，1939年西德尼·查普曼提出了第一个光化学激发方案。虽然Na d线夜光发射已经被研究了80多年，但其激发机制仍未完全了解。最近的研究发现D2/D1系比值的变化与最初的Chapman机制不一致。2005年，Slanger等人（2005）提出了一种改进的机制，分别对待NaO的不同电子激发态。这种机制已经通过D2/D1线比的地面观测进行了测试，但是线比的垂直变化——改进的Chapman方案的一个关键测试——尚未得到研究。本文提出的研究的主要科学目标是利用两种独特适合于这一目的的仪器，即奥丁卫星上的光学光谱仪和红外成像仪系统OSIRIS和Envisat上的扫描成像吸收光谱仪SCIAMACHY，通过星载肢发射观测来测试我们对Na d线激发机制的理解。具体而言，卫星观测将用于以下目的：1)OSIRIS测量-具有非常高的信噪比-将用于根据原始Chapman激发机制，通过NaO和O的反应来经验地约束Na（2P）的分支比f。这将结合使用地面激光雷达和现有卫星数据集进行独立的钠剖面观测。2) SCIAMACHY夜间边缘发射观测-允许分离D1和D2线-将用于检索真实大气中D2/D1线比的垂直变化。SCIAMACHY是唯一适合这个目的的（OSIRIS的光谱分辨率不足以分离两条Na - d线）。此外，从SCIAMACHY夜光测量中可以获得中层顶区域的原子氧密度分布，以解释推断的Na d线比率。通过这种方法，我们的研究可以为提高我们对Na d线激发机制的理解做出重要的新贡献。此外，该结果将巩固基于Na d线发射观测的中顶区域Na密度剖面的检索。这将通过提供有效查普曼激励方案的分支比f的最优值及其不确定性的估计来实现。

The Na D-line emission is one of the most prominent nightglow emission features in the terrestrial atmosphere. It was discovered in 1929 by Vesto Slipher and a first photochemical excitation scheme was proposed in 1939 by Sidney Chapman. Although the Na D-line nightglow emission has been studied for more than 80 years, its excitation mechanism is still not fully understood. Recent studies identified variations in the D2/D1 line ratio inconsistent with the original Chapman mechanism. In 2005 a modified mechanism was proposed by Slanger et al. (2005) that treats different electronically excited states of NaO separately. This mechanism has been tested using ground-based observations of the D2/D1 line ratio, but the vertical variation of the line ratio - a critical test of the modified Chapman scheme - has not yet been studied. The main science objectives of the research proposed here is to test our understanding of the Na D-line excitation mechanism using space-borne limb-emission observations with two instruments that are uniquely suited for this purpose, i.e. OSIRIS, the Optical Spectrograph and InfraRed Imager System on the Odin satellite and SCIAMACHY, the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY on Envisat.Specifically, the satellite observations will be employed for the following purposes: 1) OSIRIS measurements - which have a very high signal-to-noise ratio - will be employed to empirically constrain the branching ratio f for the production of Na(2P) through the reaction of NaO and O, according to the original Chapman excitation mechanism. This will be done in combination with independent Na profile observations with ground-based Na LIDARs and available satellite data sets.2) SCIAMACHY nighttime limb-emission observations - that allow separating the D1 and D2 lines - will be employed to retrieve the vertical variation of the D2/D1 line ratio in the real atmosphere. SCIAMACHY is uniquely suited for this purpose (the spectral resolution of OSIRIS is not sufficient to separate the two Na D-lines). In addition, atomic oxygen density profiles in the mesopause region are available from SCIAMACHY nightglow measurements to interpret the inferred Na D-line ratios.With this approach the proposed research can make important and new contributions to improve our understanding of the Na D-line excitation mechanism. In addition, the results will consolidate the retrieval of Na density profiles in the mesopause region based on observations of the Na D-line emission. This will be achieved by providing an optimum value of the branching ratio f of the effective Chapman excitation scheme together with an estimate of its uncertainty.