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

Na D线发射是地球大气中最显著的夜辉发射特征之一。1929年由Vesto Slipher发现，1939年由Sidney Chapman提出了第一个光化学激发方案。虽然Na D线夜辉发射的研究已经有80多年的历史，但其激发机制仍然没有完全弄清楚。最近的研究发现，D2/D1线比的变化与最初的查普曼机制不一致。2005年，Slanger等人提出了一种改进的机制，该机制分别处理了NaO的不同电子激发态。这一机制已经用地面观测的D2/D1线比进行了测试，但线比的垂直变化--修正的查普曼方案的一个关键测试--尚未得到研究。这里提出的研究的主要科学目标是，利用两种特别适合于这一目的的仪器，即OSIRIS（Odin卫星上的光学摄谱仪和红外成像仪系统）和SCIAMACHY（Envisat上的大气CHartographY扫描成像吸收光谱仪），利用星载临边发射观测，检验我们对Na D线激发机制的理解。卫星观测将用于以下目的：1）OSIRIS测量--其具有非常高的信噪比--将用于根据原始查普曼激发机制，通过NaO和O的反应来经验性地限制Na（2 P）的产生的分支比f。这将与地面Na激光雷达和现有卫星数据集的独立Na廓线观测相结合。2）SCIAMACHY夜间边缘发射观测--它允许分离D1和D2线--将用于反演真实的大气中D2/D1线比的垂直变化。SCIAMACHY是唯一适合于这一目的的（OSIRIS的光谱分辨率不足以分离两条Na D线）。此外，利用SCIAMACHY夜辉观测数据，还可以得到中层顶区的原子氧密度分布，从而解释Na D线的激发比，为进一步理解Na D线的激发机制做出新的贡献。此外，研究结果将巩固的Na密度剖面的反演中层顶区域的基础上观测的Na D线发射。这将通过提供有效查普曼激励方案的分支比f的最佳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.