Nitrogen is a biosignature gas that cannot be maintained in its Earth-like ratio with CO$_2$ under abiotic conditions. It has also proven to be notoriously hard to detect at optical and infrared wavelengths. Fortunately, the ultraviolet region, which has only recently started being explored for terrestrial exoplanets, may provide new opportunities to characterise exoplanetary atmospheric nitrogen. In this work, the future prospects for detecting atomic nitrogen absorption lines in the transmission spectrum of an Earth-like planet orbiting in the habitable zone of a Sun-like star with LUVOIR are explored. Using the non-local thermodynamic equilibrium spectral synthesis code Cloudy, we produce a far-ultraviolet atomic transmission spectrum for an Earth-Sun-like system, and identify several nitrogen features, including both N I and N II lines. We calculate the number of transits required for 1$\sigma$ and 3$\sigma$ detections of the planetary N{\sc i} $\lambda1200$ triplet signal with the G120M grating of the LUMOS spectrograph designed for LUVOIR, as a function of distance to the system and stellar ultraviolet emission. The minimum number of transit observations necessary for 1$\sigma$ and 3$\sigma$ detections of atomic N are 188 and 1685, respectively, for a system located at a distance of one pc with 100 times the Solar ultraviolet flux. Given that the orbital period of an Earth-Sun system is one year, it is not feasible to detect atomic N in the transmission spectrum for these systems. Future studies in this direction should therefore focus on Earth-like planets orbiting in the habitable zone of M dwarfs.

中文翻译：

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过境类地行星大气中紫外镍线的可观测性

氮是一种生物特征气体，在非生物条件下不能保持其与 CO$_2$ 的地球比例。众所周知，它在光学和红外波长下难以检测。幸运的是，最近才开始探索类地系外行星的紫外线区域，可能为表征系外行星大气氮提供新的机会。在这项工作中，探索了用 LUVOIR 探测在类太阳恒星宜居带中运行的类地行星透射光谱中原子氮吸收谱线的未来前景。使用非局域热力学平衡光谱合成代码 Cloudy，我们为类地太阳系统生成了远紫外原子透射光谱，并确定了几个氮特征，包括 NI 和 N II 线。我们使用专为 LUVOIR 设计的 LUMOS 光谱仪的 G120M 光栅计算行星 N{\sc i} $\lambda1200$ 三重态信号的 1$\sigma$ 和 3$\sigma$ 检测所需的凌日数，作为函数到系统的距离和恒星的紫外线发射。对于位于一台 pc 距离且太阳紫外线通量为 100 倍的系统，1$\sigma$ 和 3$\sigma$ 检测原子 N 所需的最小凌日观测次数分别为 188 和 1685。鉴于地球-太阳系统的轨道周期为一年，在这些系统的传输光谱中检测原子 N 是不可行的。因此，这个方向的未来研究应该集中在在 M 矮星宜居带中运行的类地行星。