A method for retrieving temperature and composition from 150 to 350 km in Earth's thermosphere using total number density measurements made via extreme ultraviolet (EUV) solar occultations by the Project for OnBoard Autonomy 2/Large Yield Radiometer (PROBA2/LYRA) instrument is presented. Systematic and random uncertainties are calculated and found to be less than 5% for the temperature measurements and 5%–20% for the composition measurements. Regression coefficients relating both temperature and the [O]/[N₂] abundance ratio with EUV irradiance at 150, 275, and 350 km are reported. Additionally, it is shown that the altitude where [O] equals [N₂] decreases with increasing solar EUV irradiance, an effect attributed to thermal expansion. Temperatures from 2010 to 2017 are compared with estimates from the MSIS empirical model and show good agreement at the dawn terminator but LYRA is markedly cooler at the dusk terminator, with the MSIS-LYRA temperature difference increasing with solar activity. Anthropogenic cooling can explain this discrepancy at periods of lower solar activity, but the divergence of temperature with increasing solar activity remains unexplained. LYRA measurements of the exospheric temperature sensitivity to EUV irradiance are compared with contemporaneous measurements made at Mars, showing that the exospheric temperature at Mars is approximately half as sensitive to EUV variability as that of Earth.

中文翻译：

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PROBA2 LYRA 掩星：热层温度和成分、对 EUV 强迫的敏感性以及与火星的比较

介绍了一种通过车载自主项目 2/大产量辐射计 (PROBA2/LYRA) 仪器通过极紫外 (EUV) 太阳掩星进行的总数密度测量来获取地球热层 150 至 350 公里范围内的温度和成分的方法。计算了系统和随机不确定性，发现温度测量值小于 5%，成分测量值小于 5%–20%。报告了与温度和 [O]/[N ₂ ] 丰度比与 EUV 辐照度在 150、275 和 350 公里处相关的回归系数。此外，还表明 [O] 等于 [N ₂] 随着太阳 EUV 辐照度的增加而降低，这是一种归因于热膨胀的效应。将 2010 年至 2017 年的温度与 MSIS 经验模型的估计值进行比较，在黎明终结者处显示出良好的一致性，但 LYRA 在黄昏终结者处明显更冷，MSIS-LYRA 温差随着太阳活动而增加。人为冷却可以解释太阳活动较低时期的这种差异，但温度随太阳活动增加的差异仍然无法解释。将 LYRA 对 EUV 辐照度的外层温度敏感性测量与在火星进行的同期测量进行比较，表明火星的外层温度对 EUV 变化的敏感度约为地球的一半。