The dayside N₂/CO₂ at 140 km altitude in the Martian thermosphere has been investigated by the imaging ultraviolet spectrograph (IUVS) aboard the Mars atmosphere and volatile evolution spacecraft during the period from October 2014 to May 2018. We find that N₂/CO₂ at 140 km altitude varies significantly in the range of 0.02–0.20 and shows a seasonal sinusoidal trend. The higher value appears during aphelion and the lower value during perihelion. Variations of observed N₂/CO₂ ratio at 140 km are mainly associated with CO₂ number density. Thus, while we found that N₂/CO₂ varies at a given altitude, we could not identify variations at a given pressure level. In order to reveal the drivers of N₂/CO₂ variations at 140 km, we examine the effects of surface N₂/CO₂, thermospheric temperature, and homopause altitude. The variations of homopause altitude could be the dominant driver of changes in N₂/CO₂. Inferred dayside homopause altitudes derived from IUVS observations show an anti‐correlation with the trend of N₂/CO₂ at 140 km. Distributions of CO₂ density at the inferred homopause altitudes suggest that dayside homopause altitude is mainly controlled by inflation and contraction of the lower atmosphere. Additionally, N₂/CO₂ shows a clear latitudinal dependence for L_s = 80°–100°. Higher N₂/CO₂ values appear in the southern winter hemisphere, corresponding to lower homopause altitude by ∼30 km. Meanwhile, the latitudinal dependence of both N₂/CO₂ and homopause altitude disappears for L_s = 340°–360°.

中文翻译：

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来自MAVEN IUVS观测的火星热圈中日间N2 / CO2比的季节性和纬度变化

2014年10月至2018年5月期间，火星大气和挥发份（MAVEN）航天器上的成像紫外光谱仪（IUVS）对火星热圈中140 km高度的日间N ₂ / CO ₂进行了研究。我们发现140 km高度的N ₂ / CO ₂在0.02至0.20的范围内变化显着，并呈季节性正弦趋势。较高的值出现在头孢菌素中，而较低的值出现在近日耳蜗中。在140 km处观察到的N ₂ / CO ₂比的变化主要与CO ₂数量密度有关。因此，虽然我们发现N ₂ / CO ₂在给定的高度变化，我们无法确定在给定的压力水平下的变化。为了揭示在140 km处N ₂ / CO ₂变化的驱动因素，我们研究了表面N ₂ / CO ₂，热圈温度和均停留高度的影响。均一高度的变化可能是N ₂ / CO ₂变化的主要驱动因素。从IUVS观测得出的推断的白天同龄动物高度与140 km处的N ₂ / CO ₂趋势呈反相关。CO _2的分布推断的同龄动物高度的密度表明，白天同龄动物的高度主要受较低大气层的膨胀和收缩控制。此外，对于L _s = 80°-100° ，N ₂ / CO ₂表现出明显的纬度依赖性。较高的N ₂ / CO ₂值出现在冬季南部的半球，与较低的同形间歇高度相对应约30 km。同时，当L _s = 340°‐ 360°时，N ₂ / CO ₂和同暂停的高度对纬度的依赖性消失。