Atomic hydrogen is one of the least-understood atmospheric constituents whose distribution is important for the studies of aeronomy and magnetospheric physics. Using 6 years of space-based daytime Lyman-α observations from 2002 to 2007, we quantify the solar cycle, seasonal, and dawn-to-dusk variations of the H density in the upper thermosphere. Our results show evident dawn-dusk asymmetry of the exobase H density that decreases nearly linearly from dawn to dusk. The observed asymmetry in terms of the dawn-dusk density ratio decreases with declining solar activity and is larger in summer than in other seasons. Such variations are not predicted by the NRLMSISE-00 model and the NRLMSIS 2.0 model. Those models predict the opposite solar cycle trend and little seasonal variation of the degree of asymmetry. Moreover, although both models and observations indicate higher densities during lower solar activity and during winter than summer, the climatological models underestimate the H density by ~50% when compared to the observations. Our results provide systematic data constraints that have long been needed for improving the geocoronal hydrogen theories.
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