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The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera on board the Proximate Object Close Flyby with Optical Navigation spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled out.

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The water production rate of a comet is one of the fundamental parameters necessary to understand cometary activity when a comet approaches the Sun within 2.5 au, because water is the most abundant icy material in the cometary nucleus. Wide-field imaging observations of the hydrogen Lyα emission in comet 67P/Churyumov–Gerasimenko were performed by the Lyman Alpha Imaging Camera (LAICA) on board the 50 kg class micro spacecraft, the Proximate Object Close Flyby with Optical Navigation (PROCYON), on UT 2015 September 7.40, 12.37, and 13.17 (corresponding to 25.31, 30.28, and 31.08 days after the perihelion passage of the comet, respectively). We derive the water production rates, , of the comet from Lyα images of the comet by using a 2D axi-symmetric Direct Simulation Monte-Carlo model of the atomic hydrogen coma; (1.46 ± 0.47) × 1028, (1.24 ± 0.40) × 1028, and (1.30 ± 0.42) × 1028 molecules s−1 on 7.40, 12.37, and 13.17 September, respectively. These values are comparable to the values from in situ measurements by the Rosetta instruments in the 2015 apparition and the ground-based and space observations during the past apparitions. The comet did not show significant secular change in average water production rates just after the perihelion passage for the apparitions from 1982 to 2015. We emphasize that the measurements of absolute based on the wide field of view (e.g., by the LAICA/PROCYON) are so important to judge the soundness of the coma models used to infer based on in situ measurements by spacecraft, like the Rosetta.

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