Apollo 16 Lyman alpha imagery of the hydrogen geocorona

Lyman α imagery of the hydrogen geocorona was obtained from the lunar surface during the Apollo 16 mission. The images are of 20° diameter fields, with 2 arc min limiting resolution, centered on the earth and about 12° upsun of the earth. The data confirm that the hydrogen geocorona is detectable above the interplanetary Lyman α background to more than 15 RE in the upsun direction. In the antisolar direction there is a pronounced ‘geotail’ effect due to solar Lyman α radiation pressure, which is markedly asymmetric about the sun line. Comparison of the data with theoretical models shows quite good agreement, particularly in the vicinity of 3 RE. In agreement with other observations the hydrogen density is lower than the theoretical density in the far-upsun direction, and very close to the earth there is a buildup of hydrogen on the dark side.

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