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Recent observations by NASA / ESA's solar and heliospheric observatory (SOHO) show that the Earth's hydrogen envelope reaches a distance of 630,000 km (391,500 miles), which is 50 times the diameter of our planet.
The extent of the terrestrial geocorone. Image credit: ESA.
Where our atmosphere merges with space, there is a cloud of hydrogen atoms called geocorona.
"The first telescope on the moon, placed by the Apollo 16 astronauts in 1972, captured an image of the geocorone surrounding the Earth and shone with ultraviolet (UV) light," said Dr. Jean- Wolf Bertaux, researcher at Versailles University. Saint-Quentin in Guyancourt, France.
"At that time, astronauts at the lunar surface did not know that they were actually integrated on the periphery of geocorona."
The SWAN (Solar Wind ANisotropies) instrument onboard SOHO has used its sensitive sensors to trace the signature of hydrogen and accurately detect the distance that separates the very suburbs from the geocorone.
"The Sun interacts with the hydrogen atoms through a particular wavelength of ultraviolet light called Lyman-alpha, which atoms can both absorb and emit," explained the Dr. Bertaux and his colleagues.
"Since this type of light is absorbed by the atmosphere of the Earth, it can only be observed from space."
"Thanks to its hydrogen absorption cell, the SWAN instrument could selectively measure the Lyman-alpha light of the geocore and reject the hydrogen atoms farther into the interplanetary space."
SWAN observations have shown that sunlight compresses hydrogen atoms in the geocorn on the land side, and also produces a region of increased density on the night side.
The denser region of day-side hydrogen is still rather rare, with only 70 atoms per cm3 37,000 miles (60,000 km) above the surface of the Earth and about 0.2 atom at the distance from the Moon.
"On Earth, we call it emptiness. This additional source of hydrogen is therefore not important enough to facilitate space exploration, "said Dr. Igor Baliukin, of the Space Research Institute.
"There is also UV radiation associated with geocorone, since hydrogen atoms scatter sunlight in all directions, but the impact on astronauts in lunar orbit would be negligible compared to the main source of radiation – the Sun, "said Dr. Bertaux.
"In contrast, the Earth's geocorone could interfere with future astronomical observations made near the Moon."
"Spatial telescopes observing the sky in UV wavelengths to study the chemical composition of stars and galaxies should take this into account."
The results were published in the Journal of Geophysical Research: Physics of Space.
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I.V. Baliukin et al. SWAN / SOHO mapping Lyman-α: the hydrogen geogenous extends well beyond the Moon. Journal of Geophysical Research: Physics of Space, published online February 15, 2019; doi: 10.1029 / 2018JA026136
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