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The Moon Flies Through The Earth's Atmosphere, SOHO Observations Reveal

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A 20-year-old data from the space-based Solar and Heliospheric Observatory (SOHO) have revealed that the outermost part of the Earth's atmosphere known as geocorona is far bigger than previously thought.

Earth's Atmosphere Extends Beyond The Moon

Researchers said the geocorona, a cloud of hydrogen atoms that glows in far-ultraviolet light, does not just wrap around our planet. It actually reaches nearly twice as far as the moon and is 50 times the Earth's diameter.

"The Moon flies through Earth's atmosphere," said Igor Baliukin, from Russia's Space Research Institute who is one of the authors of the paper that presented the findings.

Baliukin added they were not aware how big the geocorona is until they looked at observations made by the SOHO spacecraft more than two decades ago.

SWAN, one of the instruments aboard SOHO, used its sensitive sensors to trace the hydrogen signature of the geocorona and precisely detect how far this extended. Such observations, could only be done at certain times of the year when the Earth and the geocorona can be viewed from SWAN.

SWAN's hydrogen absorption cell could selectively measure Lyman-alpha, a wavelength of ultraviolet light that hydrogen atoms can both absorb and emit. Because Lyman-alpha is absorbed by Earth's atmosphere, this type of light can only be observed from space.

"The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona," researchers wrote in their study, which was published in the Journal of Geophysical Research: Space Physics.

Implications On Space Studies

The findings about the extent of the Earth's atmosphere's reach have implications in future astronomical observations conducted in the vicinity of the moon. The geocorona may interfere with the observations.

Researchers said space telescopes conducting observations within the geocorona to study the chemical composition of stars and galaxies need to take this into account. The Lyman-alpha baselines of space telescopes likely need to be adjusted for deep-space observations.

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