The gaseous layer that envelops the Earth extends up to 630,000 km, or 50 times the diameter of our planet, according to a new study based on observations from the solar and heliospheric observatory ESA / NASA , SOHO, and published in the AGU Journal of Geophysical Research: Physics of space.
"The moon is crossing the Earth's atmosphere," says Igor Baliukin of the Russian Space Research Institute, lead author of the paper presenting the results. "We did not know it before dusting off observations made by the SOHO satellite more than two decades ago."
Where our atmosphere merges with space, there is a cloud of hydrogen atoms called geocorona. SWAN, one of the spacecraft's instruments, has used its sensitive sensors to trace the signature of hydrogen and accurately detect the distance that separates the very periphery of the geocorone. These observations could only be made at certain times of the year, when the Earth and its geocorone would be visible for SWAN.
For planets containing hydrogen in their exospheres, the water vapor often appears closer to their surface. This is the case for the Earth, Mars and Venus.
"This is particularly interesting when we are looking for planets with potential water reservoirs beyond our solar system," says Jean-Loup Bertaux, co-author and former principal investigator of SWAN.
The first telescope on the moon, placed in 1972 by the Apollo 16 astronauts, captured an evocative image of the geocorone surrounding the Earth and shone with ultraviolet light.
"At that time, astronauts on the lunar surface did not know that they were actually embedded in the suburbs of the geocorone," says Jean-Loup.
The sun interacts with the hydrogen atoms via a particular wavelength of ultraviolet light called Lyman-alpha, which atoms can both absorb and emit. As this type of light is absorbed by the Earth's atmosphere, 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 geocorone and reject the hydrogen atoms further into the interplanetary space.
The new study found that sunlight squeezes hydrogen atoms into the geocorn on the land side, and also produces a region of increased density on the night side. The densest region of hydrogen at the level of the day is still rather rare, with only 70 atoms per cubic centimeter at 60,000 kilometers from the surface of the Earth and about 0.2 atoms at the distance from the moon.
"On Earth, we call it empty, so this extra source of hydrogen is not important enough to facilitate space exploration," said Igor. The good news is that these particles pose no threat to space travelers during future crew missions orbiting the moon.
"There is also ultraviolet 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 ", explains Jean-Loup Bertaux.
On the other hand, the Earth's geocorone could hinder future astronomical observations made near the moon.
"Spatial telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies should take this into account," adds Jean-Loup.
The power of archives
Launched in December 1995, the SOHO space observatory studies the sun, from its deep core to the outer crown and solar wind, for more than two decades. The satellite gravitates around the first point of Lagrange (L1), about 1.5 million kilometers from the Earth towards the sun.
This location is a good point of view to observe the geocorone from the outside. The SOHO SWAN instrument imaged the Earth and its atmosphere extended three times between 1996 and 1998.
The research team of Jean-Loup and Igor in Russia decided to extract this dataset from the archives for further analysis. These unique geocorone views from SOHO bring a new light to the Earth's atmosphere.
"Data archived many years ago can often be used for new science," says Bernhard Fleck, scientist for ESA's SOHO project. "This discovery highlights the value of the data collected over 20 years ago and the outstanding performance of SOHO."
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I. I. Baliukin et al. SWAN / SOHO mapping Lyman-α: the hydrogen geogenous extends well beyond the moon, Journal of Geophysical Research: Physics of Space (2019). DOI: 10.1029 / 2018JA026136