In the modern view, the atmosphere of our planet ends where the "official" space begins - at an altitude of about 100 km above sea level. In fact, at the end of the last century, the SOHO heliospheric observatory collected data according to which the presence of the earth's atmosphere is observed at a distance of 630, 000 km from the planet. This is puzzling - it turns out that the Moon is not a space object, but a stone in our "sky"?
The SOHO mission began in 1995 and will last until at least 2020. At present, the mission apparatus is located at the Lagrange point L1, 1.5 million km from the Earth, where it studies solar winds. Using the Solar Wind Anisotropie tool, it continuously monitors the presence of hydrogen along the Lyman-alpha line in the solar spectrum. In the same way, the device can direct an instrument to the Earth and measure the parameters of its geocorona - the farthest part of the atmosphere.
The geocorona itself was discovered a long time ago, it even got into historical photographs of the Earth, taken from the Moon during the Apollo 16 mission. The question was in measuring its size - Russian scientists were the first to do this on the basis of SOHO data. It turned out that under the influence of sunlight on the daytime side of the planet, hydrogen is compressed to a concentration of 70 atoms per cubic cm. and approaches the planet at a distance of 60, 000 km. And on the night side, it rapidly expands to a concentration of only 0.2 atoms per cubic meter. see and reaches the orbit of the moon.
It turns out that the Earth's atmosphere has a dynamic shape, but still at altitudes where satellites usually fly, it is already so rarefied that it does not create any difficulties for them. The same cannot be said about sensitive devices, especially those operating in the ultraviolet band of the spectrum. The new knowledge will help in the search for potentially useful exoplanets, where the atmosphere performs protective functions, like on Earth.