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| A cloud of hydrogen atoms called the geocorona exists where Earth’s atmosphere bleeds into outer space. New research suggests that this cloud extends far beyond the orbit of the moon, reaching up to 390,000 miles (630,000 kilometers) above Earth's surface. (Image credit: ESA) |
A recent discovery based on observations by the ESA/NASA Solar and
Heliospheric Observatory, SOHO, shows that the gaseous layer that wraps
around Earth reaches up to 630 000 km away, or 50 times the diameter of our
planet.
“The Moon flies through Earth’s atmosphere,” says Igor Baliukin of Russia’s
Space Research Institute, lead author of the paper presenting the results.
“We were not aware of it until we dusted off observations made over two
decades ago by the SOHO spacecraft.”
Where our atmosphere merges into outer space, there is a cloud of hydrogen
atoms called the geocorona. One of the spacecraft instruments, SWAN, used
its sensitive sensors to trace the hydrogen signature and precisely detect
how far the very outskirts of the geocorona are.
These observations could be done only at certain times of the year, when the
Earth and its geocorona came into view for SWAN.
For planets with hydrogen in their exospheres, water vapour is often seen
closer to their surface. That is the case for Earth, Mars and Venus.
“This is especially interesting when looking for planets with potential
reservoirs of water beyond our Solar System,” explains Jean-Loup Bertaux,
co-author and former principal investigator of SWAN.
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| Earth and its hydrogen envelope, or geocorona, as seen from the moon in ultraviolet light. This image was captured in 1972 using a camera operated by Apollo 16 astronauts on the moon. (Image credit: NASA) |
The first telescope on the Moon, placed by Apollo 16 astronauts in 1972,
captured an evocative image of the geocorona surrounding Earth and glowing
brightly in ultraviolet light.
“At that time, the astronauts on the lunar surface did not know that they
were actually embedded in the outskirts of the geocorona,” says
Jean-Loup.
Cloud of hydrogen
The Sun interacts with hydrogen atoms through a particular wavelength of
ultraviolet light called Lyman-alpha, which the atoms can both absorb and
emit. Since this type of light is absorbed by 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 from the geocorona and discard
hydrogen atoms further out in interplanetary space.
The new study revealed that sunlight compresses hydrogen atoms in the
geocorona on Earth’s dayside, and also produces a region of enhanced density
on the night side. The denser dayside region of hydrogen is still rather
sparse, with just 70 atoms per cubic centimeter at 60 000 kilometers above
Earth’s surface, and about 0.2 atoms at the Moon’s distance.
“On Earth we would call it vacuum, so this extra source of hydrogen is not
significant enough to facilitate space exploration,” says Igor.
The good news is that these particles do not pose any threat for space
travelers on future crewed missions orbiting the Moon.
“There is also ultraviolet radiation associated to the geocorona, as the
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,” says Jean-Loup Bertaux.
On the down side, the Earth’s geocorona could interfere with future
astronomical observations performed in the vicinity of the Moon.
“Space telescopes observing the sky in ultraviolet wavelengths to study the
chemical composition of stars and galaxies would need to take this into
account,” adds Jean-Loup.
The power of archives
Launched in December 1995, the SOHO space observatory has been studying the
Sun, from its deep core to the outer corona and the solar wind, for over two
decades. The satellite orbits around the first Lagrange point (L1), some 1.5
million kilometres from Earth towards the Sun.
This location is a good vantage point to observe the geocorona from outside.
SOHO’s SWAN instrument imaged Earth and its extended atmosphere on three
occasions between 1996 and 1998.
Jean-Loup and Igor’s research team in Russia decided to retrieve this data
set from the archives for further analysis. These unique views of the whole
geocorona as seen from SOHO are now shedding new light on Earth’s
atmosphere.
“Data archived many years ago can often be exploited for new science,” says
Bernhard Fleck, ESA SOHO project scientist. “This discovery highlights the
value of data collected over 20 years ago and the exceptional performance of
SOHO.”
Reference:
“SWAN/SOHO Lyman-alpha mapping: the Hydrogen Geocorona extends well beyond
the Moon” by I. Baliukin et al is accepted in Journal of Geophysical
Research: Space Physics. DOI: 10.1029/2018JA026136
Tags:
Space & Astrophysics