Earth's atmosphere is leaking. Every day, around 90 tonnes of material escapes from our planet's upper atmosphere and streams out into space. Although missions such as ESA's Cluster fleet have long been investigating this leakage, there are still many open questions. How and why is Earth losing its atmosphere – and how is this relevant in our hunt for life elsewhere in the Universe?
Given the expanse of our atmosphere, 90 tonnes per day amounts to a small leak. Earth's atmosphere weighs in at around five quadrillion (5 × 1015) tonnes, so we are in no danger of running out any time soon. However, understanding Earth's atmosphere, and how it escapes to space, is key to understanding the atmospheres of other planets, and could be crucial in our hunt for habitable planets and extraterrestrial life.
We have been exploring Earth's magnetic environment for years using satellites such as ESA's Cluster mission, a fleet of four spacecraft launched in 2000. Cluster has been continuously observing the magnetic interactions between the Sun and Earth for over a decade and half; this longevity, combined with its multi-spacecraft capabilities and unique orbit, have made it a key player in understanding both Earth's leaking atmosphere and how our planet interacts with the surrounding Solar System.
Earth's magnetic field is complex; it extends from the interior of our planet out into space, exerting its influence over a region of space dubbed the magnetosphere.
The magnetosphere – and its inner region (the plasmasphere), a doughnut-shaped portion sitting atop our atmosphere, which co-rotates with Earth and extends to an average distance of 20 000 km – is flooded with charged particles and ions that are trapped, bouncing back and forth along field lines.
At its outer Sunward edge the magnetosphere meets the solar wind, a continuous stream of charged particles – mostly protons and electrons – flowing from the Sun. Here, our magnetic field acts like a shield, deflecting and rerouting the incoming wind as a rock would obstruct a stream of water. This analogy can be continued for the side of Earth further from the Sun – particles within the solar wind are sculpted around our planet and slowly come back together, forming an elongated tube (named the magnetotail), which contains trapped sheets of plasma and interacting field lines.
However, our magnetosphere shield does have its weaknesses; at Earth's poles the field lines are open, like those of a standard bar magnet (these locations are named the polar cusps). Here, solar wind particles can head inwards towards Earth, filling up the magnetosphere with energetic particles.
Just as particles can head inwards down these open polar lines, particles can also head outwards. Ions from Earth's upper atmosphere – the ionosphere, which extends to roughly 1000 km above the Earth – also flood out to fill up this region of space. Although missions such as Cluster have discovered much, the processes involved remain unclear.
"The question of plasma transport and atmospheric loss is relevant for both planets and stars, and is an incredibly fascinating and important topic. Understanding how atmospheric matter escapes is crucial to understanding how life can develop on a planet," said Arnaud Masson, ESA's Deputy Project Scientist for the Cluster mission. "The interaction between incoming and outgoing material in Earth's magnetosphere is a hot topic at the moment; where exactly is this stuff coming from? How did it enter our patch of space?"
Initially, scientists believed Earth's magnetic environment to be filled purely with particles of solar origin. However, as early as the 1990s it was predicted that Earth's atmosphere was leaking out into the plasmasphere – something that has since turned out to be true.
Observations have shown sporadic, powerful columns of plasma, dubbed plumes, growing within the plasmasphere, travelling outwards to the edge of the magnetosphere and interacting with solar wind plasma entering the magnetosphere.
More recent studies have unambiguously confirmed another source – Earth's atmosphere is constantly leaking! Alongside the aforementioned plumes, a steady, continuous flow of material (comprising oxygen, hydrogen, and helium ions) leaves our planet's plasmasphere from the polar regions, replenishing the plasma within the magnetosphere. Cluster found proof of this wind, and has quantified its strength for both overall (reported in a paper published in 2013) and for hydrogen ions in particular (reported in 2009).
Overall, about 1 kg of material is escaping our atmosphere every second, amounting to almost 90 tonnes per day. Singling out just cold ions (light hydrogen ions, which require less energy to escape and thus possess a lower energy in the magnetosphere), the escape mass totals thousands of tonnes per year.
Cold ions are important; many satellites – Cluster excluded – cannot detect them due to their low energies, but they form a significant part of the net matter loss from Earth, and may play a key role in shaping our magnetic environment.
Solar storms and periods of heightened solar activity appear to speed up Earth's atmospheric loss significantly, by more than a factor of three. However, key questions remain: How do ions escape, and where do they originate? What processes are at play, and which is dominant?
Where do the ions go? And how?
One of the key escape processes is thought to be centrifugal acceleration, which speeds up ions at Earth's poles as they cross the shape-shifting magnetic field lines there. These ions are shunted onto different drift trajectories, gain energy, and end up heading away from Earth into the magnetotail, where they interact with plasma and return to Earth at far higher speeds than they departed with – a kind of boomerang effect.
Such high-energy particles can pose a threat to space-based technology, so understanding them is important. Cluster has explored this process multiple times during the past decade and a half – finding it to affect heavier ions such as oxygen more than lighter ones, and also detecting strong, high-speed beams of ions rocketing back to Earth from the magnetotail nearly 100 times over the course of three years.
More recently, scientists have explored the process of magnetic reconnection, one of the most efficient physical processes by which the solar wind enters Earth's magnetosphere and accelerates plasma. In this process, plasma interacts and exchanges energy with magnetic field lines; different lines reconfigure themselves, breaking, shifting around, and forging new connections by merging with other lines, releasing huge amounts of energy in the process.
Read more at: http://phys.org/news/2016-07-curious-case-earth-leaking-atmosphere.html#jCp