July 08, 2016
impression of Earth's magnetosphere
and leaky upper
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 magneto-tail), which contains
trapped sheets of plasma and interacting field lines.
of Earth's magnetosphere.
our magnetic shield
magnetosphere shield does have its
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
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
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.
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).
of the plasmasphere
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
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
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
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.
The four Cluster
crossing the northern
cusp of Earth's magnetosphere.
Here, the cold ions are thought to be important.
We know that cold ions affect the
magnetic reconnection process, for example slowing down the
reconnection rate at the boundary where the solar wind meets the
magnetosphere (the magnetopause), but we are still unsure of the
mechanisms at play.
"In essence, we need to figure out
how cold plasma ends up at the magnetopause," said Philippe
Escoubet, ESA's Project Scientist for the Cluster mission.
"There are a few different aspects
to this; we need to know the processes involved in transporting
it there, how these processes depend on the dynamic solar wind
and the conditions of the magnetosphere, and where plasma is
coming from in the first place - does it originate in the
ionosphere, the plasmasphere, or somewhere else?"
Recently, scientists modeled and
simulated Earth's magnetic environment with a focus on structures
plasmoids and flux ropes -
cylinders, tubes, and loops of plasma that become tangled up with
magnetic field lines.
These arise when the magnetic
reconnection process occurs in the magnetotail and ejects plasmoids
both towards the outer tail and towards Earth.
Cold ions may play a significant role in deciding the direction of
the ejected plasmoid.
These recent simulations showed a link
between plasmoids heading towards Earth and heavy oxygen ions
leaking out from the ionosphere - in other words, oxygen ions may
reduce and quench the reconnection rates at certain points within
the magnetotail that produce tailward trajectories, thus making it
more favourable at other sites that instead send them Earthwards.
These results agree with existing
Another recent Cluster study compared the two main atmospheric
escape mechanisms Earth experiences - sporadic plumes emanating
through the plasmasphere, and the steady leakage of Earth's
atmosphere from the ionosphere - to see how they might contribute to
the population of cold ions residing at the dayside magnetopause
(the magnetosphere-solar wind boundary nearest the Sun).
Both escape processes appear to depend in different ways on the
interplanetary magnetic field (IMF),
the solar magnetic field that is carried out into the Solar System
by the solar wind.
This field moves through space in a
spiraling pattern due to the rotation of the Sun, like water
released from a lawn sprinkler.
Depending on how the IMF is aligned, it
can effectively cancel out part of Earth's magnetic field at the
magnetopause, linking up and merging with our field and allowing the
solar wind to stream in.
Plumes seem to occur when the IMF is oriented southward
(anti-parallel to Earth's magnetic field, thus acting as mentioned
above). Conversely, leaking outflows from the ionosphere occur
during northward-oriented IMF.
Both processes occur more strongly when
the solar wind is either denser or travelling faster (thus exerting
a higher dynamic pressure).
reconnection in the tail of Earth's magnetosphere.
"While there is still much to learn,
we've been able to make great progress here," said Masson.
"These recent studies have managed
to successfully link together multiple phenomena - namely the
ionospheric leak, plumes from the plasmasphere, and magnetic
reconnection - to paint a better picture of Earth's magnetic
This research required several years
of ongoing observation, something we could only get with
we learn to other planets
Learning more about our own atmosphere can tell us much about our
planetary neighbors - we could potentially apply such research to
any astrophysical object with both an atmosphere and a magnetic
We know that planetary atmospheres play
an essential role in rendering a planet habitable or lifeless, but
there remain many open questions.
Consider the diversity seen in the planets and moons of our Solar
System, for example.
In our small patch of the Universe we
see extreme and opposite worlds:
the smog-like carbon dioxide
atmosphere of Venus
the much-depleted tenuous
atmosphere of present-day Mars
the nitrogen-rich atmosphere of
Saturn's moon Titan
the essentially airless Jovian
the oxygen-bearing atmosphere of
How do we know if these planets could
support life, or whether they may once have done so?
Mars, for example, is thought to have once had a thick,
dense atmosphere that has been considerably stripped away over time.
Although the Red Planet is
unlikely to be habitable today, it may well have been so in the
"Understanding more about our own
atmosphere will help us when it comes to other planets
throughout the Universe," said Escoubet. "We need to know more.
Why does Earth have an atmosphere that can support life, while
other planets do not?"
Cluster is a unique mission:
it comprises four spacecraft - a
format that NASA recently used for their Magnetospheric
Multiscale (MMS) mission, launched in 2015 - which allow
continuous study of Earth's magnetic field and the solar wind
from multiple locations and orientations.
Cluster has been operating since 2000,
and in that time has compiled a wealth of information about our
magnetic environment across various periods of solar and terrestrial
"Additionally, Cluster's orbit is
truly unique amongst all current missions; the fleet is on a
polar orbit, meaning they can explore our planet's dynamic polar
regions - specifically the cusps and polar caps - up close and
in unprecedented detail," added Escoubet.
"Overall, long-term space missions like Cluster are helping us
to understand a whole lot more about our planet, its atmosphere,
and atmospheric loss in general - which in turn will help us to
understand the Solar System in which we live."