Download Inside Earth`s magnetic shield

Inside Earth’s
magnetic shield
Solar wind
Bow shock
A shock wave forms as the solar
wind encounters Earth’s magnetosphere. The oncoming plasma
abruptly slows down and heats up.
An invisible structure protects Earth from
all but the Sun’s worst outbursts. Scientists
are starting to understand how it works.
Magnetosheath
Most of the hot, turbulent
plasma behind the bow
shock deflects around the
space controlled by
Earth’s magnetic field.
A
Corona
This is the Sun’s outermost
atmosphere and the solar
wind’s source.
Magnetopause
by Francis Reddy; illustration by Roen Kelly
magnetic shield envelops our planet, but the only visible evidence it exists are the rays and curtains of the
nightside aurora. Such a structure, which scientists call
a magnetosphere, forms wherever the Sun’s outflow encounters a strong planetary magnetic field.
In 1958, America’s first spacecraft, Explorer 1, began direct
study of the magnetosphere when it discovered the Van Allen
radiation belts — two regions of charged particles. Since then,
scientists have worked to understand the magnetosphere’s
structure and the complex interactions occurring within it.
The most recent exploration involves satellite fleets from
NASA and the European Space Agency (ESA). NASA’s entry
is called THEMIS, for Time History of Events and Macroscale
Interactions during Substorms. Five probes pursue orbits in
which the spacecrafts’ highest altitudes periodically align
above North American ground stations. In 2007, THEMIS
found a temporary hole in Earth’s shield. A magnetic conduit
called a flux rope formed and decayed over the course of a
few hours, channeling solar wind energy inside.
ESA’s Cluster mission flies four probes in a pyramid-shaped
formation. Findings include giant plasma swirls that form in
much the same way as the ripples of a flag in the wind.
Earlier this year, scientists announced that Cluster had
located the source of auroral kilometric radiation. The intense
50-to-500-kilohertz radio emission beams into space thousands of miles above auroral regions.
Such broadcasts appear to be common features of all planetary magnetospheres, says Robert Mutel, a Cluster scientist
at the University of Iowa. Radio observatories now under construction may one day hear this signal from far-flung alien
worlds protected by their own magnetic shields.
Sun
A flow of charged subatomic particles
(plasma) streams from the Sun at
speeds up to 2 million mph (3 million
km/h). It extends the Sun’s magnetic
field throughout the solar system.
This is the border of Earth’s magnetosphere, the boundary
that separates the solar wind from plasma within our
planet’s magnetic control. Under typical conditions, its nose
lies 40,000 miles (64,000 km) toward the Sun, but a strong
CME can push it closer than geosynchronous orbit.
THEMIS
Launched in 2007, NASA’s Time History of
Events and Macroscale Interactions during
Substorms (THEMIS) mission consists of five
identical probes in orbits at different distances.
ACE
SOHO
Coronal mass ejection (CME)
A billion-ton cloud of plasma shot
from the Sun, a CME can race through
the inner solar system at more than 5
million mph (8 million km/h).
Wind
Early warning system
The NASA/ESA Solar and Heliospheric
Observatory (SOHO) and NASA’s
Advanced Composition Explorer (ACE)
and Wind spacecraft monitor solar
gusts and gales 930,000 miles (1.5
million km) upwind of Earth.
Flux rope
Solar wind magnetic fields can
organize into a short-lived braided
bundle that directly channels
plasma into the magnetosphere.
Geosynchronous orbit
Earth
Satellites 22,240 miles (35,790 km)
high complete one orbit each day and
remain above a fixed point on Earth.
Many weather and communications
satellites operate here.
Geotail
This probe, launched in 1992, represents a
collaborative effort by NASA and Japan. It
explored the magnetotail more than 790,000
miles (1.3 million km) behind Earth.
Auroral oval
This glowing oval band, which
forms around each magnetic pole,
is where structured aurorae occur.
Cluster
Launched in 2000, four identical probes
make up this European Space Agency
mission. The probes fly polar orbits but
maintain a pyramid-shaped formation.
Plasmasphere and radiation belts
Francis Reddy is a senior editor of Astronomy.
Reconnection
The solar wind magnetic field can briefly
merge with Earth’s. These so-called
reconnection events blast magnetotail
plasma toward Earth. THEMIS data
proved these events trigger auroral
activity cycles (substorms).
Magnetotail
The solar wind stretches the nightside
magnetosphere into a tail hundreds of
Earth diameters long.
The plasmasphere contains relatively dense,
low-energy plasma. It includes the famous Van
Allen radiation belts and can be considered an
extension of Earth’s ionosphere.
Hot flow anomaly
A fast stream embedded in the solar wind strikes the bow shock
and explodes, creating pressures that briefly push back the
magnetopause. THEMIS detected such an event July 4, 2007.
Plasma sheet
This extensive area of low-energy ionized
gases in the magnetotail undergoes
considerable change during solar storms.
Kelvin-Helmholz vortices
In 2001, Cluster detected swirls of plasma 25,000
miles across (40,000 km) at the magnetopause.
The swirls form when fluids move at different
speeds across an interface. The same process
causes a flag to ripple in the wind.