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The Magnetosphere:
Earth’s Invisible Shield
Dr. Wayne R. Keith
Angelo State University
February 1, 2006 McMurry University
Introduction
• The Earth’s magnetosphere may be
invisible, but it plays an important role in
making life on Earth possible by protecting
us from the solar wind and harmful high-
energy cosmic rays.
• The lack of a global magnetic field on
Venus and Mars has contributed to their
inhospitality to life.
February 1, 2006 McMurry University
Outline
• History
– A brief account of how our understanding of
the magnetosphere and its importance has
evolved over time.
• Anatomy
– A description of the various regions.
• Applications/Science
– how the magnetosphere affects life on Earth
and the ways that we study it.
February 1, 2006 McMurry University
History
• Knowledge of some components of the
magnetosphere date back centuries, but
mostly this is a very young scientific field
in which some of the original pioneers are
still active.
• I will touch on only a few of the highlights,
but of course important contributions have
been made by many others to bring us to
our current understanding.
February 1, 2006 McMurry University
History – Magnetic Field
• It has been known
since the 1600’s that
the Earth itself is a
giant magnet.
• Convective motions
and rotation of the
iron/nickel core
generate a “dipolar”
field similar to a bar
magnet.
February 1, 2006 McMurry University
History – Comets
• Also in the 1600’s,
Johannes Kepler noted
that comet dust tails
always point away from
the sun, and guessed it
was due to sunlight.
• A comet’s second tail
remained unexplained
until the twentieth
century, when the idea of
a flow of ionized particles
from the sun, a solar
Comet Hale-Bopp wind, gained acceptance.
February 1, 2006 McMurry University
History – Solar Wind
• Kristian Birkeland was
the first to predict, in
1913, that the Sun
gives off a steady
“wind” of ionized gas,
or plasma.
• He used a Terrella
(magnetized ball) to
show that the plasma
is directed to the
poles, causing aurora.
February 1, 2006 McMurry University
History – Particle Trapping
• Carl Stormer used
Birkland’s results and
showed mathematically
that particles following
dipolar field lines can
become trapped in a
sort of “magnetic
bottle”.
February 1, 2006 McMurry University
History – Magnetic Bubble
• The modern concept of a
magnetosphere forming
by the interaction of a
neutral solar wind and the
geomagnetic field was
postulated by Chapman
and Ferraro in 1931.
• Their paper correctly
predicted that the Earth’s
magnetic field would
deflect the ionized solar
wind, forming a cavity in
the stream.
February 1, 2006 McMurry University
History – Radiation Belts
• The U.S. entered the
Space Age with
Explorer 1 in January
1958. James Van
Allen’s Geiger counter
saturates at two
altitudes.
• He has discovered
what will become
known as the Van
Allen Radiation Belts.
February 1, 2006 McMurry University
History – Today
• Dozens of spacecraft
from many different
countries have been
launched to study the
particles and fields that
surround the Earth.
• As each new level of
complexity is understood,
new questions are raised
to inspire the next
generation of missions.
IMAGE Spacecraft: EUV Instrument
February 1, 2006 McMurry University
Anatomy
• Magnetospheric physics involves a large
number of regions and sub-regions, all
with their own special terminology.
• Each region will be described separately,
and then we will put the entire picture
together.
February 1, 2006 McMurry University
Anatomy – Bow shock
• The solar wind is
supersonic, and the
Earth is a magnetic
obstacle to the flow,
so a shockwave is
formed between the
Sun and the Earth.
• Similar to the
shockwave of a
supersonic jet.
February 1, 2006 McMurry University
Anatomy – Magnetosheath
• Inside the bow shock,
the solar wind has
been heated and
decelerated.
• Most of the plasma
from the sun is
deflected and does
not enter the region
dominated by Earth’s
magnetic field.
February 1, 2006 McMurry University
Anatomy – Magnetopause
• The boundary
between the
sheath and the
magnetosphere
(the region
dominated by
Earth’s field) is
called the
magnetopause.
• Most of the solar
wind is kept
out… but some
still gets in.
February 1, 2006 McMurry University
Anatomy – Cusps
• At the point where the magnetic field lines
switch from closing on the dayside to being
swept back into the tail, there is a pair of
weak field regions called the cusps.
• Acts as a sort of plasma funnel, letting in
some of the solar wind particles.
February 1, 2006 McMurry University
Anatomy – Reconnection
• Depending on the
orientation of the
magnetic field carried
with the solar wind,
the fields can also
interconnect, allowing
plasma to pass
through the
magnetopause.
February 1, 2006 McMurry University
Anatomy – Van Allen Belts
• Some of the plasma
inside the
magnetosphere
becomes trapped in
the Earth’s magnetic
field, forming stable
regions of high-
energy particles
called radiation belts.
February 1, 2006 McMurry University
Anatomy – Magnetotail
• The force of the solar
wind sweeps the
magnetosphere into an
extended teardrop shape,
forming a tail that extends
past the orbit of the
moon.
• Particles in the equatorial
“plasma sheet” can be
very energetic, and
periodically rain down
along the field lines
towards the poles,
exciting the atmosphere
with colorful displays.
February 1, 2006 McMurry University
Anatomy – Magnetosphere
February 1, 2006 McMurry University
Applications/Science
• Understanding the magnetosphere is nice,
but how does all this affect me here in
Abilene, Texas?
• To see what the Earth might be like
without a magnetosphere, we need look
no further than our neighboring planets
Venus and Mars.
February 1, 2006 McMurry University
Applications/Science – Venus
• Venus rotates too
slowly to generate
an internal magnetic
field, so the solar
wind interacts
directly with the
ionosphere.
• Hydrogen is stripped
away, and over
time, there is very
little left to form
water.
February 1, 2006 McMurry University
Applications/Science – Mars
• Mars also lacks a
global magnetic field,
although in this case
due to the absence
of a liquid core layer.
• Most of the Martian
atmosphere has
been lost to the solar
wind, leaving it a
dead world.
February 1, 2006 McMurry University
Applications/Science – CME’s
• In addition to the
constant stream of
solar wind, the Sun
periodically has
violent storms, and
can eject huge blobs
of plasma called
CME’s (Coronal Mass
Ejections).
SOHO Spacecraft: LASCO and EIT Instruments
February 1, 2006 McMurry University
Applications/Science – Space Weather
• The magnetopause bears the brunt of these Solar
assaults, but a lot of energy still gets through, which can
cause power blackouts and satellite damage, not to
mention some very spectacular auroral displays.
February 1, 2006 McMurry University
Applications/Science – Cosmic Rays
• Another hazard from
space is cosmic rays.
These super-high-
energy particles
come from outside
the solar system at
incredible velocities.
Many are deflected,
however, some still
make it to the
atmosphere.
February 1, 2006 McMurry University
Applications/Science – Spacecraft
• The fleet of scientific
spacecraft dedicated
to studying the
magnetosphere are
helping us learn
exactly how our
invisible shield works,
and to predict when
and how “space
weather” will affect us.
February 1, 2006 McMurry University
Applications/Science – Cusps
• A major area of study is
the cusps, important
“input” regions where
much of the mass and
energy transfer from
the solar wind takes
place.
• The Cluster and DMSP
missions study the
cusps up near the
magnetopause, and
down close to the
Earth.
February 1, 2006 McMurry University
226 IL vs Emax C1,C3,F13,F14
F13
F14
DMSP Spacecraft
SSJ4 Instrument
•These Data show how
the two altitude regions
are being compared to
C1 C3 learn more about the
CLUSTER Spacecraft: CIS Instrument
entry processes.
February 1, 2006 McMurry University
Conclusions
• The Earth’s magnetosphere is a shield
that protects us from dangerous charged
particles from the Sun and elsewhere in
the cosmos.
• Understanding this region of space is
important for anticipating and protecting
against harmful effects of space storms
caused by ejections from the Sun.
February 1, 2006 McMurry University
