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Posted by baalke on October 24, 2007, 5:32 pm
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http://www.swri.org/press/2007/Jovian.htm
New theory proposes Jovian magnetosphere circulates magnetic field
remarkably different from that of Earth
Southwest Research Institute
For immediate release
PIO Contact:
Maria Martinez
maria.martinez@swri.org
(210) 522-3305
San Antonio -- Oct. 23, 2007 -- Space physicists have long assumed
that the
magnetosphere at Jupiter circulates that planet's magnetic field in
the
same way as Earth. At Earth, this circulation drives the aurora and
the
magnetic storms that cause space weather. Researchers from Southwest
Research Institute and the University of Colorado at Boulder have
developed
a new model that postulates the structure and magnetospheric processes
at
Jupiter are significantly different from those at Earth.
The invisible area of space around a planet controlled by its
magnetic
field, the magnetosphere, interacts with the high-speed solar wind in
a
complex way, particularly in the area where the magnetic field in the
solar
wind interconnects with the planetary field, through a process called
magnetic reconnection. The Dungey cycle, developed by British
scientist Jim
Dungey in 1961, is the scientifically accepted paradigm for explaining
how
magnetic reconnection circulates the Earth's magnetic field. During
this
cycle, magnetic field lines are brought up near the nose of the
magnetosphere
where they interconnect, becoming "open" and coupling the energy from
the
motion of the solar wind into the magnetosphere. That interconnection
allows
vast energy from the million mile-per-hour solar wind into the
magnetosphere,
which is the driving force behind geomagnetic storms, or space
weather, that
can seriously damage or destroy probes and satellites. Subsequent
motion of
the solar wind around the Earth's magnetosphere drags the
interconnected
field lines back over its magnetic poles where they drift down into
the
center of the magnetotail and reconnect again, but this time with
similar
field lines from the opposite hemisphere so that they are "closed" or
connected to the planet at both ends. Finally, the Dungey cycle
completes
as the newly closed field lines circulate back toward Earth, around to
its
dayside and back to its starting position at the nose of the
magnetosphere.
"For years space physicists have considered the Dungey cycle to be the
dominant circulation process in magnetospheres throughout the solar
system,
even though observations from the largest magnetosphere in the solar
system
-- Jupiter's -- didn't add up," says Dr. David McComas, senior
executive
director of the Space Science and Engineering Division at Southwest
Research
Institute.
"There are three key ways that the magnetosphere of Jupiter differs
from
that of Earth," argues Dr. Fran Bagenal, a professor of Astrophysical
and
Planetary Sciences at CU. "It's much bigger, it spins faster and it
has a
powerful source of material."
The large size of the Jovian magnetosphere means that the time it
takes for
material that reconnects in the magnetotail and moves back up to Earth
is
only about 10 hours, less than half a day. However, the process at
Jupiter
takes 750 to 1,000 hours.
"Consider that a Jupiter day is only about 10 hours," says McComas.
"That
means it would take as many as 100 Jovian days for reconnected field
lines
to move back up to Jupiter -- a staggering difference."
Furthermore, the magnetosphere of Jupiter is coupled to the spinning
planet. "Imagine stirring up a bowl of spaghetti," says Bagenal. "The
fast,
10-hour spin of the Jovian magnetic field complicates the topology of
flux
tubes that are connected to the planet on one end while the other,
open end
is swept away by the solar wind."
Another difference is that Jupiter has an active volcanic moon, Io,
which
spews out roughly a ton of material, mostly sulfur and oxygen, every
second. Half of that material is lost through a process called
charge
exchange, but the other half moves down the Jovian magnetotail as
ions
dragging the planetary magnetic field tailward. Earth has no such
counterpart to impede the return flow back toward the planet.
The new theory suggests a different geometry for closing off the
magnetic
field that has become interconnected with the solar wind --
additional
magnetic reconnection with other solar wind field lines that produce
closed
planetary field lines by reconnecting with open lines anchored back to
both
magnetic poles. This geometry at Jupiter allows for tailward flow
everywhere
in the tail and doesn't require a planetward flow, as at Earth. This
explains
why the polar aurora at Jupiter doesn't look like the terrestrial
aurora. It
also explains why observations from Ulysses showed that open flux
occurs at low
latitudes, not at the high latitudes required at Earth. The
magnetosphere
at Jupiter drags the material further down the sides so that they
occur at
lower latitudes.
"Our model matches up with the observations -- further evidence that
the
magnetospheric structure and processes at Earth and Jupiter are quite
different," says McComas.
McComas and Bagenal determined these processes for Jupiter, yet they
could
aid in understanding the magnetospheres of the other outer planets, as
well
as in other astrophysical environments where magnetic fields play an
important
role.
The paper, "Jupiter: A Fundamentally Different Magnetospheric
Interaction
with the Solar Wind," by David J. McComas and Fran Bagenal was
published in
the Oct. 24 issue of Geophysical Research Letters.
Editors: Images to accompany this article are available at
http://www.swri.org/press/2007/Jovian.htm.
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