NASA's Interstellar Boundary Explorer has observed
and described the solar system's tail for the first time.
Image Credit:
NASA/Goddard Space Flight Center
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It has long been assumed that our solar system, like a comet, has a tail.
Just as any object moving through another medium – for example, a meteor
traveling through Earth’s atmosphere – causes the particles to form a stream
trailing off behind it. But the tail of our solar bubble, called the
heliosphere, has never actually been observed, until now.
NASA’s Interstellar Boundary Explorer, or IBEX, has mapped the boundaries of
the tail of the heliosphere, something that has never before been possible.
Scientists describe this tail, called the heliotail, in detail in a paper
published on July 10, 2013, in The Astrophysical Journal. By combining
observations from the first three years of IBEX imagery, the team mapped out a
tail that shows a combination of fast and slow moving particles. There are two
lobes of slower particles on the sides, faster particles above and below, with
the entire structure twisted, as it experiences the pushing and pulling of
magnetic fields outside the solar system.
This data from NASA’s Interstellar Boundary
Explorer shows what it observed looking down the solar system’s tail. The yellow
and red colors represent areas of slow-moving particles, and the blue represents
the fast-moving particles.
Image Credit:
NASA/IBEX
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“By examining the neutral atoms, IBEX made the first observations of the
heliotail,” said David McComas, lead author on the paper and principal
investigator for IBEX at Southwest Research Institute in San Antonio, Texas.
“Many models have suggested the heliotail might be like this or like that, but
we’ve had no observations. We always drew pictures where the tail of the
heliosphere just disappears off the page, since we couldn’t even speculate about
what it really looked like.”
Other stars show tails that trail behind them like
a comet’s tail. Scientists used NASA’s Interstellar Boundary Explorer to confirm
that our solar system has one too. From top left and going counter clockwise,
the stars shown are: LLOrionis; BZ Cam; and Mira.
Image Credit:
NASA/HST/R.Casalegno/GALEX
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While telescopes have spotted such tails around other stars, it has been
difficult to see whether our star also had one. Pioneer 10 was headed in that
direction after it crossed the orbit of Neptune in 1983. However, it lost power
in 2003 before it moved into the tail so we have no data from spacecraft
directly in the tail. Watching it from afar is hard, because the particles in
the tail, and throughout the heliosphere, don’t shine, so they can’t be seen
conventionally.
IBEX, on the other hand, can map such regions by measuring neutral particles
created by collisions at the heliosphere’s boundaries. This technique, called
energetic neutral atom imaging, relies on the fact that the paths of neutral
particles aren’t affected by the heliosphere’s magnetic fields. The particles
travel in a straight line from collision to IBEX. Consequently, observing where
the neutral particles came from describes what’s going on in these distant
regions.
“Using neutral atoms, IBEX can observe far away structures, even from Earth
orbit,” said Eric Christian, IBEX mission scientist at NASA’s Goddard Space
Flight Center in Greenbelt, Md. “And IBEX scans the entire sky, so it has given
us our first data about what the tail of the heliosphere looks like, an
important part of understanding our place in and movement through the
galaxy.”
The journey for these neutral atoms begins years before it hits the IBEX
instruments. The solar wind blowing out from the sun streams out in all
directions, moving far past the furthest planets, eventually slowing down and
bending back along the tail, in response to the pressure from the inflowing
interstellar material. The particles join a mass migration of particles moving
backward inside the boundary of the heliosphere – a thin layer called the
heliopause.
While this is happening, a steady stream of slower, neutral atoms originating
from elsewhere in the galaxy, travel across the solar system. When one of these
neutral atoms collides with one of the faster charged particles, they can
exchange an electron. The result can be a slow charged particle and a fast
neutral atom. The neutral is no longer bound to the magnetic fields, and instead
speeds straight off in whatever direction it was pointed at that moment. Some of
these travel for years until they are detected by IBEX.
This animation shows the path a particle takes from
the sun, over the course of years, before colliding with one of IBEX’s
detectors. The animation shows a charged solar particle leaving the sun,
following the magnetic field lines out to the solar system’s boundary, the
heliosheath.
Image Credit:
NASA/Goddard Space Flight Center
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“By collecting these energetic neutral atoms, IBEX provides maps of the
original charged particles,” said McComas. “The structures in the heliotail are
invisible to our eyes, but we can use this trick to remotely image the outermost
regions of our heliosphere.”
Early results from IBEX on the heliotail originally suggested there might
just be a small region of slow-moving wind down the heliotail, but once
scientists had collected enough data they realized they had initially seen only
part of the picture. Based on the map of the heliotail they have now provided,
someone looking straight down the tail sees a shape a little like a four-leaf
clover. The two side leaves are filled with slow moving particles, and the upper
and lower leaves with fast ones. This shape makes sense, given the fact that the
sun has been sending out mostly fast solar wind near its poles, and slower wind
near its equator for the last few years – a common pattern in the most recent
phase of the sun’s 11-year activity cycle.
The four-leaf clover does not align perfectly with the sun, however. The
entire shape is rotated slightly, indicating that as it moves further away from
the sun and its magnetic influence, the charged particles have begun to be
nudged into a new orientation, aligning with the magnetic fields from the local
galaxy. Scientists still do not know how long the tail is.
“The tail is our footprint on the galaxy, and it’s exciting that we’re
starting to understand the structure of it,” said Christian. “The next step is
to incorporate these observations into our models and start the process of
really understanding our heliopshere.”
Scientists can test their computer simulations of the heliosphere against the
new observations and improve their models as needed. Together, data from
instruments in space and analysis at labs on the ground will continue to improve
our understanding of the comet-like tail streaming out behind us.
IBEX is a NASA Heliophysics Small Explorer. The Southwest Research Institute
leads IBEX with teams of national and international partners. Goddard manages
the Explorers Program for NASA's Science Mission Directorate in Washington.
For more information about IBEX science and mission, visit:
For high resolution media, visit:
NASA's Goddard Space Flight Center,
Greenbelt, Md.
Guillermo Gonzalo Sánchez Achutegui
ayabaca@mail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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