The
solar system moves through a local galactic cloud at a speed of 50,000
miles per hour, creating an interstellar wind of particles, some of
which can travel all the way toward Earth to provide information about
our neighborhood. Image Credit: NASA/Adler/U. Chicago/Wesleyan. Larger image
Like the wind adjusting course in the middle of a storm, scientists
have discovered that the particles streaming into the solar system from
interstellar space have most likely changed direction over the last 40
years. Such information can help us map out our place within the galaxy
surrounding us, and help us understand our place in space.
The results, based on data spanning four decades from 11 different spacecraft, were published in Science on Sept. 5, 2013.
Vestiges of the interstellar wind flowing into what's called the
heliosphere -- the vast bubble filled by the sun's own constant flow of
particles, the solar wind – is one of the ways scientists can observe
what lies just outside of our own home, in the galactic cloud through
which the solar system travels. The heliosphere is situated near the
inside edge of an interstellar cloud and the two move past each other at
a velocity of 50,000 miles per hour. This motion creates a wind of
neutral interstellar atoms blowing past Earth, of which helium is the
easiest to measure.
"Because the sun is moving though this cloud, interstellar atoms
penetrate into the solar system," said Priscilla Frisch, an
astrophysicist at the University of Chicago, Ill. and the lead author on
the paper. "The charged particles in the interstellar wind don't do a
good job of reaching the inner solar system, but many of the atoms in
the wind are neutral. These can penetrate close to Earth and can be
measured."
Frisch became interested in this subject when results in January 2012
from NASA's Interstellar Boundary Explorer, or IBEX, showed that the
interstellar wind was entering the heliosphere from a slightly different
direction than had been observed by NASA's Ulysses mission in the
1990s. Frisch and her colleagues set out to gather as much evidence from
as many sources as they could to determine whether the newer
instruments simply provided more accurate results, or whether the wind
direction itself changed over the years.
The earliest historical data on the interstellar wind comes from the
1970s from the U.S. Department of Defense's Space Test Program 72-1 and
SOLRAD 11B, NASA's Mariner, and the Soviet Prognoz 6. While instruments
have improved since the 1970s, comparing information from several sets
of observations helped the researchers gain confidence in results from
that early data. The team went on to look at another seven data sets
including the Ulysses information from 1990 to 2001, and more recent
data from IBEX, as well as four other NASA missions: the Solar
Terrestrial Relations Observatory, or STEREO, the Advanced Composition
Explorer, or ACE, the Extreme Ultraviolet Explorer, and the MErcury
Surface, Space ENvironment, GEochemistry, and Ranging mission, or
MESSENGER, currently in orbit around Mercury. The eleventh set of
observations came from the Japanese Aerospace Exploration Agency's
Nuzomi.
"The direction of the wind obtained from the most recent data does
not agree with the direction obtained from the earlier measurements,
suggesting that the wind itself has changed over time," said Eric
Christian, the IBEX mission scientist at NASA’s Goddard Space Flight
Center in Greenbelt, Md. "It's an intriguing result, which relied on
looking at a suite of data measured in a bunch of different ways."
The various sets of observations relied on three different methods to
measure the incoming interstellar wind. IBEX and Ulysses directly
measure neutral helium atoms as they coursed through the inner solar
system. IBEX's measurements are made close to Earth, while Ulysses'
measurements reach out to the orbit of Jupiter.
The earliest measurements in the 1970s observed fluorescence that
occurs when the extreme ultraviolet radiation coming from the sun
scatters off the interstellar helium wind passing by the sun. Neutral
helium atoms get caught by the sun's gravity, forming a focusing cone.
As radiation from the sun bounces off these atoms, they give off light.
Measuring the light provides information about the helium inflow
direction.
The third technique to measure the helium wind relies on the fact
that after this interaction with the sun's radiation, a fraction of
neutral helium atoms gain an electron, and thus become charged. Many
instruments in space are geared to study charged particles, such as
instruments on NASA’s STEREO and ACE. Such instruments can measure the
longitudinal direction of the particle wind, providing one last set of
historical observations to round out the picture.
From
Earth's perspective, the interstellar wind flows in from a point just
above the constellation Scorpius. Results from 11 spacecraft over 40
years show that the exact direction has changed some 4 to 9 degrees
since the 1970s. Image Credit: NASA/Goddard Space Flight Center. Larger image
The
data from these diverse sources shows that the direction of the
interstellar wind has changed some 4 to 9 degrees over the last 40
years.
"Previously we thought the local interstellar medium was very
constant, but these results show that it is highly dynamic, as is the
heliosphere’s interaction with it," said David McComas, IBEX principal
investigator at Southwest Research Institute in San Antonio, Texas.
While the reason for – and, indeed, the exact timing of – the shift
is still unclear, Frisch pointed out that scientists know our solar
system is close to the edge of the local interstellar cloud. Such an
area of the galaxy might experience turbulence, and as we hurtle through
space, the heliosphere could be exposed to different directions of
wind. While the scientists don't yet know for sure how the direction
switch happened, the team believes that additional observations should
ultimately explain its cause, giving us even more information about the
galaxy that surrounds us.
For more information about the IBEX mission, visit: www.nasa.gov/ibex
