The region that separates the Solar System from the rest of the galaxy is not smooth.
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Artist impression of interstellar space interacting with the
heliopause (blue) and termination shock (red). Image Credit: NASA |
Humanity has been able to reach interstellar space only
twice, thanks to the Voyager 1 and 2 probes, which have been traveling in space
for over 45 years. During the last decade, they crossed the boundaries known as
the termination shock and heliopause, the so-called edge of the Solar System
where plasma originated from the Sun is no longer dominant and interstellar
material becomes the major influence. Now, new research reveals something
unexpected about these boundaries. They are not smooth, they actually have
wrinkles.
The region where the solar wind is dominant is called the
heliosphere. Despite the "sphere" name, it is only spherical on one
side. The other side is elongated like the tail of a comet. The Voyager probes
have crossed the spherical side, which is much closer to Earth. First, they
went through the termination shock, where the speed of the solar wind is down
to the speed of sound. Then there is the heliopause where the solar wind is
being pushed back by the interstellar material flowing through the Milky Way.
Voyager 1 crossed this boundary in 2012 and Voyager 2
crossed it in 2018 providing crucial insights into the region. But another
mission doesn’t need to go as far to observe the heliopause. NASA’s IBEX
(Interstellar Boundary EXplorer) orbits Earth but tracks changes at the edge of
the Solar System. Astronomers made use of a particular event back in 2014 when
the pressure of the solar wind increased by 50 percent.
The data shows that the wave of particles reached the heliopause
in 2015, creating ripples across the boundary of 1.5 billion kilometers (over
900 million miles). Particles were reflected back and forth between the
termination shock and heliopause. The effect was so pronounced that the
heliopause behind Voyager 1 moved forward of about 750 million kilometers (460
million miles) compared to its position in 2012. Voyager 1 remained in
interstellar space but the heliosphere might have gotten really close to it.
Measurements in the direction of Voyager 2 are a bit more
uncertain but it might have added a bit more road for the probe to reach
interstellar space. The boundary between the Sun and the rest of the galaxy was
known to change in size as the activity of the Sun changes every 11 years. This
study shows that the changes might be more complex than previously thought and
that the heliosphere is a wrinkly, rippling region, rather than a smooth one.
NASA’s Interstellar Mapping and Acceleration Probe (IMAP)
will provide even more insight into this matter. It is expected to launch in
2025.
The study is published in Nature Astronomy.
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