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| An illustration showing the Solar System inside the heliosphere, with the termination shock and heliopause represented by two bubbles, one inside the other. (NASA) |
The bubble of space encasing the Solar System might be wrinkled, at least
sometimes.
Data from a spacecraft orbiting Earth has revealed ripple structures in the
termination shock and heliopause: shifting regions of space that mark one of
the boundaries between the space inside the Solar System, and what's outside
– interstellar space.
The results show that it's possible to get a detailed picture of the
boundary of the Solar System and how it changes over time.
This information will help scientists better understand a region of space
known as the heliosphere, which pushes out from the Sun and shields the
planets in our Solar System from cosmic radiation.
There are a variety of ways the Sun affects the space around it. One of
those is the solar wind, a constant supersonic flow of ionized plasma. It
blows out past the planets and the Kuiper Belt, eventually petering out in
the great emptiness between the stars.
The point at which this flow falls below the speed at which sound waves can
travel through the diffuse interstallar medium is called the termination
shock, and the point at which it is no longer strong enough to push back
against the very slight pressure of interstellar space is the heliopause.
Both Voyager probes have crossed the heliopause and are, effectively, now
cruising through interstellar space, providing us the first in situ
measurements of this shifting boundary. But there's another tool out in
Earth orbit that has been helping scientists map the heliopause since it
commenced operations in 2009: NASA's Interstellar Boundary Explorer (IBEX).
IBEX measures energized neutral atoms, which are created when the Sun's
solar wind collides with the interstellar wind at the Solar System boundary.
Some of those atoms are catapulted further out into space, while others are
flung back at Earth. Once the strength of the solar wind that produced them
is taken into account, energized neutral particles that return our way can
be used to map the shape of the boundary, a bit like cosmic echolocation.
Previous maps of the structure of the heliosphere have relied on long-scale
measures of the evolution of solar wind pressure and energetic neutral atom
emissions, which resulted in a smoothing of the boundary in both space and
time. But in 2014, over a period of roughly six months, the dynamic pressure
of the solar wind increased by roughly 50 percent.
A team of scientists led by astrophysicist Eric Zirnstein of Princeton
University has used this shorter-scale event to obtain a more detailed
snapshot of the shape of the termination shock and heliopause – and found
huge ripples, on the scale of tens of astronomical units (one astronomical
unit is the average distance between Earth and the Sun).
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| A three-dimensional visualization of the termination shock and heliopause, showing huge ripples in both surfaces. (Zirnstein et al., Nat. Astron., 2022) |
They also performed modeling and simulations to determine how this
high-pressure wind interacted with the Solar System boundary. They found
that the pressure front reached the termination shock in 2015, sending a
pressure wave through the region between the termination shock and the
heliopause known as the inner heliosheath.
At the heliopause, a reflected wave travels back, colliding with the
still-incoming flow of charged plasma behind the pressure front, creating a
storm of energetic neutral atoms that fills the inner heliosheath by the
time the reflected wave arrives back at the termination shock.
The team's measurements also show quite a significant shift in the distance
to the heliopause. Voyager 1 crossed the heliopause in 2012 at a distance of
122 astronomical units. In 2016, the team measured that the distance to the
heliopause in the direction of Voyager 1 was around 131 astronomical units;
at that time, the probe was 136 astronomical units from the sun, still in
interstellar space, but with a ballooning heliosphere behind it.
The team's measurement to the heliopause in the direction of Voyager 2 in
2015 is a little trickier: 103 astronomical units, with a margin for error
of 8 astronomical units on either side. At that time, Voyager 2 was 109
astronomical units from the Sun, which is still within the error margin. It
didn't cross the heliopause until 2018, at a distance of 119 astronomical
units.
Both measurements suggest that the shape of the heliopause changes, and not
insignificantly. It's not entirely clear why.
However, in 2025 a new probe will be sent into space to measure energetic
neutral atom emission with higher precision, and across a wider energy
range. That, the team said, should help answer some of the perplexing
questions about the weird, invisible, 'wrinkly' bubble that protects our
little planetary system from the strangeness of space.
The research has been published in Nature Astronomy.
Reference:
Zirnstein, E.J., Shrestha, B.L., McComas, D.J. et al. Oblique and rippled
heliosphere structures from the Interstellar Boundary Explorer. Nat Astron
(2022).
DOI: 10.1038/s41550-022-01798-6
Tags:
Space & Astrophysics