NASA's IBEX charts 11 years of change at boundary to interstellar space
Results show the shifting outer heliosphere in great detail

Date:
June 17, 2020
Source:
NASA/Goddard Space Flight Center
Summary:
Now, for the first time, scientists have used an entire solar cycle
of data from NASA's IBEX spacecraft to study how the heliosphere
changes over time. Solar cycles last roughly 11 years, as the
Sun swings from seasons of high to low activity, and back to high
again. The results show the shifting outer heliosphere in great
detail and hint at processes behind one of its most puzzling
features.



FULL STORY ==========================================================================
Far, far beyond the orbits of the planets lie the hazy contours of the
magnetic bubble in space that we call home.


==========================================================================
This is the heliosphere, the vast bubble that is generated by the Sun's magnetic field and envelops all the planets. The borders of this cosmic
bubble are not fixed. In response to the Sun's gasps and sighs, they
shrink and stretch over the years.

Now, for the first time, scientists have used an entire solar cycle of
data from NASA's IBEX spacecraft to study how the heliosphere changes
over time.

Solar cycles last roughly 11 years, as the Sun swings from seasons of
high to low activity, and back to high again. With IBEX's long record, scientists were eager to examine how the Sun's mood swings play out at the
edge of the heliosphere. The results show the shifting outer heliosphere
in great detail, deftly sketch the heliosphere's shape (a matter of
debate in recent years), and hint at processes behind one of its most
puzzling features. These findings, along with a newly fine-tuned data set,
are published in The Astrophysical Journal Supplements on June 10, 2020.

IBEX, short for the Interstellar Boundary Explorer, has been observing
the boundary to interstellar space for more than 11 years, showing us
where our cosmic neighborhood fits in with the rest of the galaxy.

"It's this very small mission," said David McComas, the principal
investigator for the mission at Princeton University in New Jersey. IBEX
is just as big as a bus tire. "It's been hugely successful, lasting
much longer than anybody anticipated. We're lucky now to have a whole
solar cycle of observations." Mapping the solar system's edge, one
particle at a time The heliosphere is filled with the solar wind, the
constant flow of charged particles from the Sun. The solar wind rushes
out in all directions, a million miles per hour, until it butts against
the interstellar medium, winds from other stars that fill the space
between them.



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As the Sun wades through the interstellar medium, it generates a hot,
dense wave much like the wave at the front of a boat coursing through
the sea. Our cosmic neighborhood is called the Local Fluff, for the cloud
of superhot gases that blooms around us. Where the solar wind and Local
Fluff meet forms the edge of the heliosphere, called the heliopause. Just inside that lies a turbulent region called the heliosheath.

Particles called energetic neutral atoms, or ENAs, that are formed in this distant region of space are the focus of IBEX's surveys. They're created
when hot, charged particles like the ones in the solar wind collide with
cold neutrals like those flowing in from interstellar space. Zippy solar
wind particles can snatch electrons from lumbering interstellar atoms,
becoming neutral themselves.

The journey of these particles begins long before IBEX detects them. Past
the planets, past the asteroid belt and the Kuiper Belt, to the edge
of the heliosphere, it takes about a year for a gust of solar wind to
race 100 times the distance between the Sun and Earth. Along the way,
the solar wind picks up ionized atoms of interstellar gases that have
wriggled in to the heliosphere.

The solar wind that arrives at the edge is not the same wind that left
the Sun a year before.

Solar wind particles might spend another six months roving the chaos of
the heliosheath, the gulf between the heliosphere's two outer boundaries.

Inevitably, some collide with interstellar gases and become energetic
neutrals.

It takes the neutral particles close to another year for the return
trip, traversing the space from the edge of the heliosphere to reach
IBEX -- if the particles happened to be heading in precisely the right direction. Of all the neutral particles formed, only a few actually make
it to IBEX. The whole trip takes two to three years for the highest-energy particles in IBEX's observing range, and even longer at lower energies
or more distant regions.

IBEX takes advantage of the fact that neutral atoms like these aren't
diverted by the Sun's magnetic field: Fresh neutral particles bound away
from collisions in nearly a straight line.



==========================================================================
IBEX surveys the skies for the particles, noting their direction and
energy.

The spacecraft only detects about one every other second. The result is
a map of the interstellar boundary, crafted from the same principle a
bat uses to echolocate its way through the night: monitor an incoming
signal to learn more about one's surroundings. By studying where the
neutrals come from, and when, IBEX can trace the remote boundaries of
our heliosphere.

"We're so lucky to observe this from inside the heliosphere," said Justyna Sokol, a visiting scientist on the Princeton team. "These are processes
that happen at very small distances. When you observe other stars that
are very far away, you observe distances of light years, from outside
their astrospheres." Even the distance between the Sun and the nose of
the heliosphere is tiny compared to many, many light years.

Using IBEX's 11-plus years of data, McComas and his team were able to
study changes that evolve over time and are key to understanding our
place in space.

The solar wind is constant, but the wind is not steady. When the wind
gusts, the heliosphere inflates like a balloon, and neutral particles
surge at the outer fringes. When the wind calms, the balloon contracts;
neutral particles dwindle. The ensuing seesaw of neutral particles,
the scientists reported, consistently echoed two to three years after
the changes in the wind - - reflecting their journey to the edge of this balloon and back.

"It takes so many years for these effects to reach the edge of the heliosphere," said Jamey Szalay, another Princeton researcher on the
team. "For us to have this much data from IBEX, finally allows us to
make these long-term correlations." Shaping up the heliosphere From
2009 to 2014, the wind blew fairly low and steady, a gentle breeze. The heliosphere contracted. Then came a surprise swell in the solar wind, as
if the Sun heaved a great sigh. In late 2014, NASA spacecraft orbiting
Earth detected the solar wind pressure increase by about 50% (it has
since remained high for several years).

Two years later, the billowing solar wind led to a flurry of neutral
particles in the heliosheath. Another two years later, they filled
most of the nose of the heliosphere. Eventually, they crested over the heliosphere's north and south poles.

These changes were not symmetric. Each observed bump traced the quirks
of the heliosphere's shape. The scientists were surprised at how clearly
they saw the tidal wave of solar wind pushing out the heliopause.

"Time and the neutral particles have really painted the distances in
the shape of the heliosphere for us," McComas said.

IBEX still hasn't observed the effects of this cosmic punch from the
back end of the heliosphere, the heliotail. That means the tail end is
much farther away from the Sun than the front; those particles are on
a much longer journey.

Maybe the solar wind surge is still hurtling toward the tail, or maybe
neutral particles are already on their way back. In the coming years,
the IBEX team will be watching for signs of their return from the tail.

"Nature set up this perfect experiment for us to better understand this boundary," Szalay said. "We got to see what happens when this one big
thing - - the solar wind push -- changes." Overall, this paints a picture
of the heliosphere that is shaped something like a comet. The shape of
the heliosphere has been a matter of debate in recent years. Some have
argued our bubble in space is spherical as a globe; others suggested it is closer to a croissant. But in this study, McComas said, IBEX data clearly
shows the heliosphere's response to the solar wind push was asymmetric --
so the heliosphere itself must be asymmetric too. The Sun is situated
close to the front, and as the Sun hurtles through space, the heliotail
trails much farther behind, something like the streaking tail of a comet.

Tackling IBEX's biggest puzzle IBEX's many years of data have also brought scientists closer to an explanation for one of the heliosphere's more
puzzling features, known as the IBEX ribbon.

The ribbon remains one of IBEX's biggest discoveries. Announced in 2009,
it refers to a vast, diagonal swath of energetic neutrals, painted
across the front of the heliosphere. It's long puzzled scientists:
Why should any part of the boundary should be so different from the
rest? Over time, IBEX has indicated that what forms the ribbon is very different than what forms the rest of the interstellar sky. It is shaped
by the direction of the interstellar magnetic field. But how are ribbon particles produced? Now, the scientists report that it's very likely a secondary process is responsible, causing the journey of a certain group
of energetic neutral particles to roughly double.

After becoming energetic neutrals, rather than ricochet back toward IBEX,
this group of particles would streak in the opposite direction, across
the heliopause and into interstellar space. There, they'd get a taste
of the Local Fluff, cruising until some would inevitably collide with
passing charged particles, losing an electron once again and becoming
tied to the surrounding magnetic field.

Another two years or so pass, and the charged particles might collide
yet again with slower peers, stealing electrons like they've done
before. After this brief migration beyond the heliosphere, the twice-born energetic neutrals might eventually re-enter, hurtling back toward home.

Extended IBEX data helped the scientists connect the ribbon to the
particles' long interstellar tour. Particles forming the ribbon have
journeyed some two years more than the rest of the neutral particles
observed. When it came to the solar wind spike, the ribbon took another
two years after the rest of the heliosphere to even start responding.

Far exceeding its initial mission of two years, IBEX will soon be joined
by another NASA mission, IMAP -- short for the Interstellar Mapping
and Acceleration Probe, for which McComas also serves as principal investigator.

The mission is scheduled to launch in late 2024.

"IMAP presents a perfect opportunity to study, with great resolution and sensitivity, what IBEX has begun to show us, so that we will really get
a detailed understanding of the physics out there," McComas said.

Video: https://www.youtube.com/ watch?time_continue=1&v=ZY8D71NW1wM&feature=emb_logo

========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Original written by Lina Tran. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. D. J. McComas, M. Bzowski, M. A. Dayeh, R. DeMajistre,
H. O. Funsten, P.

H. Janzen, I. Kowalska-Leszczyńska, M. A. Kubiak,
N. A. Schwadron, J. M. Soko'ł, J. R. Szalay, M. Tokumaru,
E. J. Zirnstein. Solar Cycle of Imaging the Global Heliosphere:
Interstellar Boundary Explorer (IBEX) Observations from
2009-2019. The Astrophysical Journal Supplement Series, 2020; 248
(2): 26 DOI: 10.3847/1538-4365/ab8dc2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200617174814.htm

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