In August 2018, NASA’s Parker Solar Probe launched to space, soon becoming
the closest-ever spacecraft to the Sun. With cutting-edge scientific
instruments to measure the environment around the spacecraft, Parker Solar
Probe has completed three of 24 planned passes through never-before-explored
parts of the Sun’s atmosphere, the corona. On Dec. 4, 2019, four new papers in
the journal Nature describe what
scientists have learned from this unprecedented exploration of our star — and
what they look forward to learning next.
These findings reveal
new information about the behavior of the material and particles that speed
away from the Sun, bringing scientists closer to answering fundamental
questions about the physics of our star. In the quest to protect astronauts and
technology in space, the information Parker has uncovered about how the Sun
constantly ejects material and energy will help scientists re-write the models
we use to understand and predict the space weather around our planet and
understand the process by which stars are created and evolve.
“This first data from
Parker reveals our star, the Sun, in new and surprising ways,” said Thomas
Zurbuchen, associate administrator for science at NASA Headquarters in
Washington. “Observing the Sun up close rather than from a much greater
distance is giving us an unprecedented view into important solar phenomena and
how they affect us on Earth, and gives us new insights relevant to the
understanding of active stars across galaxies. It’s just the beginning of an incredibly
exciting time for heliophysics with Parker at the vanguard of new discoveries.”
Though it may seem
placid to us here on Earth, the Sun is anything but quiet. Our star is
magnetically active, unleashing powerful bursts of light, deluges of particles
moving near the speed of light and billion-ton clouds of magnetized material.
All this activity affects our planet, injecting damaging particles into the
space where our satellites and astronauts fly, disrupting communications and
navigation signals, and even — when intense — triggering power outages. It’s
been happening for the Sun’s entire 5-billion-year lifetime, and will continue
to shape the destinies of Earth and the other planets in our solar system into
the future.NASA’s Parker Solar Probe mission has returned unprecedented data
from near the Sun, culminating in new discoveries published on Dec. 4, 2019, in
the journal Nature. Among the findings are new understandings of how the Sun’s
constant outflow of material, the solar wind, behaves. Seen near Earth — where
it can interact with our planet’s natural magnetic field and cause space
weather effects that interfere with technology — the solar wind appears to be a
relatively uniform flow of plasma. But Parker Solar Probe’s observations reveal
a complicated, active system not seen from Earth.Credits: NASA’s Goddard Space
Flight Center.
“The Sun has
fascinated humanity for our entire existence,” said Nour E. Raouafi, project
scientist for Parker Solar Probe at the Johns Hopkins Applied Physics
Laboratory in Laurel, Maryland, which built and manages the mission for NASA.
“We’ve learned a great deal about our star in the past several decades, but we
really needed a mission like Parker Solar Probe to go into the Sun’s
atmosphere. It’s only there that we can really learn the details of these
complex solar processes. And what we’ve learned in just these three solar
orbits alone has changed a lot of what we know about the Sun.”
What happens on the
Sun is critical to understanding how it shapes the space around us. Most of the
material that escapes the Sun is part of the solar wind, a continual outflow of
solar material that bathes the entire solar system. This ionized gas, called
plasma, carries with it the Sun’s magnetic field, stretching it out through the
solar system in a giant bubble that spans more than 10 billion miles.
The dynamic solar wind
Observed near Earth,
the solar wind is a relatively uniform flow of plasma, with occasional
turbulent tumbles. But by that point it’s traveled over ninety million miles —
and the signatures of the Sun’s exact mechanisms for heating and accelerating
the solar wind are wiped out. Closer to the solar wind’s source, Parker Solar
Probe saw a much different picture: a complicated, active system.
“The complexity was
mind-blowing when we first started looking at the data,” said Stuart Bale, the
University of California, Berkeley, lead for Parker Solar Probe’s FIELDS
instrument suite, which studies the scale and shape of electric and magnetic
fields. “Now, I’ve gotten used to it. But when I show colleagues for the first
time, they’re just blown away.” From Parker’s vantage point 15 million miles
from the Sun, Bale explained, the solar wind is much more impulsive and
unstable than what we see near Earth.
Like the Sun itself,
the solar wind is made up of plasma, where negatively charged electrons have
separated from positively charged ions, creating a sea of free-floating
particles with individual electric charge. These free-floating particles mean
plasma carries electric and magnetic fields, and changes in the plasma often
make marks on those fields. The FIELDS instruments surveyed the state of the
solar wind by measuring and carefully analyzing how the electric and magnetic
fields around the spacecraft changed over time, along with measuring waves in
the nearby plasma.
These measurements
showed quick reversals in the magnetic field and sudden, faster-moving jets of
material — all characteristics that make the solar wind more turbulent. These
details are key to understanding how the wind disperses energy as it flows away
from the Sun and throughout the solar system.
One type of event in
particular drew the eye of the science teams: flips in the direction of the
magnetic field, which flows out from the Sun, embedded in the solar wind. These
reversals — dubbed “switchbacks” — last anywhere from a few seconds to several
minutes as they flow over Parker Solar Probe. During a switchback, the magnetic
field whips back on itself until it is pointed almost directly back at the Sun.
Together, FIELDS and SWEAP, the solar wind instrument suite led by the
University of Michigan and managed by the Smithsonian Astrophysical
Observatory, measured clusters of switchbacks throughout Parker Solar Probe’s
first two flybys.
Parker Solar Probe observed
switchbacks — traveling disturbances in the solar wind that caused the magnetic
field to bend back on itself — an as-yet unexplained phenomenon that might help
scientists uncover more information about how the solar wind is accelerated
from the Sun.Credits: NASA’s Goddard Space Flight Center/Conceptual Image
Lab/Adriana Manrique Gutierrez
“Waves have been seen in the solar
wind from the start of the space age, and we assumed that closer to the Sun the
waves would get stronger, but we were not expecting to see them organize into
these coherent structured velocity spikes,” said Justin Kasper, principal
investigator for SWEAP — short for Solar Wind Electrons Alphas and Protons — at
the University of Michigan in Ann Arbor. “We are detecting remnants of
structures from the Sun being hurled into space and violently changing the
organization of the flows and magnetic field. This will dramatically change our
theories for how the corona and solar wind are being heated.”
The exact source of the switchbacks
isn’t yet understood, but Parker Solar Probe’s measurements have allowed
scientists to narrow down the possibilities.
Among the many particles that
perpetually stream from the Sun are a constant beam of fast-moving electrons,
which ride along the Sun’s magnetic field lines out into the solar system.
These electrons always flow strictly along the shape of the field lines moving
out from the Sun, regardless of whether the north pole of the magnetic field in
that particular region is pointing towards or away from the Sun. But Parker
Solar Probe measured this flow of electrons going in the opposite direction,
flipping back towards the Sun — showing that the magnetic field itself must be
bending back towards the Sun, rather than Parker Solar Probe merely
encountering a different magnetic field line from the Sun that points in the
opposite direction. This suggests that the switchbacks are kinks in the
magnetic field — localized disturbances traveling away from the Sun, rather
than a change in the magnetic field as it emerges from the Sun.
Parker Solar Probe’s observations of
the switchbacks suggest that these events will grow even more common as the
spacecraft gets closer to the Sun. The mission’s next solar encounter on Jan.
29, 2020, will carry the spacecraft nearer to the Sun than ever before, and may
shed new light on this process. Not only does such information help change our
understanding of what causes the solar wind and space weather around us, it
also helps us understand a fundamental process of how stars work and how they
release energy into their environment.
The rotating solar wind
Some of Parker Solar Probe’s
measurements are bringing scientists closer to answers to decades-old
questions. One such question is about how, exactly, the solar wind flows out
from the Sun.
Near Earth, we see the solar wind
flowing almost radially — meaning it’s streaming directly from the Sun,
straight out in all directions. But the Sun rotates as it releases the solar
wind; before it breaks free, the solar wind was spinning along with it. This is
a bit like children riding on a playground park carousel – the atmosphere
rotates with the Sun much like the outer part of the carousel rotates, but the
farther you go from the center, the faster you are moving in space. A child on
the edge might jump off and would, at that point, move in a straight line
outward, rather than continue rotating. In a similar way, there’s some point
between the Sun and Earth, the solar wind transitions from rotating along with
the Sun to flowing directly outwards, or radially, like we see from Earth.
Exactly where the solar wind
transitions from a rotational flow to a perfectly radial flow has implications
for how the Sun sheds energy. Finding that point may help us better understand
the lifecycle of other stars or the formation of protoplanetary disks, the
dense disks of gas and dust around young stars that eventually coalesce into
planets.
Now, for the first time — rather
than just seeing that straight flow that we see near Earth — Parker Solar Probe
was able to observe the solar wind while it was still rotating. It’s as if
Parker Solar Probe got a view of the whirling carousel directly for the first
time, not just the children jumping off it. Parker Solar Probe’s solar wind
instrument detected rotation starting more than 20 million miles from the Sun,
and as Parker approached its perihelion point, the speed of the rotation
increased. The strength of the circulation was stronger than many scientists
had predicted, but it also transitioned more quickly than predicted to an
outward flow, which is what helps mask these effects from where we usually sit,
about 93 million miles from the Sun.
“The large rotational flow of the
solar wind seen during the first encounters has been a real surprise,” said
Kasper. “While we hoped to eventually see rotational motion closer to the Sun,
the high speeds we are seeing in these first encounters is nearly ten times
larger than predicted by the standard models.”
Dust near the Sun
Another question approaching an
answer is the elusive dust-free zone. Our solar system is awash in dust — the
cosmic crumbs of collisions that formed planets, asteroids, comets and other
celestial bodies billions of years ago. Scientists have long suspected that,
close to the Sun, this dust would be heated to high temperatures by powerful
sunlight, turning it into a gas and creating a dust-free region around the Sun.
But no one had ever observed it.
For the first time, Parker Solar
Probe’s imagers saw the cosmic dust begin to thin out. Because WISPR — Parker
Solar Probe’s imaging instrument, led by the Naval Research Lab — looks out the
side of the spacecraft, it can see wide swaths of the corona and solar wind,
including regions closer to the Sun. These images show dust starting to thin a
little over 7 million miles from the Sun, and this decrease in dust continues
steadily to the current limits of WISPR’s measurements at a little over 4
million miles from the Sun.
Parker Solar Probe saw cosmic dust
(illustrated here) — scattered throughout our solar system — begin to thin out
close to the Sun, supporting the idea of a long-theorized dust-free zone near
the Sun.Credits: NASA’s Goddard Space Flight Center/Scott Wiessinger
“This dust-free zone was predicted
decades ago, but has never been seen before,” said Russ Howard, principal
investigator for the WISPR suite — short for Wide-field Imager for Solar Probe
— at the Naval Research Laboratory in Washington, D.C. “We are now seeing
what’s happening to the dust near the Sun.”
At the rate of thinning, scientists
expect to see a truly dust-free zone starting a little more than 2-3 million
miles from the Sun — meaning Parker Solar Probe could observe the dust-free
zone as early as 2020, when its sixth flyby of the Sun will carry it closer to
our star than ever before.
Putting space weather under a
microscope
Parker Solar Probe’s measurements
have given us a new perspective on two types of space weather events: energetic
particle storms and coronal mass ejections.
Tiny particles — both electrons and
ions — are accelerated by solar activity, creating storms of energetic
particles. Events on the Sun can send these particles rocketing out into the
solar system at nearly the speed of light, meaning they reach Earth in under
half an hour and can impact other worlds on similarly short time scales. These
particles carry a lot of energy, so they can damage spacecraft electronics and
even endanger astronauts, especially those in deep space, outside the
protection of Earth’s magnetic field — and the short warning time for such
particles makes them difficult to avoid.
Understanding exactly how these
particles are accelerated to such high speeds is crucial. But even though they
zip to Earth in as little as a few minutes, that’s still enough time for the
particles to lose the signatures of the processes that accelerated them in the
first place. By whipping around the Sun at just a few million miles away,
Parker Solar Probe can measure these particles just after they’ve left the Sun,
shedding new light on how they are released.
Parker Solar Probe has made
new observations of energetic particles — like those seen here impacting a
detector on ESA and NASA’s Solar and Heliospheric Observatory — which will help
scientists better understand how these events are accelerated.Credits:
ESA/NASA/SOHO
Already,
Parker Solar Probe’s ISʘIS instruments, led by Princeton University, have
measured several never-before-seen energetic particle events — events so small
that all trace of them is lost before they reach Earth or any of our near-Earth
satellites. These instruments have also measured a rare type of particle burst
with a particularly high number of heavier elements — suggesting that both
types of events may be more common than scientists previously thought.
“It’s
amazing – even at solar minimum conditions, the Sun produces many more tiny
energetic particle events than we ever thought,” said David McComas, principal
investigator for the Integrated Science Investigation of the Sun suite, or
ISʘIS, at Princeton University in New Jersey. “These measurements will help us
unravel the sources, acceleration, and transport of solar energetic particles
and ultimately better protect satellites and astronauts in the future.”
Data
from the WISPR instruments also provided unprecedented detail on structures in
the corona and solar wind — including coronal mass ejections, billion-ton
clouds of solar material that the Sun sends hurtling out into the solar system.
CMEs can trigger a range of effects on Earth and other worlds, from sparking
auroras to inducing electric currents that can damage power grids and
pipelines. WISPR’s unique perspective, looking alongside such events as they
travel away from the Sun, has already shed new light on the range of events our
star can unleash.Parker Solar Probe’s imagers look out sideways from behind the
spacecraft’s heat shield, watching structures as they develop in the corona.
Credits: NASA/JHUAPL/Naval
Research Lab/Parker Solar Probe
https://www.nasa.gov/feature/goddard/2019/nasas-parker-solar-probe-sheds-new-light-on-the-sun
https://www.nasa.gov/feature/goddard/2019/nasas-parker-solar-probe-sheds-new-light-on-the-sun
Journal article: https://www.nature.com/articles/s41586-019-1818-7
https://www.nature.com/articles/s41586-019-1807-x
https://www.nature.com/articles/s41586-019-1813-z
https://www.nature.com/articles/s41586-019-1807-x
https://www.nature.com/articles/s41586-019-1813-z
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