Early results findings in
three early passes of the sun
NASA/Goddard Space Flight Center –
December 4, 2019 -- 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.
"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.
"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.
"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.
Already, Parker Solar Probe's IS?IS
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 IS?IS, 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.
"Since Parker Solar Probe was
matching the Sun's rotation, we could watch the outflow of material for days
and see the evolution of structures," said Howard. "Observations near
Earth have made us think that fine structures in the corona segue into a smooth
flow, and we're finding out that's not true. This will help us do better
modeling of how events travel between the Sun and Earth."
As Parker Solar Probe continues on its
journey, it will make 21 more close approaches to the Sun at progressively
closer distances, culminating in three orbits a mere 3.83 million miles from
the solar surface.
"The Sun is the only star we can
examine this closely," said Nicola Fox, director of the Heliophysics
Division at NASA Headquarters. "Getting data at the source is already
revolutionizing our understanding of our own star and stars across the
universe. Our little spacecraft is soldiering through brutal conditions to send
home startling and exciting revelations."
Data from Parker Solar Probe's first two
solar encounters is available to the public online:
Parker Solar Probe is part of NASA's
Living with a Star program to explore aspects of the Sun-Earth system that
directly affect life and society. The Living with a Star program is managed by
the agency's Goddard Space Flight Center in Greenbelt, Maryland, for NASA's
Science Mission Directorate in Washington. Johns Hopkins APL designed, built
and operates the spacecraft.
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