A major milestone and new results from NASA’s Parker Solar Probe were announced on Dec. 14 in a press conference at the 2021 American Geophysical Union Fall Meeting in New Orleans. The results have been published in Physical Review Letters and accepted for publication in the Astrophysical Journal.
For the first time in history, a
spacecraft has touched the Sun. NASA’s Parker Solar
Probe has now flown through the Sun’s
upper atmosphere – the corona – and sampled particles and magnetic fields
there.
The new milestone marks one major step for Parker
Solar Probe and one giant leap for solar science. Just as landing on the Moon
allowed scientists to understand how it was formed, touching the very stuff the
Sun is made of will help scientists uncover critical information about our
closest star and its influence on the solar system.
"Parker Solar Probe “touching the Sun” is a
monumental moment for solar science and a truly remarkable feat," said
Thomas Zurbuchen, the associate administrator for the Science Mission
Directorate at NASA Headquarters in Washington. "Not only does this
milestone provide us with deeper insights into our Sun's evolution and it's
impacts on our solar system, but everything we learn about our own star also
teaches us more about stars in the rest of the universe.”
As it circles closer to the solar surface, Parker is
making new discoveries that other spacecraft were too far away to see,
including from within the solar wind – the flow of particles from the Sun that
can influence us at Earth. In 2019, Parker discovered that magnetic zig-zag
structures in the solar wind, called switchbacks, are plentiful close to the
Sun. But how and where they form remained a mystery. Halving the distance to
the Sun since then, Parker Solar Probe has now passed close enough to identify
one place where they originate: the solar surface.
The first passage through the corona – and the promise
of more flybys to come – will continue to provide data on phenomena that are
impossible to study from afar.
“Flying so close to the Sun, Parker Solar Probe now
senses conditions in the magnetically dominated layer of the solar atmosphere –
the corona – that we never could before,” said Nour Raouafi, the Parker project
scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.
“We see evidence of being in the corona in magnetic field data, solar wind
data, and visually in images. We can actually see the spacecraft flying through
coronal structures that can be observed during a total solar eclipse.”
Closer Than Ever
Before
Parker Solar Probe launched in 2018 to explore the mysteries of the Sun by
traveling closer to it than any spacecraft before. Three years after launch and
decades after first conception, Parker has finally arrived.
Unlike Earth, the Sun doesn’t have a solid surface. But it does have a
superheated atmosphere, made of solar material bound to the Sun by gravity and
magnetic forces. As rising heat and pressure push that material away from the
Sun, it reaches a point where gravity and magnetic fields are too weak to
contain it.
That point, known as the Alfvén critical surface, marks the end of the
solar atmosphere and beginning of the solar wind. Solar material with the
energy to make it across that boundary becomes the solar wind, which drags the
magnetic field of the Sun with it as it races across the solar system, to Earth
and beyond. Importantly, beyond the Alfvén critical surface, the solar wind
moves so fast that waves within the wind cannot ever travel fast enough to make
it back to the Sun – severing their connection.
Until now, researchers were unsure exactly where the Alfvén critical
surface lay. Based on remote images of the corona, estimates had put it
somewhere between 10 to 20 solar radii from the surface of the Sun – 4.3 to 8.6
million miles. Parker’s spiral trajectory brings it slowly closer to the Sun
and during the last few passes, the spacecraft was consistently below 20 solar
radii (91 percent of Earth’s distance from the Sun), putting it in the position
to cross the boundary – if the estimates were correct.
On April 28, 2021, during its eighth flyby of the Sun, Parker Solar Probe
encountered the specific magnetic and particle conditions at 18.8 solar radii
(around 8.1 million miles) above the solar surface that told scientists it had
crossed the Alfvén critical surface for the first time and finally entered the
solar atmosphere.
“We were fully expecting that, sooner or later, we would encounter the corona for at least a short duration of time,” said Justin Kasper, lead author on a new paper about the milestone published in Physical Review Letters, and deputy chief technology officer at BWX Technologies, Inc. and University of Michigan professor. “But it is very exciting that we’ve already reached it.”
For the first time in history, a spacecraft has touched the Sun. NASA’s Parker Solar Probe has now flown through the Sun’s upper atmosphere – the corona – and sampled particles and magnetic fields there. Credits: NASA's Goddard Space Flight Center/Joy Ng Download this video in HD formats from NASA Goddard's Scientific Visualization Studio
Into the Eye of the
Storm
During the flyby, Parker Solar Probe passed into and out of the corona
several times. This is proved what some had predicted – that the Alfvén
critical surface isn’t shaped like a smooth ball. Rather, it has spikes and
valleys that wrinkle the surface. Discovering where these protrusions line up
with solar activity coming from the surface can help scientists learn how
events on the Sun affect the atmosphere and solar wind.
As Parker Solar Probe passed through the corona on encounter nine, the spacecraft flew by structures called coronal streamers. These structures can be seen as bright features moving upward in the upper images and angled downward in the lower row. Such a view is only possible because the spacecraft flew above and below the streamers inside the corona. Until now, streamers have only been seen from afar. They are visible from Earth during total solar eclipses. Credits: NASA/Johns Hopkins APL/Naval Research Laboratory
At one point, as Parker Solar Probe dipped
to just beneath 15 solar radii (around 6.5 million miles) from the Sun’s
surface, it transited a feature in the corona called a pseudostreamer.
Pseudostreamers are massive structures that rise above the Sun’s surface and
can be seen from Earth during solar eclipses.
Passing through the pseudostreamer was like flying
into the eye of a storm. Inside the pseudostreamer, the conditions quieted,
particles slowed, and number of switchbacks dropped – a dramatic change from
the busy barrage of particles the spacecraft usually encounters in the solar
wind.
For the first time, the spacecraft found itself in a
region where the magnetic fields were strong enough to dominate the movement of
particles there. These conditions were the definitive proof the spacecraft had
passed the Alfvén critical surface and entered the solar atmosphere where
magnetic fields shape the movement of everything in the region.
The first passage through the corona, which lasted
only a few hours, is one of many planned for the mission. Parker will continue
to spiral closer to the Sun, eventually reaching as close as 8.86 solar radii
(3.83 million miles) from the surface. Upcoming flybys, the next of which is
happening in January 2022, will likely bring Parker Solar Probe through the
corona again.
“I’m excited to see what Parker finds as it repeatedly
passes through the corona in the years to come,” said Nicola Fox, division
director for the Heliophysics Division at NASA Headquarters. “The opportunity
for new discoveries is boundless.”
The size of the corona is also driven by solar
activity. As the Sun’s 11-year activity cycle – the solar cycle – ramps up, the outer edge of the corona will expand, giving
Parker Solar Probe a greater chance of being inside the corona for longer
periods of time.
“It is a really important region to get into because
we think all sorts of physics potentially turn on,” Kasper said. “And now we're
getting into that region and hopefully going to start seeing some of these
physics and behaviors.”
Narrowing Down
Switchback Origins
Even before the first trips through the corona, some surprising physics was
already surfacing. On recent solar encounters, Parker Solar Probe collected
data pinpointing the origin of zig-zag-shaped structures in the solar wind,
called switchbacks. The data showed one spot that switchbacks originate is at
the visible surface of the Sun – the photosphere.
By the time it reaches Earth, 93 million miles away, the solar wind is an
unrelenting headwind of particles and magnetic fields. But as it escapes the
Sun, the solar wind is structured and patchy. In the mid-1990s, the
NASA-European Space Agency mission Ulysses flew over the
Sun’s poles and discovered a handful of bizarre S-shaped kinks in the solar
wind’s magnetic field lines, which detoured charged particles on a zig-zag path
as they escaped the Sun. For decades, scientists thought these occasional
switchbacks were oddities confined to the Sun’s polar regions.
In 2019, at 34 solar radii from the Sun, Parker discovered that switchbacks
were not rare, but common in the solar
wind. This renewed interest in the features and raised new
questions: Where were they coming from? Were they forged at the surface of the
Sun, or shaped by some process kinking magnetic fields in the solar atmosphere?
The new findings, in press at the Astrophysical Journal, finally confirm
one origin point is near the solar surface.
The clues came as Parker orbited closer to the Sun on its sixth flyby, less
than 25 solar radii out. Data showed switchbacks occur in patches and have a
higher percentage of helium – known to come from the photosphere – than other
elements. The switchbacks’ origins were further narrowed when the scientists
found the patches aligned with magnetic funnels that emerge from the
photosphere between convection cell structures called supergranules.
In addition to being the birthplace of switchbacks, the scientists think
the magnetic funnels might be where one component of the solar wind originates.
The solar wind comes in two different varieties – fast and slow – and the funnels
could be where some particles in the fast solar wind come from.
“The structure of the regions with switchbacks matches up with a small
magnetic funnel structure at the base of the corona,” said Stuart Bale,
professor at the University of California, Berkeley, and lead author on the new
switchbacks paper. “This is what we expect from some theories, and this
pinpoints a source for the solar wind itself.”
Understanding where and how the components of the fast solar wind emerge,
and if they’re linked to switchbacks, could help scientists answer a longstanding solar
mystery: how the corona is heated to millions of degrees, far
hotter than the solar surface below.
While the new findings locate where switchbacks are made, the scientists
can’t yet confirm how they’re formed. One theory suggests they might be created
by waves of plasma that roll through the region like ocean surf. Another contends
they’re made by an explosive process known as magnetic reconnection, which is
thought to occur at the boundaries where the magnetic funnels come together.
“My instinct is, as we go deeper into the mission and lower and closer to
the Sun, we're going to learn more about how magnetic funnels are connected to
the switchbacks,” Bale said. “And hopefully resolve the question of what
process makes them.”
As Parker Solar Probe ventures closer to the Sun, it’s crossing into uncharted regimes and making new discoveries. This image represents Parker Solar Probe's distances from the Sun for some of these milestones and discoveries. Credits: NASA's Goddard Space Flight Center/Mary P. Hrybyk-Keith
Now that researchers know what to look for, Parker’s closer passes may
reveal even more clues about switchbacks and other solar phenomena. The data to
come will allow scientists a glimpse into a region that’s critical for
superheating the corona and pushing the solar wind to supersonic speeds. Such
measurements from the corona will be critical for understanding and forecasting
extreme space weather events that can disrupt telecommunications and damage
satellites around Earth.
“It’s really exciting to see our advanced technologies succeed in taking
Parker Solar Probe closer to the Sun than we’ve ever been, and to be able to return
such amazing science,” said Joseph Smith, Parker program executive at NASA
Headquarters. "We look forward to seeing what else the mission discovers
as it ventures even closer in the coming years."
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. The Johns Hopkins University Applied Physics Laboratory in Laurel,
Maryland, manages the Parker Solar Probe mission for NASA and designed, built,
and operates the spacecraft.
By Mara Johnson-Groh
NASA’s Goddard Space Flight Center in Greenbelt, Md
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