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.
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.
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.
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.
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.
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.
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.
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.
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.
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.