High-speed particles spew out of the sun like water from a shower
head, scientists reported last week.
Data from the Parker Space Probe, a NASA spacecraft that
launched in 2018 to gather readings of the sun’s out atmosphere, or corona, is
providing clues about how the sun generates the solar wind.
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It reported 1 million mph stream of electrons, protons, and
other charged particles rushing outward into the solar system.
The solar wind research ties into a mystery that has long
perplexed scientists: Why is the corona, where temperatures soar to millions of
degrees, so much hotter than the surface of the sun, which is a relatively cool
5,537 degrees Celsius
The Parker probe is named after Eugene N. Parker, a
University of Chicago astrophysicist who first predicted the existence of the
solar wind in 1958.
The forces balance so that the sun neither collapses nor
blows apartThe sun has an atmosphere of tenuous gases that is dragged
downward by gravity while pressure generated by fusion reactions within the sun
pushes upward.
Overall, the forces balance so that the sun neither
collapses nor blows apart. However, the forces do not cancel perfectly
everywhere, and Parker’s calculations show how the sun can act like a leaky
balloon.
“If you put enough pressure in the system,” said Stuart
Bale, a physicist at the University of California, Berkeley, “the atmosphere
can escape, and as it escapes, it becomes energized.”
In a paper published in the journal Nature, Bale, who leads
an instrument on the Parker Solar Probe that measures electric and magnetic
fields in the solar wind, and his colleagues reported that the streams of solar
wind match patterns of hot gases rising and cooler gases falling within the
sun.
A phenomenon that resembles a thunderstorm
This phenomenon of convection, essentially the same thing
that occurs in a thunderstorm, produces up-and-down flows of hydrogen within
the sun, and the pattern of flows. The flows are almost like thunderstorms
packed next to one another, known as supergranulation.
The convection of charged particles generates shifting
magnetic fields that stretch until they snap and reconnect, releasing energy
that contributes to the heating of the corona.
That reconnection appears to accelerate the solar wind
particles.
Earlier observations of the sun had already indicated that
solar wind comes out of what are known as coronal holes, regions where the
magnetic field continues far outward into space instead of wrapping around and
coming back down at another point on the sun.
Imagine a simple bar magnet, which generates a magnetic
field similar in shape to the one that surrounds Earth. At the poles, the
magnetic fields go straight up and down; those are the coronal holes.
During the sun’s quiet periods — solar activity varies on an
11-year cycle, from comparatively calm to hyperactive — the sun’s magnetic
field possesses this bar magnet configuration. When the Parker spacecraft
launched, the sun was near its minimum.
As the sun approaches the maximum of its cycle, when the
magnetic field is in the throes of reversing direction, the structure of the
field becomes more complex, and more coronal holes appear.
Like jets from a shower head
The Parker spacecraft’s instruments detected that the solar
wind was not uniform over coronal holes. Instead, the particles emerged in
“micro streams,” like jets from a shower head.
The space probe’s sensors “started seeing that the solar
wind had a huge amount of structure,” said James Drake, a professor of physics
at the University of Maryland and another author of the Nature paper.
The periodic pattern of the micro streams matched that of
the supergranulation, suggesting that magnetic reconnection near the sun’s
surface plays a key role in the acceleration of the particles.
“I could figure out all the characteristics of reconnection,”
Drake said. “I could figure out how much heating was going on. And once we
figured out how much heating, I found out it was enough to power the wind.”
He added, “We didn’t have this before at all.”
Gary Zank, director of the Center for Space Plasma and
Aeronomic Research at the University of Alabama in Huntsville, said the new
results were “one crucial and important step in answering the puzzle of why the
solar corona is a million degrees hotter compared to its very relatively cold
surface.” Zank was not involved in the research, but he served as one of the
scientists who reviewed the paper for Nature editors.
“It basically says, here is the mechanism by which we can
start to understand how that transfer of energy takes place,” Zank said.
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