Holey Corona! Sun-grazing spacecraft pries solar secrets loose
Parker Solar Probe scientists release results—and wow the solar physics world
More than 60 years ago, when University of Chicago physicist Eugene Parker proposed that the sun sent out a supersonic “solar wind” of charged particles that bathed the solar system, other scientists greeted his theory with skepticism. But spacecraft observations soon proved him right.
Now his namesake, NASA’s Parker Solar Probe spacecraft, has swooped closer to the sun than any other human-made object—just 36 solar radii [15 million miles], half Mercury’s distance.
Launched in August 2018, the probe seeks to solve the two biggest mysteries of the solar corona. The first is to find the mechanism that heats the corona to more than 100 times the already toasty temperature—10,000 degrees Fahrenheit—of the solar surface.
“Whatever heats the corona has to be electric and magnetic fields. That’s all there is,” says physicist Keith Goetz, the principal investigator for the University of Minnesota’s Parker Solar Probe team.
The second is to identify the coronal dynamo that creates the solar wind by accelerating subatomic particles from the sun’s surface to speeds up to a million miles per hour and spewing them out in a 3-D stream. The solar wind can energize particles trapped in Earth’s magnetic field, leading to beautiful auroras. But it can also lead to damage to the electrical grid and telecommunications, and can even pose a danger to satellites and astronauts in orbit.
A deeper understanding of how the sun creates the solar wind will help scientists predict events and minimize damage.
In December 2019 mission scientists, including the University of Minnesota team from the College of Science and Engineering, released the first batch of results in the journal Nature—and wowed the solar physics world. Read the article on the journal website.
A prime suspect in heating the corona was vibrations in the sun’s magnetic fields, which resemble the vibrations of a plucked harp string. The spacecraft picked up stronger vibrational waves in the fields as it approached the sun, but when especially strong waves hit, the solar wind sped up by 300,000 miles per hour in just seconds.
The strongest waves also sharply reversed the direction of the magnetic field, contorting it into an S-shaped pattern the scientists dubbed a switchback, after the hairpin turns on a steep mountain road.
Scientists are excited about these events because they believe they hint at how the energy that heats the corona is transported up from the solar surface.
Bursts of solar wind faster than a million miles per hour have been detected escaping from cooler regions of the corona—coronal “holes”— near the sun’s polar regions. But slower gusts are common, and now the Parker Solar Probe has caught some issuing from coronal holes nearer the solar equator.
Earth-based instruments could not have sorted out the origins of the fast and slow particles because they intermingle in transit and arrive all jumbled up. But with the sun-grazing orbiter, scientists can observe the various species of solar wind at their birth.
University of California, Berkeley physics professor Stuart Bale said in a news release that trying to study the solar wind from Earth is like trying to divine the source of a waterfall by looking near the bottom of the cascade.
“Now, with the Parker Solar Probe, we are getting closer and closer to the top of the waterfall, and we can see that there is underlying structure,” said Bale, who received bachelor’s, M.S. and Ph.D. degrees from the University of Minnesota and leads the probe’s study of electric and magnetic fields in the solar wind.
Working with Bale are Goetz, University physics professors Cynthia Cattell and John Wygant, and retired professor Paul J. Kellogg.
The probe also found that the cocoon of cosmic dust that envelops the sun and corona is thinnest close to the star. The dust, left by comets and asteroids, was expected to be largely swept away by the solar wind.
“The so-called dust-free zone or cut-off might be smooth or sharp,” said Goetz.
“With Parker, camera observations together with modeling suggest that maybe the dust-free zone starts below 17 solar radii [7.3 million miles],” Goetz explained. “This would be exciting because Parker will eventually descend well below that limit, going down to 10 solar radii [4.3 million miles]. When that happens, we’ll be able to observe the dust presence or absence or cut-off directly with our instrument. No modeling required.”
Read an earlier story about the Parker Solar Probe on the College of Science and Engineering website.
Story by Deane Morrison