Lauren Fuge
Astrophysicists have just announced that NASA’s Parker Solar Probe has done what was once thought impossible – touch the Sun.
On April 28, 2021, the spacecraft became the first to ever enter the Sun’s outer atmosphere, the corona. This is the aura of blisteringly hot plasma you can see from Earth during a solar eclipse, extending millions of kilometres out from the surface of the Sun itself. It’s an extreme environment, with strong magnetic fields and high temperatures of more than one million degrees Celsius – mysteriously much hotter than Sun’s 5,500°C surface.
The probe spent five hours directly immersed in the corona’s plasma. The measurements it made will lead to new insights about the physics of the corona and how the solar wind forms there.
“This marks the achievement of the primary objective of the Parker mission and a new era for understanding the physics of the corona,” says Justin C. Kasper from the University of Michigan.
Kasper is first author on the study reporting on the probe’s success, published in Physical Review Letters and announced at the American Geophysical Union Fall Meeting 2021.
The Parker Solar Probe was launched in 2018 and quickly became the fastest human-made object in history, soaring towards the Sun at up to 700,000km/h. It swung around Venus to slow down and then entered into orbit around the Sun. Now, during its eighth close encounter with the Sun, the spacecraft has dipped close enough to “touch” it.
The Sun’s outer atmosphere truly begins at the Alfvén critical surface. In the corona, strong magnetic fields bind plasma and prevent solar wind from escaping, but at the Alfvén point, the solar winds can exceed a critical speed and blast off out through the Solar System towards interstellar space.
“If you look at close-up pictures of the Sun, sometimes you’ll see these bright loops or hairs that seem to break free from the Sun but then reconnect with it,” explains Michael Stevens, an astrophysicist at the Harvard Center for Astrophysics who helped build the instrument that verified the probe had entered the corona.
“That’s the region we’ve flown into – an area where the plasma, atmosphere and wind are magnetically stuck and interacting with the Sun.”
According to Gary Zank, a co-investigator on the probe’s Solar Wind Electrons Alphas and Protons (SWEAP) instrument, it’s “hard to overstate” the significance of what the Parker Solar Probe has now achieved.
“This event is what many heliophysicists have dreamed about for most of their careers,” he says.
“For over 50 years, since the dawn of the space age, the heliospheric community has grappled with the unanswered problem of how the solar corona is heated to well over a million degrees to drive the solar wind.
“The first measurements of the sub-Alfvénic solar wind may represent the most major step forward in understanding the physics behind the acceleration of the solar wind since the formative model by Parker.”
During the five hours the probe spent below the Alfvén critical surface, it made several interesting discoveries reported in the paper led by Kasper. To the surprise of the researchers, the critical surface was found to be “wrinkled”.
The largest such wrinkle was produced by a pseudostreamer: a large magnetic structure, pushing the Alfvén critical surface away from the Sun, though the researchers don’t know why.
The probe also found fewer “switchbacks” – sudden reversals in the Sun’s magnetic field – below the Alfvén critical surface, which might mean that these reversals don’t emerge from the corona.
There were also hints of a “power boost” inside the corona that might speed up the solar winds.
“We have been observing the Sun and its corona for decades, and we know there is interesting physics going on there to heat and accelerate the solar wind plasma,” says Nour E. Raouafi, a project scientist at the Johns Hopkins University Applied Physics Laboratory, which is leading the mission.
“Still, we cannot tell precisely what that physics is. With Parker Solar Probe now flying into the magnetically dominated corona, we will get the long-awaited insights into the inner workings of this mysterious region.”
After reaching this first milestone, the probe will now descend deeper and deeper into the Sun’s atmosphere and spend longer amounts of time there.
