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“We were flying through the source of the solar wind,” Raouafi says. But if you’re traveling through them, some would appear to fly above your head, while others pass below your feet.” “When you see structures from afar, they look like they’re in the same location in space. “Imagine you’re riding on the Parker Solar Probe,” says Nour Raouafi (Johns Hopkins Applied Physics Laboratory). And not only did it image the structures, but it actually flew through them.

WISPR revealed structures in the solar wind that scientists had never seen from their previous vantage points. NASA / Johns Hopkins APL / Naval Research Laboratory The sequences of images show the Parker Solar Probe flying through the solar wind, with coronal streamers passing above (top sequence) and below (bottom sequence) the spacecraft.

These WISPR images show mostly sunlight scattered off electrons in the solar wind brighter areas signify regions with higher solar wind density. That difference was visible not just to the SWEAP and FIELDS instruments, which measure particles and electric and magnetic fields, respectively, but also to the probe’s WISPR imager. Parker saw plasma waves moving back and forth instead of flowing outward. What’s clear is that inside the Sun’s atmosphere, conditions are different than just outside. The reason for this wrinkly surface is still an open question, though the researchers suspect the crossing over a pseudostreamer lower in the corona pushed the boundary out to enable the first crossing. “It’s not fuzzy - it’s well-defined while we’re under it - but the surface has some structure to it.” So while the probe sees a smooth change in conditions while crossing the boundary, where the boundary is can change. Then, on its way outward, the spacecraft once again skimmed beneath the surface for more than an hour. Even as it continued flying toward the Sun, though, it popped back out, only to submerge again more deeply when it was at its closest approach - but briefly, that time exiting after just half an hour. The first time Parker passed the Alfvén surface was the longest it flew through the atmosphere for about five hours. Read more about the Parker Solar Probe mission in the November 2020 issue of Sky & Telescope, and subscribe to support more stories like this one. Kasper led a study on the crossings, published December 14th in Physical Review Letters, and announced the results with other team members at the ongoing meeting of the American Geophysical Union. The probe actually entered the magnetized atmosphere a total of three times during its eighth pass around the Sun, explains Justin Kasper (University of Michigan, BWX Technologies, Inc.), primary investigator of the particle–detecting SWEAP instruments aboard the spacecraft. The magnetic field outside this surface is weaker, and plasma takes charge, dragging magnetic field lines with it as it streams outward in the form of the solar wind. Inside, the Sun’s magnetic field has a tight hold on the plasma, governing its movements. That boundary is known as the Alfvén surface. On April 28, 2021, NASA’s Parker Solar Probe slipped past that boundary, touching plasma and magnetic waves that are attached to the Sun.

But it does have a region of space around it where it governs its crackling plasma beyond that, it cedes control. Unlike Earth, the Sun doesn’t have a solid surface. In this artist's concept, the Parker Solar Probe hovers above the Alfven surface, a hilly boundary that defines the region around the Sun where the solar magnetic field governs plasma motions.