Unlike Earth, the Sun doesn’t have a stable floor. But it does have a area of space round it the place it governs its crackling plasma; past that, it cedes management. On April 28, 2021, NASA’s Parker Solar Probe slipped previous that boundary, touching plasma and magnetic waves which can be connected to the Sun.
That boundary is named the Alfvén floor. Inside, the Sun’s magnetic subject has a decent maintain on the plasma, governing its actions. The magnetic subject outdoors this floor is weaker, and plasma takes cost, dragging magnetic subject traces with it because it streams outward in the type of the solar wind.
The probe truly entered the magnetized environment a complete of thrice throughout its eighth go round the Sun, explains Justin Kasper (University of Michigan, BWX Technologies, Inc.), main investigator of the particle–detecting SWEAP devices aboard the spacecraft. Kasper led a examine on the crossings, revealed December 14th in Physical Review Letters, and introduced the outcomes with different group members at the ongoing assembly of the American Geophysical Union.
Read extra about the Parker Solar Probe mission in the November 2020 issue of Sky & Telescope, and subscribe to assist extra tales like this one.
How to Touch the Sun
The first time Parker handed the Alfvén floor was the longest; it flew by way of the environment for about 5 hours. Even because it continued flying towards the Sun, although, it popped again out, solely to submerge once more extra deeply when it was at its closest method — however briefly, that point exiting after simply half an hour. Then, on its way outward, the spacecraft as soon as once more skimmed beneath the floor for a couple of hours.
“[The Alfvén surface] has to be wrinkly,” Kasper says. “It’s not fuzzy — it’s well-defined while we’re under it — but the surface has some structure to it.” So whereas the probe sees a clean change in circumstances whereas crossing the boundary, the place the boundary is can change. The motive for this wrinkly floor remains to be an open query, although the researchers suspect the crossing over a pseudostreamer decrease in the corona pushed the boundary out to allow the first crossing.
What’s clear is that inside the Sun’s environment, circumstances are totally different than simply outdoors. Parker noticed plasma waves shifting backwards and forwards as a substitute of flowing outward. That distinction was seen not simply to the SWEAP and FIELDS devices, which measure particles and electrical and magnetic fields, respectively, but additionally to the probe’s WISPR imager.
WISPR revealed buildings in the solar wind that scientists had by no means seen from their earlier vantage factors. And not solely did it picture the buildings, nevertheless it truly flew by way of them. “Imagine you’re riding on the Parker Solar Probe,” says Nour Raouafi (Johns Hopkins Applied Physics Laboratory). “When you see structures from afar, they look like they’re in the same location in space. But if you’re traveling through them, some would appear to fly above your head, while others pass below your feet.”
“We were flying through the source of the solar wind,” Raouafi says.
The view inside the corona could make clear the Parker Solar Probe’s two essential science aims: the origin of the solar wind and the warmth supply for the million-degree corona. Analysis to handle these aims is ongoing and can quickly embody Parker’s ninth and tenth passes. Preliminary evaluation signifies that the ninth go, too, took Parker inside the Alfvén floor; knowledge nonetheless hasn’t been downlinked from the tenth go, which occurred on November twenty first.
Sussing Out Switchbacks
From the probe’s insider view, scientists might also discover an evidence for switchbacks, “S”-shape curves in the magnetic subject. Though identified to exist since the Nineteen Nineties, Parker renewed curiosity in these buildings as a result of they have been so ubiquitous in the solar wind.
“Are they forged on the surface of the Sun? Or are they shaped by some process that kinks the field on the way out from the Sun?” wonders Stuart Bale (University of California, Berkeley), principal investigator of the FIELDS instrument suite.
While the solutions to these questions aren’t identified simply but, the new observations present that the switchbacks happen in discrete patches, they usually are inclined to have a better proportion of helium ions. That connects them to the photosphere, the highest degree of the Sun the place most of the photons we see come from.
“It gets more interesting,” Bale says. He notes that the patches of switchbacks and their helium ions appear to be related to magnetic funnels, areas between the large convection cells in the Sun’s boiling plasma. While it’s nonetheless too quickly to rule out any eventualities for switchbacks’ origins, he provides, “We’ll get there!”
The Parker Solar Probe will proceed its spiral in towards the Sun, with its subsequent closest method in February. As it comes nearer, solar exercise can be ramping up, promising extra insights in the future. “Humanity has ‘touched’ the Sun,” says program scientist Kelly Korreck (NASA HQ), “and revolutionary discoveries have just begun.”