NASA announced today (December 14) that the Parker Solar Probe (PSP), launched in 2018, has entered the corona of the Sun, becoming the first spacecraft ever to do so.
The PSP has now made 10 close approaches, the most recent on November 21, when it got within 5.3 million miles of the solar surface. But on its 8th approach back in April, we now know by the data retrieved from the craft that for a period of about five hours, it was within the Sun’s corona. It was expected that these close approaches would accomplish that at some point, but up to now it had been unclear exactly where the boundary of the corona is.
Parker Solar Probe on November 21, at the time of its 10th of 24 planned orbits around the Sun, each making a progressively closer approach
There is a certain altitude above the Sun’s surface where the magnetic and gravitational forces aren’t strong enough to corral the thermal expansion of charged particles (which happens because the corona is really, really hot), and those particles escape as the solar wind. Below that altitude, magnetism reels the particles back in, containing them within the Sun’s atmosphere. That imaginary spherical boundary, called the Alfvén critical surface, is where the Sun “begins”.
The PSP has been imaging the solar wind with its wide-field solar probe (WISPR) camera in visible light, and in fact we have some footage from that 8th encounter:
I should mention, because the Geminid meteor shower is happening now, that the PSP’s WISPR camera also detected in 2019 the dust trail that is responsible for this annual sky show. That’s at least in part the debris left by the asteroid Phaethon, a trail 12 million miles long and 60,000 miles wide:
This PSP image highlights the location of the very faint dust trail observed following the asteroid Phaethon's orbit (between the red arrows)
Back to the coronal entry, though — the piece of data used to mark that boundary is the Alfvén Mach number (MA), which can be derived directly from white-light imaging. If MA is below 1, you are “sub-Alfvénic”, magnetic forces are dominant, and thus you are in the corona. This happened for the PSP during three intervals (shaded gray) between April 28 and April 30 of this year:
Alfvén Mach number (MA) during the PSP’s 8th solar encounter
That first interval lasted over 5 hours, too sustained a period for noise to have been the explanation. With that, PSP had its first-ever entry into the corona, becoming the first human-made craft ever to achieve this.
“Flying so close to the Sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere – the corona – that we never could before,” said Nour Raouafi, the Parker project scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images. We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse.”
Based on all the fluctuations and spikes in MA, the conclusion is that the coronal boundary isn’t at all smooth, but quite wrinkly:
The Parker Solar Probe passes in and out of the corona as it orbits in this artist’s conception
Now that we are getting into the corona, on future flybys we’ll be able to take advantage of a cool feature of PSP’s orbits: When it gets very close to the Sun, it is travelling so screamingly fast (up to 430,000 mph in future flybys) that for a while it goes faster than the Sun rotates. There are a couple of intervals called co-rotation periods where PSP hovers over one spot above the surface for several hours, enabling it to make measurements in the corona in a single spot over the Sun’s surface for a sustained period. That’s illustrated here:
Coronal entry will probably happen again on the next close flyby, in January 2022, and there should be plenty more after that.
“I’m excited to see what Parker finds as it repeatedly passes through the corona in the years to come,” said Nicola Fox, division director for the Heliophysics Division at NASA Headquarters. “The opportunity for new discoveries is boundless.”
