NASA’s Parker Solar Probe is an ambitious spacecraft that seeks to get closer to the Sun than any prior craft. And wow, has it done that. This week, NASA reported that the Parker Solar Probe got within 15 solar radii, or about 10 million kilometers, of the surface of the Sun. That puts the spacecraft within the solar corona, a feat never before accomplished. And the probe answered a long-standing question – how far out does the Alfvén critical surface start?

The Alfvén critical surface is where the solar atmosphere ends and the solar wind begins. Researchers had estimated that this point occurs somewhere between 10 and 20 solar radii from the surface, and on April 28, 2021, the Parker Solar Probe found the correct conditions for this transitional location at 18.8 solar radii or just over 13 million kilometers out from the surface. The results of this finding were published in the Physical Review Letters, and lead author Justin Kasper noted: We were fully expecting that, sooner or later, we would encounter the corona for at least a short duration of time, but it is very exciting that we’ve already reached it.

Additionally, when the Parker Solar Probe reached that 15 solar radii point, it ran across what is known as a pseudostreamer. If you’ve ever looked at photos of a solar eclipse at totality, you’ve seen these streamers coming out of the Sun’s surface from behind the disk of the Moon. The probe discovered that flying through these streamers is similar to flying through the eye of a hurricane – everything quiets down, particles move more slowly, and there are fewer magnetic switchbacks.

The Parker Solar Probe will continue to spiral closer and closer to the Sun, eventually getting as close as 8.86 solar radii. The next flyby happens in January 2022 and will bring the spacecraft through the corona again. We’ll bring you the results of that flyby here on Daily Space.

More Information

NASA press release

“Parker Solar Probe Enters the Magnetically Dominated Solar Corona,” J. C. Kasper et al., 2021 December 14, Physical Review Letters