NASA's Parker Solar Probe passed through an invisible boundary between the sun's atmosphere and the rest of space, making it the first spacecraft ever to do so. Here's everything you need to know.
The probe was first sent up to space on April 28, 2021, and Parker reported back the boundary measurements, known as the Alfvén critical surface. The findings were finally shared during a news briefing at the American Geophysical Union. "We have finally arrived. Humanity has touched the sun," said Nicola Fox, the director of NASA's Heliophysics Science Division in Washington, D.C. Fox revealed that Alfvén's measurements were 13 million kilometers above the sun's surface.
One of Parker's purposes was to gather more information regarding this significant layer of the sun. There are a few important reasons why scientists as NASA believe that understanding the Alfvén critical surface is needed. One is that this area is where plasma packets detach from the sun and assimilate into the solar wind. The solar wind is a stream of charged particles that have come down from the sun's outer atmosphere, also known as the corona. Since the stream can affect life on Earth and satellites, researchers would like to see how the solar wind works specifically.
Parker also documented that the magnetic field strength increased as it passed into the boundary, but the density of charged material decreased. Under the Alfvén critical surface charges, particles don't move away from the sun like they usually do; instead, they bend back towards the surface. Kasper explained, "The surface clearly has some structure and warp to it."
The second reason researchers are looking closely at the Alfvén critical surface is to find out why the corona, which reaches nearly a million degrees Celsius, is significantly hotter than the sun's surface, which is a few thousand degrees Celsius. It's unusual because temperatures are typically higher as you get closer to a heat source. Solar physicist Justin Kasper of the University of Michigan in Ann Arbor said, "We'll now be able to witness directly how coronal heating happens."
The probe will continue its course around the sun to gather more research. The goal is to have Parker get 6 million kilometers away from the surface. "The expectation is that as we fly closer and closer to the sun, we'll keep crossing this boundary," said solar physicist Nour Raouafi from Johns Hopkins Applied Physics Laboratory.