Thanks to the Parker Solar Probe we’re up close and personal with our host star. And just one step closer to understanding space weather and so much more about how the Sun impacts life right here on Earth.
So the thought of traditionally “touching” the Sun is vastly different than, let's say, going to the surface of Mars. And that’s because the Sun doesn’t have a solid surface that we can physically touch, even if we had some ridiculously “fire-radiation proof” suit to make it happen! Instead, the upper atmosphere is an extension of its surface held in place by the overwhelming gravitational pull and magnetic forces of the Sun.
The further you get from the core, the weaker these forces become. Eventually reaching a point known as the Alfvén Critical Surface where the atmosphere of the Sun ends. And from here we begin to observe solar wind. Solar wind is made up of streams of charged particles and gas that are propelled by the Sun at millions of kilometers per second throughout the Solar System.
These particles can ultimately damage our satellites, like those used for GPS and phone calls. But one the most important questions that Parker needed to answer is where this Alfvén Critical surface is. And in 2021, Parker made a historic discovery.
After spending five hours within the upper atmosphere, the probe observed that both the energy and pressure of the Sun’s magnetic field were stronger than that of the particles within it. This meant that the forces from the Sun were strong enough to maintain control of the particles, essentially keeping them close to the center of the sun.
However, when Parker rose further away, the reverse was apparent. The forces were no longer strong enough to trap the particles, and they were propelled into the Solar System. This was evidence that Parker passed through the Alfvén Critical surface, like flying into the eye of a hurricane, where it’s most calm, and then returning to a barrage of wind. And even more surprising, the critical surface is not a perfect sphere. Parker detected that it’s actually made up of spikes and valleys.
But overall, scientists finally determined the end of the Sun's atmosphere, at approximately 13 million kilometers from the surface. As Parker continued deeper into the upper atmosphere, it made additional observations of detailed features, like Pseudostreamers. And you might recognize these as the huge ribbon like structures from photos of solar eclipses here on Earth. But one interesting fact that scientists observed within the pseudostreamer region, was a reduction in what we call switchbacks, which might be why we see such unpredictable, powerful bursts of energetic particles from the Sun.
In 1995, scientists observed S-shaped kinks in the magnetic field of solar wind near the Sun's poles, thanks to the Ulysses mission. Then in 2019 Parker’s data revealed an unexpected discovery. Before the new insight from Parker, scientists believed that switchbacks were rare and only occurred at the Sun's poles. However, Parker saw familiar signs of magnetic field lines that zig-zagged back and forth throughout the upper atmosphere. And now scientists have evidence to support that switchbacks are common in solar wind…at least near the Sun’s surface. And since solar wind is constantly flowing from the Sun, scientists think that understanding switchbacks will help us fully understand space weather.
As Parker continues to spiral closer to the Sun prepping for its closest flyby in 2025 at 6.2 million kilometers, researchers now have a better idea of what to look for. And I have no doubt that we’ll slowly begin to unravel even more about the mysteries of switchbacks, the inner workings of the Alfvén Critical Surface and how the Sun ultimately impacts our satellites and sometimes even life here on Earth.