The Parker Solar Probe

Today, December 24th 2024, the Parker Solar Probe got 7 times closer to the Sun than any spacecraft ever has, going faster than any spacecraft ever has—690,000 kilometers per hour. WHEEEEEE!!!!!!!

But the newspapers are barely talking about the really cool part: what it’s like down there. The Sun doesn’t have a surface like the Earth does, since it’s all just hot ionized gas, called ‘plasma‘. But the Sun has an ‘Alfvén surface’—and the probe has penetrated that.

What’s the Alfvén surface? In simple terms, it’s where the solar wind—the hot ionized gas emitted by the Sun—breaks free of the Sun and shoots out into space. But to understand how cool this is, we need to dig a bit deeper.

After all, how can we say where the solar wind “breaks free of the Sun”?

Hot gas shoots up from the Sun, faster and faster due to its pressure, even though it’s pulled down by gravity. At some point it goes faster than the speed of sound! This is the Alfvén surface. Above this surface, the solar wind becomes supersonic, so no disturbances in its flow can affect the Sun below.

It’s sort of like the reverse of a black hole! Light emitted from within the event horizon of a black hole can’t get out. Sound emitted from outside the Alfvén surface of the Sun get get in.

Or, it’s like the edge of a waterfall, where the water starts flowing so fast that waves can’t make it back upstream.

That’s pretty cool. But it’s even cooler than this, because ‘sound’ in the solar wind is very different from sound on Earth. Here we have air. The Sun has ions—atoms of gas so hot that electrons have been ripped off—interacting with powerful magnetic fields. You can visualize these fields as tight rubber bands, with the ions stuck to them. They vibrate back and forth together!

You could call these vibrations ‘sound’, but the technical term is Alfvén waves. Alfvén was the one who figured out how fast these waves move. Parker studied the surface where the solar wind’s speed exceeds the speed of the Alfvén waves.

And now we’ve gone deep below that surface!

This realm is a strange one, and the more we study it, the more complex it seems to get.

You’ve probably heard the joke that ends “consider a spherical cow”. Parker’s original model of the solar wind was spherically symmetric, so he imagined the solar wind shooting straight out of the Sun in all directions. In this model, the Alfvén surface is the sphere where the wind becomes faster the Alfvén waves. There are some nice simple formulas for all this.

But in fact the Sun’s surface is roiling and dynamic, with sunspots making solar flares, and all sorts of bizarre structures made of plasma and magnetic fields, like spicules, ‘coronal streamers’ and ‘pseudostreamers’… aargh, too complicated for me to understand. This is an entire branch of science!

So, the Alfvén surface is not a mere sphere: it’s frothy and randomly changing. The Parker Solar Probe will help us learn how it works—along with many other things.

Finally, here’s something mindblowing. There’s a red dwarf star 41 light years away from us, called TRAPPIST-1, which may have six planets beneath its Alfvén surface! This means the planets can create Alfvén waves in the star’s atmosphere. Truly the music of the spheres!

For more, check out these articles:

• Wikipedia, Alfvén wave.

• Wikipedia, Alfvén surface.

and this open-access article:

• Steven R. Cranmer, Rohit Chhiber, Chris R. Gilly, Iver H. Cairns, Robin C. Colaninno, David J. McComas, Nour E. Raouafi, Arcadi V. Usmanov, Sarah E. Gibson and Craig E. DeForest, The Sun’s Alfvén surface: recent insights and prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH), Solar Physics 298 (2023).

A quote:

Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfvén surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfvén surface along any radial ray.