Artist's impression of the Parker Solar Probe, a small satellite, shooting through fiery streaks of hot gas in front of the enormous Sun.

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   John Carlos Baez (johncarlosbaez@mathstodon.xyz)'s status on Wednesday, 25-Dec-2024 03:55:04 JST John Carlos Baez

   Today the Parker Solar Probe will get 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 really cool part is what the newspapers are barely talking about: what it's like down there. The Sun doesn't have a surface like the Earth does, since it's all just hot gas. But it 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 gas emitted by the Sun - breaks free of the Sun and shoots out into space. But to understand how cool it 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.

   But it's even cooler than that, 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!

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