The spotted face of the Sun

June 26, 2023   Issue #582

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Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

Normally, these “Pics o’ the Letter” are from national observatories, space missions, or astrophotographers who are experts in their field.

But this one (really two) is (are) not: They’re from… me!

I had to set up my telescope to do some upkeep, and I figured as long as I’m out during the day, I’d stick the ol’ Celestron solar filter on it and check out the Sun. I’ve seen a bunch of recent images other folks have taken so I expected to see some sunspots, and holy heliosphere did our nearest star not disappoint.

I took this with my Celestron 20 cm telescope, filtered (of course) and my widest angle eyepiece, holding my phone cam up to the eyepiece (the circle around it surrounded by blackness is the edge of the field of view in the eyepiece). As you can see, the Sun’s been busy! Dozens of sunspots are visible, arranged in various clusters. That’s typical; they usually appear in groups or at least pairs.

I check online at the Solar and Heliospheric Observatory (SOHO) site and found a map of the current sunspots; they’re numbered.

[Note this image is backwards relative to mine].

Technically these are called active regions, and they’re numbered as they’re discovered, officially with a “1” in front. I put in a higher-power eyepiece and got a close-up of AR 13341:

The contrast isn’t great but you can see the dark central part of the spot (called the umbra) surrounded by a lighter-toned area (the penumbra). There are other spots nearby too. If you look carefully you can see texture to the Sun’s surface, including irregular regions of very slightly lighter tone near the spots. That’s real! They’re called faculae.

Great! But what is all this?

The Sun has a magnetic field, but it’s not like Earth’s. Our own planet’s field is like a bar magnet, with magnetic north and south poles and donut-shaped field lines connecting them. The magnetic field is generated by currents of iron in our planet’s core, which create the overall field.

The Sun’s not like that though. It’s a gas — well, technically a plasma; a gas where the atoms have their electrons all stripped off, and the distinction is important. Material near the Sun’s core is extremely hot, and hot gas/plasma rises. When it reaches the surface it radiates its energy away into space, cools, and sinks back down. These towers of moving plasma are called convection cells, and there are thousands of them across the Sun, each a conveyor belt of material.

But plasma has an electric charge due to the loss of the atoms’ electrons, and when you move something with an electric charge it generates a magnetic field. So each of these towers is generating its own field! There is no real overall magnetic field to the Sun; instead it has thousands of local ones.

The magnetic field lines connecting the local magnetic field poles on the Earth are fairly well organized. But on the Sun they’re a mess, interfering with each other and sometimes getting tangled up. When the convecting plasma reaches the surface of the Sun, those tangled lines can act like an net, preventing the plasma from sinking again. This material still cools but cannot sink. Cooler material doesn’t emit as much light as the stuff around it, so it appears dark in comparison: sunspots.

The inner part of the sunspot is where the magnetic field lines are most crowded together and the towering loops most vertical as they penetrate the surface, so most intense, and that’s why the umbra is dark. At the edges the lines are angled more, so not as intense, and the penumbra not as dark.

Where two convection towers meet, the downwelling gas still has a magnetic field, and these regions can actually heat up. They glow slightly brighter than the surface around them, creating wiggly lighter features: faculae (which means “little torches”). Sunspots darken the Sun by a bit, but the faculae more than make up for that, so even when there are lots of sunspots the Sun is actually a tiny bit brighter, by about 0.1%.

Do you know someone who would be excited to read about this? Then hey, please share this issue with them!

But those magnetic field lines… they contain a lot of energy — a lot — and when they tangle up they can suddenly snap and reconnect, releasing that energy. This doesn’t happen often in a sunspot, but it does happen. The blast of energy can send out intense light as well as a massive wave of subatomic particles moving at a decent fraction of the speed of light. These are solar flares. They can be mind-crushingly powerful, radiating as much as 10% of the Sun’s total energy!

There are also coronial mass ejections, where the magnetic field launches up to a billion tons (!!) of hydrogen plasma into space at high speed. If a flare is local and intense like a tornado, a CME is like hurricane. Far more powerful, but spread out.

Both can cause all sorts of grief here on Earth if we get in the way. I’ve written about this many times on The Old Blog™ and you can read about it there. I recommend this article about a simply terrifying storm in 2012 if you dare.

The Sun also has a cycle to its magnetic activity; every 12 years or so it goes from being very weak to very strong and then weak again. At minimum there are few if any sunspots, then ~6 years later at maximum there might be many dozens marring the Sun’s face. Right now we’re in the part of the cycle where the activity is ramping up, so we see more spots.

Astronomers have been keeping records of sunspots for centuries; we’re currently in Cycle 25, and it’s looking to be a bit more twitchy than predicted. In the graph below you can see there are already more sunspots than predicted, so the peak will likely be more active than Cycle 24. The graph below it puts it in more historical perspective.

It’s hard to predict what the cycle will do, both for sunspot numbers and for the violent activity. In 2003 there were huge flares that actually threatened Earth-orbiting satellites, an no one really knows if this cycle will do that or not. We just have to keep an eye on the Sun — well, only kinda sorta literally, since NASA and other groups do observe the Sun constantly — and see how it behaves. Hopefully we won’t see a repeat of 2012!

But when these flares and CMEs do happen we learn a lot about the Sun, so in that sense it’s good when they happen, as long as they don’t put the hurt on us 150 million kilometers away. I know I’ll be watching for solar storm alerts (especially since they can generate incredible aurorae), and my friends who study the Sun for a living will too. If anything interesting happens, rest assured I’ll let you know here on BAN and on my social media!

Et alia

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