Why Is The Sun’s Corona Hotter Than Its Surface?

16:35 minutes

If you want to study something, the best way to do it is to go straight to the source. That goes for bodies in our solar system as well. Over the last several decades, NASA has sent space probes to study Mars, Saturn, Jupiter, Venus, Pluto, and the objects beyond them. But in our never-ending quest to explore the solar system, we can’t leave out our sun.

On August 11th, NASA will launch the Parker Solar Probe, the latest mission to study our nearest star—and every other star in the universe. It won’t be the first spacecraft to get a close look at our sun, but it will be the nearest we’ve ever come—about 3.8 million miles.

[We wouldn’t be doing our job right if we didn’t talk about the bacteria behind your blues and bries.]

If that still sounds far, picture this: If the sun and the earth were on opposite ends of a yardstick, the Parker Solar Probe would be hanging out around the four-inch mark. That’s close enough to measure the sun’s magnetic field. Hopefully, the probe will help scientists answer decades-long mysteries about the sun’s corona.

Alex Young, associate director for science in the heliophysics science division at NASA Goddard, joins Ira discuss the mission goals for the solar probe and what will keep the spacecraft from meeting an Icarian fate.

Segment Guests

Alex Young

Alex Young is associate director for science in the heliophysics science division at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

As you know, over the last several decades, NASA has sent space probes to study Mars and Saturn, Jupiter, Venus, even to Pluto and its moons, and beyond. In our quest to explore our solar system, we’re at last turning inward, to the key to all life on our planet which is, yes, our sun.

On August 11th, or about that time, NASA will launch the Parker Solar Probe. It won’t be the first spacecraft to get up close to look at our sun, but it will be the nearest we’ve ever come to touching it. Just 3.8 million miles away.

So how close is that? OK, picture this. If the sun and the Earth were on opposite sides of a yardstick, the Solar Probe would be hanging out at about four inches– four inches away from the sun. Wow. Close enough to measure the sun’s magnetic field, help scientists answer decades-long mysteries about the sun’s corona.

Well, how will it do all of this and not become a modern day Icarus? You can send your burning questions about NASA’s solo mission to Scifri– S-C-I-F-R-I, OR give us a call at 844- 724- 8255. 844 SCI TALK.

Alex Young is Associate Director for Science in the Heliophysics Science Division at Goddard Space Flight Center. He joins us now. Alex, welcome back to Science Friday.

ALEX YOUNG: Thanks for having me again.

IRA FLATOW: So tell us about this mission. Where exactly is this spacecraft going? And how long will it take to get there? What’s it going to do? I’m going to put my feet up on the table. You take it.

ALEX YOUNG: All right. So, on there abouts August 11th, we’re going to launch the mission from what’s called the Delta Heavy. It’s our biggest rocket. We’re sending it, using Venus to help us along the way. We’re going to get into a very, very close orbit. As you mentioned, 3.8 million miles.

It’s going to take us several years. We have 24 orbits. We get to this closest orbit in 2024. And then, we’re going to be flying through the corona, the region where all the action is, where all the cool stuff is happening, so we can answer these fundamental questions about why is the corona so hot?

Why does its solar wind stream away at millions of miles an hour? And what is causing all of this crazy activity we call space weather? Solar flares, coronal mass ejections, these things that have a huge impact on our technological society.

IRA FLATOW: Let’s talk about the heat of the corona. The corona is hotter than the surface of the sun. Is that right? And how hot is that?

ALEX YOUNG: That’s right. So, the surface of the sun, the visible part or photosphere, is about 10,000 degrees. But as you go up higher into the corona, the temperatures quickly skyrocket up to many millions of degrees Fahrenheit. So just unbelievable temperatures, as these particles are moving super, super fast.

IRA FLATOW: And we don’t have any idea why that is?

ALEX YOUNG: Well, we have some idea. We have some very good theories. They’re all based on the release of magnetic energy. The sun’s corona is threaded with magnetic fields. These are coming out of sunspots from the surface. They get all twisted up, almost like the way rubber bands get twisted.

And they release this energy in the form of these little, tiny explosions. There’s waves that are traveling through. And we think there’s some sort of combination, or maybe one of those two mechanisms, but we really don’t understand the details. And that’s why we have to go there, because we’ve been looking at it from a distance. We can only learn so much.

IRA FLATOW: OK. You know the next question I’m going to ask you. I’m going to let one of our listeners do it for us. Let’s go Duffy in West Georgia. Hi, Duffy.

DUFFY: Hello.

IRA FLATOW: Hey, there.

DUFFY: Yeah, my question is, how does that thing even get close before it’s turned into a cinder?

IRA FLATOW: Yeah, we all knew that was coming. Thanks for calling.

ALEX YOUNG: That is the $64,000 question. So, that is great. Not only is this an amazing science mission, but it is a technological marvel. Engineers have done amazing things. And the reason we haven’t been able to do it until now is we didn’t have the technology.

So, there’s a couple of things that are allowing us to do it. The first is a 4 and 1/2 inch thick carbon shield. It is a composite of carbon fiber, carbon foam. It’s actually mostly air. And that is coated with a special coating that reflects a lot of the sunlight.

And then, we also have a cooling system that cools the spacecraft. In particular, it also cools the solar panels. And you know what’s crazy about that is, it actually uses just regular water. A gallon of regular water. And so, the combination of those two things do an amazing job.

The front of the spacecraft, the radiation heats it up to about 2,500 degrees Fahrenheit. But behind that shield, and because of the shield and the radiator, it’s actually a cozy 85 degrees Fahrenheit where those instruments are.

IRA FLATOW: I’m picturing a radiator cap on top of that water.

ALEX YOUNG: Pretty much.


IRA FLATOW: No pit stops.


For we geeks in the audience, the shield, you said about four inches thick? Is it sort of an aerogel? I remember from–

ALEX YOUNG: It’s very– yes. It is similar to aerogel. And it’s really, really cool. I’ve gotten to see a piece of it. And actually, they do a fun demonstration where they take a blowtorch, and put the blowtorch on the front. You can see it getting heated up. And you can put your hand behind it, and you don’t feel anything.

IRA FLATOW: Sounds like the tiles on the space shuttle, were made sort of the same way.

ALEX YOUNG: It’s all very similar. And you know, it looks so simple. It’s mind boggling to think, when you see it up close, that this piece of foam is actually protecting it from all of that intense heat and radiation.

IRA FLATOW: Cool stuff. Let’s move on to the solar wind that you talked about. You want to study the solar wind. What is that? And why do we care about it?

ALEX YOUNG: So, the corona is being heated. The corona is made up of mostly hydrogen. Actually, the protons from hydrogen. There are also the electrons. They’ve all separated from each other because they’re so hot. There’s helium, and there are some bigger elements. But all of that stuff is energized in the corona, and it’s streaming away, and it’s carrying the magnetic field of the sun with it.

And this solar wind bathes the solar system. It’s covering– it’s impacting everything in the solar system. And that wave of stuff that is flying away, it’s traveling at speeds of around two million miles an hour. So, we expect a high corona to expand. But why is it accelerating so much in that lower part of the sun’s atmosphere?

That is really the question. And the reason we want to know that is the solar wind interacts with atmospheres. With us, we get the Aurora Borealis from it. But the solar wind is stripping away atmospheres of all planets, both those that have magnetic fields, and those that don’t. And so, understanding the solar wind, where it comes from, not only important for understanding the star itself, but it’s an important part of life in the solar system.

IRA FLATOW: How far out– is the solar wind connected to the corona, at all?

ALEX YOUNG: It is. There’s a part at which the solar wind becomes distinctly different. So there’s a region where it kind of changes over, and there’s no more information that’s coming out of it. And that’s actually one of the reasons why we have to go there in person, because we don’t have all the information we need.

When we measure it from a distance, there is a bit of a disconnect. And so, a lot of the information, the waves and whatnot, that are coming out, are not there anymore. And so, we have to go in where the acceleration is happening, to really understand the detail physics.

IRA FLATOW: Where does the corona end? Does it stop above the surface?

ALEX YOUNG: It’s a little bit of– it’s not completely certain. There have been some recent papers. I want to say, roughly, about 15 million miles, or so. And that’s based on some of the recent research, actually from a couple of colleagues of mine. In particular, Craig DeForest from Southwest Research. So that’s kind of the general idea of about where it’s happening.

IRA FLATOW: Could you go out there? Do you have instrumentation on board that could discover something you didn’t know existed around the sun? And how would you know if you discovered it?


ALEX YOUNG: Well, there are things we don’t completely know. We know that there is a certain type of dust around the sun that we can’t see very easily. It’s very, very small. We’ll be looking at it, both with these instruments to measure the fields, as well as the particles. And we even have some cameras that are looking at stuff that’s streaming away.

I suspect we’re definitely going to find something we didn’t expect. And we have these cameras. We may, in fact, see something we did we didn’t expect to see.

I think that’s the coolest thing about science, is we always go in with questions, and we always come out with more questions. And that’s what keeps us going.

IRA FLATOW: I think one of the coolest things I learned from the years of covering sun science– and this just boggles my mind– is how long it takes. You talk about the surface of the sun being 10,000 degrees. No. I mean the corona 10,000. Surface, 10,000. But the interior of the sun, because there’s a nuclear reaction going on, is 100 million, or something like that.

ALEX YOUNG: We talked about this last time. Yeah. This is so cool.

IRA FLATOW: And not only that, but the light particles inside the sun, it’s estimated could take anywhere from 100,000 to a million years for them to make their way out to the surface?

ALEX YOUNG: Yes. The sun is a nuclear furnace. And protons are being squeezed together, making helium, and they’re producing gamma rays. And those gamma rays are slowly wandering out. They actually encounter another atom, and then that atom gives off another light particle. And they randomly walk their way out to the surface.

And it takes hundreds of thousands of years. It’s crazy. So the light we’re seeing is actually not the light that was created at the center.

IRA FLATOW: How is the– how do you know when you will achieve success with this mission? What will success be. In this mission?

ALEX YOUNG: Well, success is certainly answering some of these fundamental questions of, is it the waves that that’s creating the heating? Is it these tiny, little explosions, called nano-flares? It is some combination of those? Are we seeing the signatures, when we measure the plasma? So when we measure the solar corona, we’re actually going to be able to measure the detailed ways in which that plasma is distributed.

IRA FLATOW: Interesting.

ALEX YOUNG: And so, does that structure match with the theories that we have? Or are there new things that we see, that we then have to go back and take our theories and rework them? And so, this is sort of iterative process.

IRA FLATOW: Talking with Alex Young of the Heliophysics Science Division at the famous Goddard Space Flight Center. I’m Ira Flatow. This is Science Friday, from WNYC Studios.

Now, will the space probe be on the other side of the sun, a long time from the Earth? And if it’s out of touch, who controls it?

ALEX YOUNG: Well, that’s a great question, because this is one of the things that’s unique about this spacecraft. This is the most autonomous spacecraft ever, because it will be out of touch.

It’s primary science phase is roughly 11 days or so, when it’s over in that region on the other side of the sun. And so, the spacecraft has to be completely self-sufficient. She is her own spacecraft. She takes care of herself. And that is one of the amazing engineering aspects of it.

IRA FLATOW: Let’s go to the phones. Let’s go to Jessie in Anchorage. Hi, Jessie.

JESSIE: Hello.

IRA FLATOW: Hey, there. Go ahead.

JESSIE: OK. I was wondering, the electronics on the probe, is the heat shield and cooling system, does it cool everything down enough? Or did they have to come up with new technology for the motherboards, and wiring, and everything, on the probe?

ALEX YOUNG: Well, I don’t know all the details. But I know that, in general, space technology is a little bit behind the stuff that you can buy off the shelf. This all has to be radiation hard. A lot of the technology here has been designed for planetary missions.

The kind of environment that we actually see for places like Jupiter is actually even– in some ways, the radiation is even worse inside Jupiter’s magnetosphere. All of that instrumentation has been designed for that. So that’s not really new, in terms of the missions we already have.

The cooling system does have to make sure that everything is behind that, and in the shadow of it, to protect it. But the instrumentation itself is really pretty well established.

IRA FLATOW: Here’s a tweet from Laura. And she says, isn’t the sun like a big candle? As a child, I figured out that when flame met wick, that was not as hot as the spot over the apex. Seems like corona would be the same. Why waste money and resources to play Icarus? Stick a thermometer over a candle.

ALEX YOUNG: Well, the problem is that the heat source, when you walk away from it, the temperature goes down. When you have an atmosphere, when you move away from the Earth, from the ground, as you move up higher, it gets cooler. As you move away from the flame, as you move away from the heat source– the heat source is really the big ball of nuclear furnace. And when you move away from it, it should get cooler. And it doesn’t do that.

IRA FLATOW: And that’s really one of the mysteries.

ALEX YOUNG: Exactly. And that’s a mystery that’s been around for a long, long time.

IRA FLATOW: It was discovered a while back.

ALEX YOUNG: That’s right. This is actually a pretty cool year because 1958, 60 years ago, was really when a lot of the genesis of a lot of this– this is when Eugene Parker wrote his famous paper about the solar wind. This is when NASA was formed. This was when the Simpson Commission came together, with a bucket list of things to do. And this is one of the key items that finally being achieved on that bucket list.

IRA FLATOW: Is this when the Van Allen belt was discovered?

ALEX YOUNG: Exactly. And this was the beginning of Explorer 1, 60 years ago. So this is such an amazing time, for the culmination of all that incredible work to come to fruition.

IRA FLATOW: And so, you’re sending this probe to our own star. How much can you extrapolate to every other star in the universe?

ALEX YOUNG: You can extrapolate a lot. And that’s one of the things that really makes this so much more than just a heliophysics, a solar mission. This is a universe mission. This is a mission that’s telling us about the stars in the universe.

The lucky thing is, we have a star close by. We can study it in detail. So it’s a fundamental laboratory. And it is going to provide– and it does provide– information that we then do extrapolate to other stars, and other solar systems.

You had a great discussion earlier about bacteria, and about the formation of life. Understanding the sun, its history, applies to this solar system, but it also applies to other solar system, extrasolar planets.

IRA FLATOW: Yeah, because that’s where all our energy comes from.

ALEX YOUNG: That’s right.

IRA FLATOW: That’s a good way to end, Alex.

Alex Young, Associate Director for Science in the Heliophysics– that means solar, sun, or something like that, right? Heliophysics Science Division at the Goddard Space Flight Center. Alex, always a pleasure to talk to you.

ALEX YOUNG: Same here. I love it. And we can’t wait to talk again sometime.

IRA FLATOW: We’ll do it again.

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