Parker Solar Probe

Why is the Sun's corona hotter than its surface?
14 August 2018

Interview with 

Nicky Fox, Johns Hopkins University


This week, a very special spacecraft is beginning an extraordinary journey into the heat of the Sun’s corona (or outer atmosphere). The Parker Solar Probe launched on August 12th and is setting off to uncover some of the mysteries of the hottest part of the Sun, and hopefully, won’t melt in the process. Katie Haylor has been finding out more…

Katie - On the 11th August, 2018 the Parker Solar Probe - 60 years in the making - older than NASA itself is set to launch from Cape Canaveral in Florida. It’s quite a small craft; imagine an hexagonal compartment about 1 by 1½ metres - that’s the main body where most of the scientific kit is housed. About 1½ metres above is the heat shield.

Nicky - It is made of a carbon-carbon composite. It’s very similar to a graphite epoxy that you might find in golf clubs or a nice tennis racket. And it’s a sandwich structure; there are two very thin face sheets and then there about 4½ inches of a carbon-carbon foam about 97 percent air when it’s here on Earth. And then on the very front surface of that we have a plasma sprayed alumina coating that actually reflects most of the sunlight. The front side of the heat shield will get to about 1400 degrees celsius, but the instruments on the main body of the spacecraft just a little bit above room temperature.

Katie - The mission's project scientist, Nicky Fox. So, equipped with this rather impressive shield, once in space the probe will journey through the Solar System using gravity assists from Venus to do a series of fly-bys through the Sun’s corona across the seven years of the mission focusing its orbit closer and closer to the Sun and getting to, at its closest point, a mere 3.8 million miles above the Sun’s surface. This probe will get closer than ever before to our Sun and will, hopefully, survive long enough to beam back data to us here on Earth. So why attempt something so daring? Well, there are a few mysteries about the corona that have been bothering scientists for decades. First up, the corona itself is about 300 times hotter than the surface of the Sun despite being further away from the heat source, which is rather puzzling…

Nicky - The other mystery is why it's accelerated so fast in this region. So where you see this incredible heating, the plasma itself gets energised and it does accelerate away from the Sun at supersonic speeds out and bathing all of the planets. It carries with it the Sun’s magnetic fields; so the Sun is rotating, and the magnetic field of the Sun is rotating with the Sun and all of that material is kind of stuck on those magnetic field lines. Where the plasma gets super-energised it is so energetic that it actually grabs the magnetic field of the Sun and pulls it away from the Sun with it as it streams out towards the edge of our Solar System. The Earth has a magnetic field and those two fields can interact, when the Solar wind arrives at the Earth, and cause large scale changes and it can lead to big space weather events. And by going and making these measurements in this region it will finally enable us to put the physics behind the drivers of the solar wind - the stuff that is coming and impacting our planet. It will make transformational improvements in our ability to predict the impact that the Sun has on our Earth every day.

Katie - So what kind of equipment is on board this probe, which is going to capture the information needed to answer these questions?

Nicky - I mentioned the magnetic field as being something that’s changing so we carry three magnetometers with us to make sure that we cover the full frequency range. Where there’s a magnetic field that's changing there’s an electric field that’s changing, so we carry antennas on the spacecraft and they will also measure plasma waves. And so when you have all these particles moving around they generate waves and the characteristic wave will tell us a lot about the physics that is going on. If we see one type of wave we know it’s likely to be one type of physics, etc.

Katie - The probe will also capture some key characteristics of the continually streaming Solar wind. Things like speed, temperature, composition and density, so scientists can learn more about the plasma close to the Sun…

Nicky - And then we also carry high energy particle detectors with us that measure those very high energy particles associated with those transients, so the flares, coronal mass ejection shocks in the solar wind. And then, last but not least, we have a white light imager that is taking images of, essentially, the Solar wind right in front of the spacecraft - so what the spacecraft is about the plough through - so that will help us to put into context all of these wonderful in-situ measurements that we’re making with the rest of the payload.

Katie - There are big expectations for this little craft. I asked Nicky about what challenges lay in store…

Nicky - We do have to worry, of course, about the heat, the dust. We have to keep our solar panels cool. Another really huge challenge: it takes light eight minutes to get from the Sun to the Earth so there’s no way we can "joystick" this mission. If anything goes wrong, she is totally programmed to figure out what it is that is going wrong and fix it on orbit. You’re sending a spacecraft into this very challenging environment and she’s very very small, but she’s very independent. It’s an amazing team that we have that have really put her through her paces. And it is rather like sending your kids to college; you do the best you can, you bring them up, and then you send them out into the world, and you just hope that they can take care of themselves...


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