Science at the Sun
The Solar Orbiter mission has been designed to answer four key questions about the solar wind. What are they?
1. How is the solar wind accelerated by the corona’s magnetic field?
By studying the correlation between the solar wind and the changing properties of the magnetic fields in the Sun’s corona, the mission hopes to reveal the mechanism by which particles in the solar wind are accelerated to speeds of up to 800km/s.
2. How do transient features on the Sun drive variability?
Sudden events on the Sun’s surface, such as solar flares and coronal mass ejections, can have a huge impact on the intensity of the solar wind and can dramatically affect space weather on Earth. By observing these events at close range, the Solar Orbiter will help us understand the origin of these events with the ultimate aim of being able to predict them.
3. How do solar eruptions fill the heliosphere with energetic particles?
The Sun is the most powerful particle accelerator in the Universe. As well as the constant flow of the solar wind it also regularly emits storms of particles that travel at close to the speed of light. These high-energy particles can be detected at Earth’s surface and can affect radio transmissions and air travel, but we don’t know exactly where they come from on the Sun. The Solar Orbiter will attempt to identify the source of these particles.
4. How does the solar dynamo work?
All the solar activity we see is ultimately generated by the Sun’s magnetic field. We know the magnetic field varies over an 11-year cycle, but we don’t know how the magnetic field is generated by the ‘solar dynamo’. Solar Orbiter should help solve this mystery by providing the first detailed observation of the magnetic fields in the Sun’s polar regions.
the solar wind in the area immediately around the spacecraft – characterising its make-up and plotting how it changes at different distances from the Sun.
These instruments are mostly fitted to the outside of the spacecraft and on the boom that extends 4.4m behind it. Here, just metres from the blistering temperatures at the front of the heat shield, it will be so cold that the instruments need heating to maintain their operational temperatures.
The other six remote sensing instruments are a selection of cameras and telescopes that will study the Sun itself at a variety of wavelengths. Unfortunately, to see these features the instruments need to look through the heat shield – and so each has a special channel driven through the shield protected by a sliding door which will open only while the instruments are collecting their measurements.
The remote instruments will be looking for the source of the solar wind, transient magnetic features on the Sun’s surface – like the coronal loops that carry arches of superheated plasma high above the surface and are often the source of the vast eruptions of material known as coronal mass ejections.
Surface features
One of the mission’s main aims is to understand how these features develop over time. And so as the spacecraft makes its closest approach each orbit it will match the rotation rate of the Sun, effectively hovering over the same section of the Sun’s surface for several days allowing the instruments to observe the surface features as they evolve.
“What I want to know is, how is the magnetic field evolving in the lead-up to coronal mass ejections,” says Lucie Green. “The longer we can watch, the better.”
Over the course of its mission, the Solar Orbiter should revolutionise our understanding of the solar wind and heliosphere. Especially since it will not be working alone.
NASA also has a spacecraft in close orbit around the Sun. The Parker Solar Probe launched in August 2018 and is on an even more daredevil mission to study the source of the solar wind.
Travelling at 109km/s it is the fastest spacecraft ever built. On 29 January it passed within 20 million km of the Sun’s surface, closer than any other spacecraft has been before. By the time it makes its final orbits of the sun in 2025 it will pass within just 6 million km of the solar surface – flying through the outer layers of the corona itself. In these regions, very close to the Sun, the spacecraft will experience temperatures of up to 1,400˚C. The radiation will be so intense that it will be impossible to take images of the Sun itself. Instead, the probe will take detailed measurements of the Sun’s magnetic field and sample the particles of solar wind at their origin as they are accelerated into the Solar System.
By combining the data from the instruments on both the Solar Orbiter and the Parker Solar Probe, scientists hope to be able to understand how the solar wind is created and controlled, and learn how it develops as it moves away from the Sun.
“The solar wind has a dramatic impact on the rest of the Solar System,” says Lucie Green. “The heliosphere really means something for the planets. It’s the strong winds in the heliosphere that strip off the atmosphere of Mars and it is the changes in the heliosphere that drive space weather at Earth, and it all starts at the Sun.”
The spacecraft’s main defence is its heat shield – which will almost always be pointed towards the Sun – leaving the rest of the Solar Orbiter protected in its shadow