Understanding sun spicules

27 June 2017

Interview with

Professor Lucie Green, UCL

We don't really know that much about how our local star works, but a recent paper in the journal Science has shed light on Sun spicules. These are long strands of high energy plasma that are fired off into space at high speed, but thier origins are something of a mystery. Graihagh Jackson spoke to space scientist Lucie Green from UCL about sun spicules, and about how a group of scientists have built a model of them in action.

Lucie - I think it’s worth setting the scene that our Sun is a ball of gas, and it’s a ball of seething, roiling gas. The surface of the Sun isn’t a solid surface at all, it has a very, very low gas density and because of all this energy coming from the interior of the Sun, the surface gas is moving around all the time and within that, the magnetic fields of the Sun are also being moved around. So these spicules are being formed in a very, very dynamic part of the Sun.

Graihagh - What happens; why do they suddenly just fire off like that?

Lucie - That’s been a question that’s been worked on for a long time, and that’s one of the finding of this paper is to help us understand what mechanisms are at play. So you need to be able to come up with a way that you can accelerate gas away from the surface of the Sun, so against the Sun’s gravity and, at the same time, you need to be able to heat the gas. The reason that spicules are so interesting and important for the Sun is they seem to be a way by which you can send hot gas into the atmosphere of the Sun and do things like heat the corona to the millions of degrees that you mentioned, and supply the mass for the solar wind.

What this paper has done is to be able to say well actually, there’s a particular evolution of the magnetic field at the bottom of the spicules that can store tension, and release tension in the magnetic field. So imagine the magnetic field being almost like a guitar string that has a tension, and when you release that you can accelerate the plasma - a bit like a slingshot effect.

Graihagh - Oh I see. Why didn’t we really know much about this before? I imagine the Sun - well we’re told not to look at it directly so I imagine it’s quite hard to study?

Lucie - It is hard to study and it’s hard, particularly in this layer of the Sun’s atmosphere lower down and that’s because the physics there is incredibly complex. It’s interesting to me because the upper atmosphere of the Sun (the corona) we focused on for a long time, and the lower layer that this paper is focusing on has, perhaps, not received the focus. Not because we’re not interested in it but because it’s a really complex region. The plasma conditions there are very interesting, they’re relatively high, and the magnetic field evolves in very interesting ways. So we’ve needed to make this step wise development in the physics to be able to get to the point where we can start to understand why spicules form, and the model that’s been developed by this team is absolutely a world calls model and leading the way in our understanding.

Graihagh - They took data from NASA’s spacecraft IRIS and a telescope, and then put it all into this model and I understand it took a year to run this model because it was so complicated. The thing I want to ask you is you’re not involved in this study so you’re not biased, so can you just sum up how important these studies are in terms of your field?

Lucie - Absolutely. I’ve been an admirer of this model, this simulation for a long time and it’s becoming more and more realistic, and I think it’s the best model we have. It’s such an important area of the Sun to understand because the region of the Sun that they’re looking at is kind of the gateway to the atmosphere. It regulates how mass and energy flow into the atmosphere of the Sun.

What’s been really important about this team is that they have, right from the start, recognised that when you launch a space mission you need not only to have that data from the mission, but you also need to develop the model to go alongside it. So, within your computer, you create the conditions as best you can and then you make predictions about what the observations should show if you’ve got particular physical processes happening and that allows you to test your physics models, constrain the models with the data. Working in parallel with the simulations and the data has been a big focus of the IRIS mission and a big focus of this team, and now it’s really paying off.

Graihagh - That’s really interesting. But what are the implications for us here on Earth?

Lucie - I think when it comes to applying this knowledge to us here on the ground, really the link is between what the Sun’s doing and something what we call space weather. I think that his study will help us understand more the science of space weather, and by that I mean understanding how the Sun’s atmosphere is varying over time. For example, the temperature of the atmosphere and the outflow of something called the solar wind, and it’s the solar wind flowing towards the Earth that is the driver of a lot of the space weather that we receive. So I think this study is really about understanding and developing a much more detailed picture of how the plasma, the gas, and the magnetic field in our local star evolves.

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