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STEREO is a mission to help us understand what we call space weather,

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how the activity on the sun can affect us here

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at Earth. The sun will blow out what we call coronal mass ejections.

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They're a kind of solar storm, a billion tons of matter, coming at us

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at a million miles and hour. They can affect our technology here on Earth, affect power systems,

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affect spacecraft. And, so understanding them is not just

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a case of their scientific interest, but it also has practical application.

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These two spacecraft are giving us two views on the sun instead of just one.

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That's really important, because the structures that we are looking at on the sun

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that are in the solar wind, which is the sun's atmosphere, blowing out into space,

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are really three dimensional objects. To really understand what we are seeing,

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we have to look at them from more than one point of view.

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CMEs, looking at them, they are kind of like big puffs of

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smoke. They have structure, but you can look right through them. In general,

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you can sort of think of CMEs as a kind of big, bent Slinky

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By looking at it in more than one point of view at the same time, we are able to

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confirm that that's roughly what CMEs look like.

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The sun's super hot atmosphere is flowing out into space constantly

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and this is called the solar wind. Coronal mass ejections are kind of like

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storms in the solar wind. We can see from a distance the large

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scale of these storms, blowing out into the solar system.

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But we can actually sense, in a number of locations, throughout the solar system,

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STEREO and other ones, the actual disturbance as it goes by.

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So, we can sample the solar wind, see what that's like, sample the CME as it goes by.

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Putting all those together, the remote sensing data, the local

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conditions, has given a really good, complex picture

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of coronal mass ejections, that you can't really get from a single spacecraft.

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Comet tails respond to the solar wind,

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and we've known that for a long time. They are basically charged gas, and they're interacting

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with the magnetic field in the solar wind.

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What we see here is all this dynamism in this tail. And you can see it's not just sort of a tail,

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but that it's responding -- you see there? Right there -- It's responding to this coronal mass ejection

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that's coming out from the sun. It helps us understand

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how the comet interacts with the solar wind, we can actually

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use the comet sort of as a way to probe the solar wind.

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STEREO allowed us to see the far side of the sun

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for the first time. You look at the sun and you see these bright regions

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these active regions, that's where things like solar flares occur, and

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really big coronal mass ejections will often come from near those. You might not see

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anything on the side of the sun facing you, but something really interesting might be

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developing on the far side of the sun. It can be a surprise when it comes around, you know, we might

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some inklings that something is going on back there, but we can't really see it without, for instance,

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STEREO.

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So this was a major, major solar storm. We would have known something had happened, but we wouldn't have

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understood its magnitude, just from data from the point of view of the Earth. And if you

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look in the coronagraph image to the right, you see that suddenly, there are these little bright

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like, bits of snow that show up on the detector. Well, those are

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called solar energetic particles. What they are

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are particles accelerated almost to the speed of light. Those can be quite

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hazardous to, for instance, spacecraft or astronauts.

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They are related to these coronal mass ejections and other kinds of solar activity

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call flares. We would never have even known that it was happening

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without spacecraft on the far side of the sun.

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By putting all these different points of view onto the sun and space weather

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we've been able to understand our star in a way we've never been able to

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understand before.

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