The primary science goal of the STEREO mission is to understand the origin and consequences of Coronal Mass Ejections, the most energetic type of eruption that occurs on the Sun, and a primary driver of space weather on Earth.

Other specific objectives, that are needed to complete the primary goal, are to

• 1. Understand the causes and mechanisms that trigger the eruption of CMEs
• 2. Characterize how CMEs move through the heliosphere
• 3. Develop a three-dimensional, time dependent model of the magnetic topology, temperature, density, and velocity structure of the solar wind
• 4. Discover the mechanisms and sites of solar energetic particle acceleration

Let's start with a question. Why do we care about CME eruptions on the Sun?

The reason is a phenomenon called "space weather".

The Sun, the Earth, and the space in-between are all connected. The outer atmosphere of the sun, what we call its corona , expands billions of miles into outer space. The expansion is not static, but dynamic -- the flow out of the sun reaches speeds of millions of miles per hour. This coronal material that is flowing out of the sun, is what we call the solar wind. The solar wind fills up interplanetary space. It continues its flow out past all the known planets, out past even Pluto (a former planet), filling a region called the heliosphere.

The solar wind contains mass, and that mass is moving, so it exerts pressure. The solar wind also drags the magnetic field from the surface of the Sun out into space. This means the solar wind has both kinetic and magnetic energy. When the solar wind flows by the Earth, both the kinetic and the magnetic forces in the solar wind interact with the Earth's own region of space, the region of space known as the Earth's magnetosphere.

Coronal mass ejections (CMEs) are solar wind that has violently erupted off the sun. It is basically a blob of highly ionized gas (what we call a plasma). These eruptions fling off 1 to 10 billion tons of material out of the solar atmosphere into the solar wind. This type of solar wind typically travels 1-3 million miles per hour, such that it reaches the Earth's orbit at 1 AU (93 million miles from the Sun) in typically 2-4 days.

When the magnetic field around the Earth becomes disturbed from being hit by a CME, there tends to be movement of particles and changes in the electric fields in the space surrounding the Earth, resulting in a magnetic storm (or in some cases a substorm).

Magnetic activity causes the aurora (northern and southern lights). No one minds the aurora. But because we are a technological society, magnetic storms can also cause damage to our infrastructure. Damage or loss of satellites used for communication, weather, or defense. Damage from surges in power grids, causing blackouts or brownouts. Increased radiation over the poles of the Earth, limiting safe travel by airplanes over those routes. Radiation exposure for our astronauts.

We care about CME eruptions on the Sun because they are the primary cause for major magnetic storms that affect our society.

Specific STEREO Mission Objectives:

• Objective 1. Understand the causes and mechanisms that trigger the eruption of CMEs

What is puzzling to us is that we do not understand what causes the eruption in the first place. Yes, we know that it involves the magnetic forces on the Sun, typically in so-called solar active regions, but why does it erupt today and not yesterday, or tomorrow? Why are some eruptions fast, while others are slow? Why are some eruptions more likely to create a major magnetic storm at Earth, while other eruptions create hardly any disturbance?

All of the instruments on STEREO are needed to answer these questions. While SECCHI will be taking remote images of the CME as it erupts from the Sun and travels into space, SWAVES detects the traveling shock ahead of the CME through radio bursts, and the in situ experiments PLASTIC and IMPACT take samples of the CME as it flows past the spacecraft.

With the information we obtain from STEREO, we will understand the causes and mechanisms that trigger CMEs such that we can begin to start FORECASTING that an event will occur in the future. Right now, we can only do NOWCASTING, after we see the eruption.

• Objectives 2 and 4: Characterize how CMEs move through the heliosphere and develop a three-dimensional, time dependent model of the magnetic topology, temperature, density, and velocity structure of the solar wind

CMEs do not always head toward the Earth. Even those that originally are seen by coronagraphs as headed our way can be deflected, slowed down, sped up, or undergo other changes in the space between the Sun and the Earth. This is because that space is filled with the solar wind that was already there. This is sometimes called the ambient solar wind.

If the solar wind were always the same, we would have a steady state situation for the ambient solar wind. Life would be boring, and frankly we would be studying something else.

But the solar wind is very changeable. The amount of material (density), how fast it is going (speed), the direction it is coming from (flow direction), what it is made out of (composition), and the strength and direction of the embedded solar magnetic field are all variable. Most of these parameters depend on what feature on the Sun from which the wind originated. Some of these parameters stay unchanged as they travel through space – for example the composition. Other parameters can be modified depending on what it encounters in its journey. For example, a parcel of fast solar wind slamming into the rear end of a parcel of slow solar wind that was ahead of its path causes that fast solar wind to slow down and compress, changing its speed, density, and even flow direction.

Think of a fast solar wind (such as a CME) as a Ferrari, cruising at high speeds until it hits rush hour traffic. Traffic slows down (change in speed). Cars bunch up (change in density), and some cars move into other lanes or even exit to get past obstacles (change in flow direction).

We use both the remote and the in situ instruments to characterize how the CMEs move through the heliosphere. We collect the data from the in situ solar wind and energetic particle instruments even when a CME has not yet occurred, so that we will know what it will be traveling through when it does occur. PLASTIC and IMPACT make these in situ measurements. PLASTIC measures the density, speed, flow, and material of the solar wind, while IMPACT measures its electrons, embedded magnetic fields, and more energetic particles.