Coronal Mass Ejections

A few times every day during solar maximum conditions, the Sun can let loose a titanic blast of material. For days, a heated cloud of plasma can be suspended by magnetic pressures just above the photosphere in a region called the chromosphere. Then, for reasons not fully understood, this billion-ton cloud can become unhinged and be propelled away from the Sun. The cloud may only have started off as a gentle puff of plasma. As it enters the lower reaches of the solar corona, the Sun's outer atmosphere, the cloud expands and accelerates enormously to speeds of millions of kilometers per hour. Within a few days, the cloud has reached the orbit of the Earth, while parts of the cloud itself still envelope the orbits of Venus and Mercury. In time, these coronal mass ejections, or CMEs as they are called, cause interplanetary space to be filled with a changing patina of cloud fragments and magnetic field blobs, millions of kilometers across, and flowing outward in a great pinwheeling pattern, out beyond the orbit of Pluto.

No two CMEs are exactly the same, so astronomers describe these explosions by average properties, just as we often say that the average human being is about 6 feet tall. CMEs are actually not very dense by the time they reach the Earth's orbit. As they expand through space, their density falls from millions of particles per cubic centimeter near the Sun, to barely a dozen particles per cubic centimeter near Earth. Most of them travel at nearly one million kilometers per hour and take 2-3 days to get to the Earth's orbit. The fastest ones can travel at nearly three times this speed and get to Earth within a day. Many of them are actually quite hollow and resemble enormous soap bubbles blown into space by the Sun.

The outer surface still contains some of the Sun's original magnetic field, though weakened by over a million times as it is stretched across the orbits of Mercury and Venus.

As spectacular as these solar storms can be, there is little cause for concern that the Earth's atmosphere will be 'blown away' by them. A CME blast wave is actually a better vacuum than what you would find in a television picture tube, but this doesn't mean that they are completely without any consequence.

Many systems in our solar system, and even in the Milky Way galaxy, are like pencils balanced on their points. The subtle pressure changes that CMEs bring with them into the depths of the solar system can affect the delicate balances in other physical systems elsewhere in space.


Teacher Notes:

How do scientists use information about coronal mass ejections to determine the impact on the Sun-Earth system?

As the scale of complexity increases, we resort to summary characteristics. Systems of complexity may show characteristics that are not predictable. The full explanation of complexity is reduced to a scale that is outside of our direct experience.

• (K-2) Things in nature and things people make have very different sizes, weights, ages, and speeds.

• (3-5) Almost anything has limits on how big or small it can be.

• (6-8) As complexity of any system increases, gaining an understanding of it depends increasingly on summaries, such as averages and ranges, and on description of typical examples of that system.

• (9-12) Representing large numbers in terms of powers of ten makes it easier to think about them and to compare things that are greatly different.


Science Nuggets:

2000...WIND, Ulysses and Voyager 2 use helium ions to trace a CME beyond the orbit of Pluto.

2000...IMP-8 measures solar wind speeds up to 1,050 kilometers per second during the July 14th ‘Bastille Day’ solar storm.

2000...SOHO detects solar storms on the far side of the Sun using sound waves.

1999...Yohkoh scientists observe 'sigmoid' features in solar magnetic field which are preludes to coronal mass ejection events.

1998...ACE measurements of oxygen ion abundances show that some CMEs are spawned by processes on the solar surface where magnetic and thermal pressures are nearly in exact balance.

1995...Yohkoh discovers a dimming of the hot corona as seen from the Earth, during a coronal mass ejection.