A series of coronal mass ejections (CMEs) from April 2-6. Images taken by the LASCO instrument aboard the SOHO spacecraft. (Click to launch movie.)

A few times every day during solar maximum conditions, the Sun can let loose a titanic blast of material. For days, a 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 is only a small part of a larger blast of coronal material that forms a large loop or bubble rising through 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 Earth, having passed Venus and Mercury on its way. In time, these coronal mass ejections, or CMEs as they are called, cause interplanetary space to be filled with a changing mosaic of cloud fragments and magnetic field blobs, millions of kilometers across, and flowing outward in a great pinwheel pattern, to beyond the orbit of Pluto.

Each CME Unique
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 2-4 days to get to 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.

Earth's Protective Shield
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. As they sweep past Earth, they distort Earth's magnetic field causing its Sun-ward side to be pushed inside the orbits of geosynchronous communication satellites. Satellite outages and other problems can then ensue as delicate electronics are battered by hailstorms of energetic CME particles. In other parts of Earth's magnetic field, powerful currents of particles become energized and flow into the polar regions causing the Northern and Southern Lights.

Find out more about the Sun-Earth Connection at the Sun-Earth Connection Education Forum Web site.

Text adapted from the Sun-Earth Connection Tutorial courtesy of NASA, originally written by Dr. Sten Odenwald. Images and videos courtesy of NASA unless otherwised noted.