"The CME on November 6, 1997, was the fastest and most energetic one yet observed with SOHO. The image above (left) shows the CME exploding at a speed of approximately 4.5 million miles per hour. This CME produced an intense radiation storm of high-energy protons that hit SOHO and caused the ''blizzard'' that appears in the image (right)."

What are CMEs?
"The sun is constantly kicking out material from its atmosphere, generating what's known as the solar wind. The solar wind travels through space and is usually deflected by the earth's magnetosphere. Compared to the gentle breeze from the solar wind, fast-moving CMEs are a hurricane of plasma and coiled magnetic field lines spinning through space. Until recently, scientists didn't know what caused these violent magnetic eruptions."

"Normally, magnetic field lines move up through the sun slowly, at about one mile a second. This slow, continuous movement creates the solar wind. When a CME takes off, however, it can move thousands of miles a second. The mystery, says astrophysicist Spiro Antiochos, is: What allows a CME to build energy in the sun's atmosphere until it bursts through in a violent explosion?"

"A new theory, developed by Antiochos and colleagues at the Naval Research Lab, involves competing magnetic field lines that hold down the developing CME like tethers hold down an expanding helium balloon. Antiochos compares these tethering field lines to rubber bands that can stretch but never break. Through a process called magnetic reconnection, opposing magnetic field lines merge and cancel. Then, with no tether holding the ballooning material in place, the CME erupts out of the sun's atmosphere."

"The longer magnetic fields keep the CME pinned down, the more energy it builds and the faster and more violent the explosion when it's finally set free. Fast CMEs can reach speeds of 1250 miles per second and eject a billion tons of hot, ionized gas into space."
http://www.exploratorium.edu/eclipse/cmes3.html

So what triggers this release from the magnetic field than pins down these toroidal balls of plasma?

SOLAR MAGNETIC FIELD STRENGTH: Many models of the solar magnetic field used prior to Ulysses assumed that the solar magnetic field was similar to that of a dipole; field lines near the solar equator were thought to form closed loops whereas field lines from the poles were dragged far into interplanetary space by the solar wind. For a dipole, the field strength over the poles is twice that at the equator. Ulysses found that the amount of outward magnetic flux in the solar wind did not vary greatly with latitude, indicating the importance of pressure forces near the sun for evenly distributing magnetic flux. (MAG experiment, A. Balogh, Imperial College; E. Smith, Jet Propulsion Laboratory).