The eruptions create huge clouds of energetic particles that can trigger magnetic storms, disrupting power grids, and satellite communications.
Much of the time, these outbursts are directed away from the Earth, but some inevitably come our way. When they do, the particles, and the magnetic fields they carry, can have highly undesirable effects. When a big storm hits and the conditions are just right, power grids and spacecraft are affected.
The particles in a CME are hazardous to astronauts; and even airline companies that fly polar routes are concerned about this because CMEs can black-out aircraft communications and irradiate crew members or passengers.
Dr Chris Davis from the UK's Rutherford Appleton Laboratory underlined the power of CMEs.
"The energy in a CME is typically about 10-to-the-power-of-24 joules. That is the same as a bus hitting a wall at 25mph a billion, billion times. It's 100 times the energy stored in the world's nuclear arsenal," he said.