coronal mass ejection
/ CME /
A coronal mass ejection, usually shortened to CME, is a colossal cloud of magnetised plasma that the Sun hurls out into space during a storm. Where a solar flare is a flash of light, a CME is a physical eruption of matter — billions of tonnes of the Sun's gas blasted outward at high speed.
A CME erupts when a large, unstable structure of magnetic field and plasma in the corona loses its balance and is flung off, carrying its own tangled magnetic field with it. A typical CME can carry on the order of a billion tonnes of material and race outward at hundreds to a few thousand kilometres per second. Unlike the light from a flare, which arrives in 8 minutes, the plasma cloud of a CME takes longer — typically one to three days — to cross the distance to Earth, which gives forecasters a precious window of warning. When a CME aimed at Earth arrives, its magnetic field can slam into and shake the Earth's own magnetic field, triggering a geomagnetic storm.
CMEs matter because they are the main driver of severe space weather at Earth. A strong one can supercharge the auroras, induce damaging electric currents in long power lines and pipelines, knock out satellites, and endanger astronauts. Tracking CMEs as they leave the Sun, and predicting whether and when they will hit Earth, is a central task of modern space-weather forecasting.
In 1989 a CME-driven geomagnetic storm overloaded the power grid in Quebec, Canada, and left about six million people without electricity for around nine hours.
Space weather is not just a sky show — it can reach down to the power outlets in our homes.
A CME and a flare are not the same event. They often erupt together from the same active region, but each can occur without the other, and they affect Earth in different ways and on different timescales.