Certain spectral lines (e.g. the red line of hydrogen) are particularly useful, because they come from high levels in the Sun's atmosphere. When the Sun is observed in the light of such lines, a sudden brightening is now and then seen near sunspots, at times expanding tens of thousands of kilometers in a matter of minutes. Such abrupt events were named solar flares, and what made them especially interesting was that occasionally, after a prominent flare, a magnetic storm would erupt on Earth. It suggested that the flare released some disturbance which traveled through space and took a day or two to reach Earth. Considering the vastness of interplanetary space, a great amount of energy had to be released in a very short time to produce this effect.

    A century later we know a great deal more about flares, but have yet to trace the process of their energy release. It is generally held that the energy is drawn from the magnetic field, not just because of the close association between flares and sunspots, but also because theoretically only magnetic fields are capable of releasing energy so quickly. Unfortunately, it apparently happens at the higher levels of the Sun's atmosphere, and no good way exists of observing the magnetic field there. Starting in 1942, bursts of ions with energy comparable to that of cosmic rays were observed from some flares, and after x-ray telescopes were placed in orbit (x-rays are stopped by the atmosphere) also bursts of x-rays, sometimes rising within a second or two.

    By that time spacecraft had already found the cause of flare-associated magnetic storms at Earth. It was the arrival of interplanetary shock fronts, marking the front edge of expanding plasma clouds which, it was then believed, arose in flares. Once CMEs were discovered, it seemed obvious that here were probably similar clouds, at the start of their journey outwards from the Sun.

    Some CMEs come with flares, some don't, and many scientists nowadays feel that CMEs are more frequently associated than flares with the rapid release of magnetic energy on the Sun. The real mystery, of course, is the release process itself.