A solar flare is a flash of light. A coronal mass ejection is something with actual mass behind it — up to 10 billion tons of magnetized solar plasma, physically hurled off the Sun at speeds that can exceed a thousand kilometers per second. If a flare is the Sun raising its voice, a CME is the Sun throwing something. It's also the single biggest driver of the geomagnetic storms covered elsewhere in this wiki.

A CME is a large-scale eruption of plasma and magnetic field from the Sun's corona, its outer atmosphere. It happens when the Sun's magnetic field, twisted into an unstable configuration above an active region, snaps and reconfigures — not just releasing radiation, as in a flare, but physically launching the tangled magnetic structure and the plasma trapped within it out into space. CMEs and large flares frequently occur together, launched by the same magnetic instability, but a flare can happen without a CME, and a CME can occur without a notable flare.

Unlike flares, which have a clean letter-based scale (A through X), CMEs don't have an official, universally adopted classification system — though speed is the dividing line researchers use informally. CMEs slower than 500–800 km/s are typically called "slow"; faster ones are "fast." A proposed formal system, the SCORE scale, has been suggested to give CME speed the kind of clear public shorthand the flare scale already has, though it hasn't become the standard the way flare classes have.

What matters more in practice is a CME's shape as seen from Earth: a halo CME appears to expand in a full ring around the Sun in coronagraph images, which happens when the eruption is heading either directly toward or directly away from Earth. Halo CMEs are, on average, more than twice as fast as non-halo CMEs and tend to come from the most energetic eruptions — which is exactly why they get the most attention from forecasters. A halo CME heading toward Earth is the clearest early warning sign of an incoming geomagnetic storm.

CME speeds span a wide range, from around 250 km/s for the gentlest eruptions to over 3,000 km/s for the most extreme recorded events. At typical fast-CME speeds of 500–1,500 km/s, the 150 million km trip from Sun to Earth takes roughly 1 to 3 days — the advance warning window that makes geomagnetic storms considerably more forecastable than the flares that often accompany them.

CMEs are observed using coronagraphs — instruments that block out the Sun's overwhelmingly bright disk to reveal the much fainter corona around it, the same way an eclipse briefly reveals it naturally. NASA and NOAA maintain the DONKI catalog (Database Of Notifications, Knowledge, Information), which logs each detected CME along with modeled arrival estimates at Earth, generated using models like ENLIL that simulate how the ejection will expand and travel through interplanetary space.

A CME arriving at Earth first encounters the bow shock, then compresses the magnetopause — the outer boundary of Earth's magnetosphere. What happens next depends heavily on the CME's own internal magnetic field orientation: if it points opposite to Earth's field (southward Bz), the two connect through magnetic reconnection, and the CME's energy pours into the magnetosphere, triggering a geomagnetic storm. If the orientation points the same direction as Earth's field, much of that same CME can arrive and produce comparatively little disturbance — which is why CME speed and mass alone don't fully predict storm strength; the magnetic orientation on arrival matters just as much.

Once a CME's energy connects with Earth's magnetosphere, the resulting geomagnetic storm can induce currents in power grids, degrade GPS accuracy, disrupt high-frequency radio, and push aurora to lower latitudes — the same established effects covered in this wiki's geomagnetic storms entry, since a CME is typically the trigger behind them.

Because CME arrival is what drives most strong geomagnetic storms, the same possible health correlations discussed under geomagnetic storms and meteoropathy — disrupted sleep, headaches, fatigue — tend to cluster around CME arrival windows specifically. The 1-to-3-day travel time means these effects, if they occur, show up a day or more after the originating flare or eruption was actually observed on the Sun, not on the day of the eruption itself.

Solar Cycle 25's extended maximum has kept CME activity frequent through 2026, often with several tracked simultaneously in DONKI at once — clusters of CMEs launched days apart converging on Earth in overlapping arrival windows, which complicates forecasting since a later, faster CME can catch up to and merge with an earlier, slower one before either arrives. Multiple Earth-directed CMEs arriving within the same 48-hour window have become a fairly regular occurrence during this stretch of the cycle, each capable of contributing its own glancing or direct hit to the resulting geomagnetic activity.