What is a coronal mass ejection?
A coronal mass ejection — a CME — is the Sun throwing a piece of itself at space: a cloud of magnetized plasma, typically a billion tons or more, blasted out of the Sun's outer atmosphere at anywhere from 250 to over 3,000 kilometers per second. When one is aimed at Earth, it arrives one to three days later and can set off a geomagnetic storm — the thing that pushes aurora toward the equator, rattles power grids, and gives this site its reason to exist.
1The eruption itself
The Sun's outer atmosphere is threaded with magnetic field lines that store enormous energy. When a tangled region of that field snaps into a new configuration — usually above a sunspot group — it can fling the overlying plasma outward all at once. That's a CME: not light, not radiation, but matter — electrons, protons, and the magnetic field frozen into them, expanding as it goes until the cloud is far larger than the Sun itself.
The Sun produces CMEs constantly — a few per week at solar minimum, several per day near solar maximum. Most miss Earth entirely; space is big and Earth is a small target. The ones that matter here are the Earth-directed ones, and especially the fast ones.
2CME vs. solar flare — the flash and the cannonball
The two get mixed up constantly because they often happen together, in the same eruption. The distinction is simple:
- A flare is a flash. Light and X-rays. It reaches Earth in 8 minutes — there is no outrunning it, and by the time you see it, its main effect (a shortwave radio blackout on the sunlit side) has already happened.
- A CME is a cannonball. A physical cloud of plasma that takes a day or three to cross the 93 million miles. It's the CME, not the flare, that drives geomagnetic storms and aurora — and its travel time is why storm arrival can actually be forecast.
Either can happen without the other. A big flare with no CME is a radio event and nothing more; a "stealth CME" with no flare can still deliver a storm.
3What happens when one hits Earth
Earth sits inside a magnetic bubble — the magnetosphere. When a CME arrives, the first sign is a sudden jump in solar wind speed and density at the L1 monitoring spacecraft, about 1.5 million km upstream. Then the crucial variable: the CME's magnetic field orientation. If it points southward (negative Bz, in the jargon), it links up with Earth's field and pours energy in; if northward, even a fast CME can pass with barely a ripple. That single variable is why arrival forecasts are easier than storm-strength forecasts.
The storm that follows is measured on NOAA's G-scale, from G1 (minor — aurora at high latitudes) to G5 (extreme — aurora near the tropics, grid and satellite stress). What that looks like in practice:
- Aurora, pushed to lower latitudes than usual — the one effect anyone can walk outside and see (our aurora tonight page turns live data into a visibility answer).
- Power grids feel induced currents; extreme storms have tripped grids — Québec went dark in 92 seconds in March 1989.
- Satellites ride through a swollen, denser upper atmosphere — drag that dropped 38 Starlink satellites in 2022 during a mere G1.
- GPS and HF radio degrade while the ionosphere is disturbed.
And people on the ground? Nothing. The atmosphere and magnetosphere absorb it all — a geomagnetic storm is completely safe to stand outside in. The danger is to infrastructure, which is why the historic extremes — Carrington 1859, the Halloween storms of 2003 — read as engineering stories, not medical ones.
4How CMEs are detected — and how this site tracks them
You can't see a CME leave the Sun with your eyes; the Sun's disk is a million times brighter than the eruption. Coronagraphs — telescopes that blot out the disk with an occulting mask — do it from space: SOHO/LASCO at L1 and the STEREO-A spacecraft off to the side. From those images, NASA's DONKI catalog logs each CME's launch time, speed, and direction. A CME aimed straight at Earth appears as an expanding halo around the whole Sun — the ominous "halo CME."
CME Tracker plots every cataloged CME on a live map of the inner solar system and runs a drag-based physics model for each Earth-directed one: fast CMEs plow into the slower background solar wind and decelerate, slow ones get dragged along faster. That yields an arrival-time estimate — honestly uncertain by around half a day, which is the state of the field, not a shortcut. The model is validated against NASA's measured shock arrivals across hundreds of events; the error bars are published, not hidden, on the science page.
5Quick answers
Is a CME heading to Earth right now? That changes week to week — the CME activity today page has the current answer, server-fresh.
How often do CMEs hit Earth? Glancing blows are routine — a few every month. Storms worth a headline (G3+) happen a handful of times a year near solar maximum. G5 extremes are once-a-decade events; the last was the Gannon storm of May 2024.
Could a Carrington-class CME happen again? Yes — one crossed Earth's orbit in July 2012 and simply missed us. It's a when, not an if, which is why grid operators take space weather seriously.
How do I see one? You see its effects: the aurora. Subscribe on the tracker for storm alerts, or check aurora tonight when a storm is inbound.
Watch one cross space
The best way to understand a CME is to watch one travel. The live map plots every CME of the last 90 days moving through the inner solar system — or replay a famous storm from launch to impact.