What is a CME (coronal mass ejection)?

A coronal mass ejection is a huge cloud of magnetized plasma, typically a billion tons or more of electrically charged gas, that erupts from the Sun's outer atmosphere (the corona) and travels outward through space at anywhere from about 250 to over 3,000 km per second. When one is aimed at Earth, it can trigger a geomagnetic storm and aurora when it arrives one to three days later.

What is the difference between a CME and a solar flare?

A flare is a flash of light and radiation; a CME is an eruption of actual matter. The flare's light reaches Earth in 8 minutes and can black out HF radio on the sunlit side immediately. The CME's plasma cloud takes one to three days to arrive, and that is what drives geomagnetic storms and aurora. The two often happen together in one eruption, but either can occur without the other.

How long does a CME take to reach Earth?

Usually one to three days, depending on launch speed. Slow CMEs (~400 km/s) take three days or more; a fast one (1,500+ km/s) can arrive in under a day. The record is the August 1972 event at about 14.6 hours. Arrival-time forecasts are honestly uncertain, typically off by around half a day even for good models.

Are CMEs dangerous to people on the ground?

Not directly. Earth's magnetic field and atmosphere absorb the impact; standing outside during a geomagnetic storm is completely safe, and the visible result is the aurora. The real risks are technological: strong storms can stress power grids, disrupt HF radio and GPS accuracy, and increase satellite drag. The historic extremes, Carrington 1859 and Québec 1989, are grid and infrastructure stories, not health stories.

Is there a CME heading to Earth right now?

That changes week to week. Check the CME activity today page at cmetracker.ai/today for the current answer, or the live map at cmetracker.ai/live, which plots every CME NASA has cataloged over the last 90 days and estimates arrival times for Earth-directed ones.

What is the CME Scoreboard?

The CME Scoreboard is a NASA CCMC project where research teams publicly log their predicted arrival times for each Earth-directed CME, so forecasting methods can be compared against what actually happened. CME Tracker runs the same kind of self-scoring: our drag-based model is validated against NASA's shock-arrival catalog across hundreds of events, with the honest error bars published on our science page.

What is a coronal mass ejection?

A coronal mass ejection, or CME, is the Sun throwing a piece of itself into space: a cloud of magnetized plasma weighing a billion tons or more, leaving 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: aurora far from the poles, stressed power grids, satellites pushed off course.

A coronal mass ejection leaving the Sun on August 31, 2012: a bright loop of plasma with a dark hollow and a bright core, seen by the SOHO coronagraph, with the Sun from SDO at the center — Real imagesAugust 31, 2012. A long filament of plasma lifts off the Sun and leaves at about 1,450 km/s: four and a half hours in 23 frames. The black disk is part of the camera. It hides the Sun so the faint cloud can be seen. The Sun in the middle is a separate picture, to scale. This one only grazed Earth, and lit aurora three nights later. Images: SOHO LASCO C2 (ESA and NASA) and NASA SDO, via Helioviewer.

1How the Sun makes one

The Sun's outer atmosphere, the corona, is threaded with magnetic field. Above sunspot groups that field is strong and tangled, and it holds arches of plasma down against the surface. The surface never stops moving, so the arches get twisted and stressed. Energy builds up in them like a spring being wound.

Loops of magnetic field arch over a sunspot group. Under them sits a twisted rope of field holding cool, dense plasma. It can sit like this for days.

IllustrationThe standard picture of an eruption, simplified. Not to scale: the loops over a real sunspot group stand many Earths tall.

So a CME is not light and not radiation. It is matter: electrons, protons, and the magnetic field frozen into them. It keeps expanding as it goes, until the cloud is far bigger than the Sun it came from.

The Sun in ultraviolet on August 31, 2012, with a long filament of plasma tearing away from its lower left edge — Real imageThe rope letting go. This is the same eruption as the one at the top of the page, seen in ultraviolet light by NASA's Solar Dynamics Observatory. Image: NASA SDO, via Helioviewer.

1 billion+ tons

Typical mass thrown out

250 to 3,000 km/s

Launch speed (fast ones are rare)

1 to 3 days

Usual travel time to Earth

14.6 hours

The record (August 1972)

The Sun does this constantly: a few CMEs a week when it is quiet, several a day near the peak of its 11-year cycle. Most miss Earth. Space is big and Earth is a small target. You can see where on the Sun they come from and every day's count since 2016.

Try it on the real thing. Module 1 of our free course puts today's CME map on the page: pick an eruption and call whether it is aimed at Earth. Six minutes, no account. Start Module 1 →

2Flare or CME? The flash and the cannonball

The two get mixed up because they often come out of the same eruption. They are different things, and they reach Earth at very different times.

IllustrationOne eruption, three arrivals. The clock speeds up as it goes: minutes, then an hour, then two days.

A flare is a flash. Light and X-rays. It reaches Earth in 8 minutes. There is no warning, and by the time anyone sees it, its main effect has already happened: a shortwave radio blackout on the daylit side of the planet.
Fast protons come next. The eruption and its shock wave fling some particles to a good fraction of light speed. They arrive in tens of minutes to hours and matter to astronauts, polar flights and satellite electronics.
The CME is a cannonball. A physical cloud that takes a day or three to cross 150 million km. It is the CME, not the flare, that drives geomagnetic storms and aurora. Its travel time is why a storm can be forecast at all.

Either can happen without the other. A big flare with no CME is a radio event and nothing more. A quiet "stealth" CME with no flare can still deliver a storm.

3The trip to Earth

A CME does not coast. The space between the planets is filled with the solar wind, a thin stream of plasma already flowing outward at about 400 km/s. A fast CME plows into it and slows down. A slow one gets swept along and speeds up. Pick a launch speed and watch what the wind does to it.

Our modelThis runs the same drag model the live tracker uses, with the solar wind set to 430 km/s. Distances are to scale. The cloud spreads as it travels: by Earth's orbit it is tens of millions of km deep, and Earth takes about a day to pass through it.

That is why an arrival time can be forecast, and also why it is uncertain. The answer depends on the wind the cloud meets on the way, and nobody measures that wind until it gets here. A good forecast is still off by about half a day. We score ours in public on the live scoreboard.

LiveToday's traffic: every CME NASA has cataloged in the last 90 days, where our model puts each one right now. Click the map to open the full tracker.

4What actually arrives

A CME is not an even puff of gas. When a fast one crosses Earth, the monitoring spacecraft parked 1.5 million km upstream sees three things, in order. Drag the slider to push one across.

IllustrationA textbook passage, drawn from the pattern real ones follow. Real ones are messier, and only about a third show a clean magnetic cloud.

The shock. Speed and density jump in seconds. This is the first warning, 15 to 60 minutes before Earth itself feels it.
The sheath. Hours of solar wind the cloud has swept up and squeezed. Dense, turbulent, with a magnetic field that flips around.
The magnetic cloud. The rope itself. The field is strong and turns smoothly over a day or so. If it points south for hours, that is when the big storms happen.

The tracker draws this same picture from real measurements. Open the storm anatomy chart on the live map, or on any famous storm's replay.

5At Earth: why one storm rages and the next one fizzles

Earth sits inside its own magnetic field, which holds the solar wind off about 60,000 km above the daylit side. What a CME does to that shield depends mostly on one thing: which way the cloud's magnetic field points when it gets here.

IllustrationSeen from the side, with the Sun off to the left. Earth is drawn far too big. Forecasters call this direction Bz: negative means south.

Nobody can measure that direction until the cloud reaches the monitor upstream, less than an hour before it reaches us. That is why "when will it arrive" can be forecast days ahead and "how strong will the storm be" cannot.

The storm that follows is rated on NOAA's G-scale:

G1Minor: aurora in the far north

G2Moderate

G3Strong: aurora at mid latitudes

G4Severe

G5Extreme: aurora near the tropics

What that means in practice:

Artwork of Earth's magnetic field glowing green as a solar storm arrives

Aurora

Pushed far from the poles. It is the one effect anyone can walk outside and see.

Aurora tonight →

Artwork of red and green aurora over a line of power pylons

Power grids

Long lines pick up stray currents. Québec went dark in 92 seconds in March 1989.

The Québec blackout →

Satellites

The upper air heats and swells, and drag goes up. A mere G1 brought down 38 Starlinks in 2022.

The Starlink storm →

Artwork of a solar eruption reaching toward Earth

Radio and GPS

A disturbed upper atmosphere bends and absorbs signals. Shortwave fades and GPS loses accuracy.

Radio blackouts now →

And people on the ground? Nothing. The air and the magnetic field 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 and the Halloween storms of 2003, read as engineering stories, not medical ones.

6How we see them coming

You cannot see a CME leave the Sun with your eyes. The Sun's disk is a million times brighter than the cloud. A coronagraph is a telescope with a small disk in front of it that blots the Sun out, like holding up your thumb, so the faint light scattered by the cloud shows up.

Wide coronagraph view of the August 31, 2012 CME as a large pale loop spreading to the left of the Sun — Real imageSeen from the side. The same 2012 eruption four hours in. This frame is 25 Suns wide and the cloud is already bigger than the Sun. Images: SOHO LASCO C2 and C3 (ESA and NASA), NASA SDO.

Coronagraph image from October 28, 2003 showing a halo CME as a bright ring all the way around the Sun, the picture speckled with white dots — Real imageSeen head-on. October 28, 2003. A cloud coming straight at the camera shows on every side of the Sun at once: a *halo CME*. This one reached Earth in 19 hours. Image: SOHO (ESA and NASA).

Coronagraphs on SOHO, GOES-19, SOLAR-1 and STEREO-A watch the Sun around the clock. From their images, NASA's DONKI catalog logs each CME's launch time, speed and direction. Meet the spacecraft.

CME Tracker plots every cataloged CME on a live map of the inner solar system and runs the drag model from section 3 for each one aimed at Earth. That gives an arrival time with an honest error bar of about half a day, which is where the whole field stands. The model is checked against NASA's measured arrivals across hundreds of storms, and the misses are published on the science page.

The CME Scoreboard. NASA's CCMC runs a public CME Scoreboard where forecasting teams log their arrival predictions before each impact, so methods can be compared with what happened. It is the right instinct: predictions should be scored in public. We hold ourselves to it too. Every arrival window this site issues is scored against what arrived on our live scoreboard.

7Quick answers

Is a CME heading to Earth right now? That changes week to week. The CME activity today page has the current answer.

How often do CMEs hit Earth? Glancing blows are routine, a few every month. Storms worth a headline (G3 and up) 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 is 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 you can replay a famous storm from launch to impact.

Open the live tracker → Replay a famous storm → 🎓 Try it: Module 1, 6 min →