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Coronal mass ejections (CMEs), explained

What CMEs are, how fast they travel, how forecasters spot them and time their arrival, and how accurate arrival forecasts really are. Sourced to NOAA SWPC and peer-reviewed studies.

What a CME is

A coronal mass ejection is a huge cloud of plasma and magnetic field thrown out of the Sun's corona. It can carry billions of tons of material and a magnetic field stronger than the normal solar wind. [1] Webb & Howard (2012) review several decades of coronagraph observations of CMEs. [2]

Many fast CMEs start when twisted magnetic structures (flux ropes) in active regions become unstable and reconfigure through magnetic reconnection, often together with a solar flare. Some begin when a filament or prominence erupts. [1]

Speed and travel time

NOAA gives CME speeds from below 250 km/s to nearly 3,000 km/s. The fastest Earth-directed CMEs can arrive in about 15 to 18 hours, and slower ones take several days. [1]

CMEs that move faster than the surrounding solar wind drive a shock ahead of them. The shock can accelerate particles and add to radiation-storm risk. [1]

How forecasters see and time them

Forecasters estimate a CME's size, speed and direction mainly from coronagraphs, the LASCO instrument on SOHO and the coronagraph on STEREO-A. They then run models such as WSA-Enlil to predict arrival. [1] [3]

The final warning comes from the DSCOVR spacecraft at L1, which usually sees the shock 15 to 60 minutes before it reaches Earth. How severe the storm becomes depends mostly on whether the CME's magnetic field points southward, and for how long. [1]

How accurate are arrival forecasts?

Riley et al. (2018) analysed forecasts from 32 models submitted to NASA's CCMC CME Scoreboard between 2013 and mid-2018. On average the models predicted shock arrival to within about ±10 hours, but with standard deviations often above 20 hours. The best performers had a mean absolute error of about 13 hours, and accuracy did not improve over the period. [4]

So an arrival time like 'Thursday afternoon' is best read as a window of about half a day either side. Recent CMEs are listed on our live dashboard from NASA's DONKI database. [5]

Sources

  1. NOAA SWPC: Coronal Mass Ejections https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections
  2. Webb, D. F. & Howard, T. A. (2012). Coronal Mass Ejections: Observations. Living Reviews in Solar Physics. doi:10.12942/lrsp-2012-3
  3. NOAA SWPC: WSA-Enlil Solar Wind Prediction https://www.swpc.noaa.gov/products/wsa-enlil-solar-wind-prediction
  4. Riley, P. et al. (2018). Forecasting the Arrival Time of Coronal Mass Ejections: Analysis of the CCMC CME Scoreboard. Space Weather. doi:10.1029/2018SW001962
  5. NASA CCMC: DONKI (Space Weather Database Of Notifications, Knowledge, Information) https://ccmc.gsfc.nasa.gov/tools/DONKI/

Sources were checked on 2026-10-21. Text on this page is original. Quoted figures are attributed and linked to the source. General education only, not for safety or operational decisions. Disclaimer.