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Answer: A coronal mass ejection (CME) is a huge cloud of plasma and magnetic field thrown off the Sun's corona. Fast Earth-directed CMEs can arrive in 15 to 18 hours (most take days) and cause the largest geomagnetic storms, which bring aurora but can disrupt GPS, satellites and power grids.

What a CME is

CMEs can carry billions of tons of material at speeds from under 250 km/s to nearly 3000 km/s. Explosive ones begin when twisted magnetic structures in the lower corona snap into a calmer shape (magnetic reconnection), often with a flare.

Watching them

Forecasters estimate size, speed and direction from coronagraph images, mainly SOHO's LASCO. The DSCOVR satellite at L1 then gives 15 to 60 minutes' warning of arrival.

Geomagnetic storms

A storm occurs when energy transfers efficiently from the solar wind into Earth's magnetosphere, especially when the CME's magnetic field points south. Effects include:

• extra drag on low-orbit satellites,
• GPS and navigation errors,
• induced currents in power grids and pipelines,
• auroras.

Claims

• id: C1 claim: CMEs are large expulsions of plasma and magnetic field from the corona. source: S1 evidence: "Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun’s"
• id: C2 claim: The fastest Earth-directed CMEs can arrive in 15-18 hours. source: S1 evidence: "The fastest Earth-directed CMEs can reach our planet in as little as 15-18 hours."
• id: C3 claim: DSCOVR can give 15 to 60 minutes advance warning. source: S1 evidence: "This can often provide 15 to 60 minutes advanced warning of shock arrival at Earth"
• id: C4 claim: The largest geomagnetic storms are associated with CMEs. source: S2 evidence: "The largest storms that result from these conditions are associated with solar coronal mass ejections"
• id: C5 claim: Storms can disrupt navigation systems such as GNSS. source: S2 evidence: "they also can disrupt navigation systems such as the Global Navigation Satellite System (GNSS)"

Sources

• [S1] Coronal Mass Ejections. NOAA. https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Geomagnetic Storms. NOAA. https://www.swpc.noaa.gov/phenomena/geomagnetic-storms . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_zqyv2hce,
  author = {{iinventedlight}},
  title = {{Coronal mass ejections and geomagnetic storms}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/coronal-mass-ejections/},
  note = {Permanent ID iil:note:zqyv2hce. License: CC-BY-4.0.},
}

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