Solar Flares & CMEs: How the Sun Triggers a Geomagnetic Storm

Two eruptions, one cause

A solar flare and a coronal mass ejection (CME) are often triggered by the same event — a sudden rearrangement of the Sun's magnetic field above an active sunspot region — but they release energy in different forms and travel to Earth on completely different timescales.

Magnetic reconnection: the energy release mechanism

Sunspot regions host tightly twisted, stressed magnetic field lines, stretched and sheared by the differential rotation of the Sun's plasma beneath them. When oppositely directed field lines are pushed close enough together, they can suddenly snap and reconnect into a lower-energy configuration — magnetic reconnection — converting the magnetic energy that had been stored in the stressed field directly into particle acceleration, intense heating and radiation across the whole electromagnetic spectrum, from radio waves to hard X-rays and gamma rays, within minutes.

Classifying flares: the GOES X-ray scale

Flares are classified by the peak flux they produce in the 1-8 Angstrom X-ray band, measured by GOES satellites, on a logarithmic letter scale:

class   peak flux (W/m^2)         relative size
A       10^-8 to 10^-7            background level
B       10^-7 to 10^-6
C       10^-6 to 10^-5            minor, little Earth effect
M       10^-5 to 10^-4            can cause radio blackouts
X       10^-4 and above           major, strongest on record ~X28 (2003)

Each letter is a factor of 10 in peak flux, and the number after the letter is a linear multiplier within that decade, so an X2 flare is twice as powerful as an X1, and ten times an M2. This scale measures the flare's X-ray output specifically — it does not by itself say whether the event launched a CME, or which direction that CME is headed.

The coronal mass ejection: a billion tonnes of plasma

A CME is a distinct, much slower-acting eruption: a huge bubble of magnetised plasma, often a billion tonnes or more, thrown outward from the corona at speeds ranging from a few hundred to around 3,000 kilometres per second. Not every flare produces a CME and not every CME is accompanied by a strong flare, but the largest, most geoeffective events tend to involve both together, launched from the same reconnection event.

Because the CME must physically cross the roughly 150-million-kilometre gap to Earth, it takes far longer to arrive than the flare's light and X-rays, which reach Earth in about 8 minutes: a fast CME can arrive in under a day, while a typical one takes one to three days. That delay is what gives space-weather forecasters real, if imperfect, lead time to issue warnings once a CME's speed and direction are measured by coronagraphs.

What happens when it hits: geomagnetic storms and aurorae

If an Earth-directed CME's magnetic field is oriented southward, opposite to Earth's own northward-pointing field at the point of contact, it couples efficiently into the magnetosphere via reconnection at the dayside boundary, driving a geomagnetic storm. Energy pumped into the magnetosphere accelerates charged particles that precipitate down field lines into the upper atmosphere near the poles, colliding with oxygen and nitrogen atoms and exciting them to emit light — the aurora, normally confined to high latitudes but pushed toward the equator during the strongest storms. Storm intensity is tracked with the planetary Kp index (0-9); large storms can also induce currents in long conductors — power grids, pipelines, undersea cables — which is the practical reason space weather is monitored as closely as ordinary weather.

Frequently asked questions

Try it live

Everything above runs in your browser — open Solar Flare & CME and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.