Module 2 of 6 · 7 min

Why the Sun erupts

Module 1 showed what a CME is. This is the machine behind it: where the energy to launch a billion tons comes from, and what finally lets it go. Start with a real one.

ReplayMay 8, 2024. An X1.0 flare from sunspot group AR 3664, and the CME it launched.

The flare bursts at the Sun at stop 1. Press play. The older wedges are dimmed: the new one arrives bright, right at the Sun.

Your call

Play from the flash and pause the moment the bright new wedge appears at the Sun. How long after the flash did the cloud show up? Press I see it and the page reads the clock for you, or type the minutes yourself.

The idea: a spring, wound for years

The Sun isn't solid, so its equator spins faster than its poles. The magnetic field is frozen into that plasma, so a tidy pole-to-pole field gets dragged, stretched and wound like a spring, year after year. Where the wound-up field bursts through the surface, its footpoints are sunspots, and above them it arches into loops that hold plasma down.

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.

IllustrationOne process, two signatures. The flash is the flare, at Earth in eight minutes. The escaping cloud is the CME, at Earth in one to three days. Not to scale.

Flares and CMEs run on this stored magnetic energy, not on heat. That is why they come from sunspot groups, and why a tangled group is watched more closely than a tidy one.

A real one, leaving

This is not a drawing. On August 31, 2012 a long rope of plasma lifted off the Sun and left at about 1,450 km/s. Play it, or drag the slider.

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 imagesFour 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. Images: SOHO LASCO C2 (ESA and NASA) and NASA SDO, via Helioviewer.

The Sun in ultraviolet on August 31, 2012, with a long filament of plasma tearing away from its lower left edge — Real imageThe same eruption a few minutes into the escape, in ultraviolet light. Image: NASA SDO, via Helioviewer.

The pairing of flash and cloud isn't automatic. A confined flare flashes under a strong overlying field that holds the cloud in, so no storm follows. A stealth CME slips away with almost no flash, which is a forecaster's least favorite kind. That is why "X-class flare!" in a headline tells you almost nothing about storms by itself.

Optional · a deep dive

Deep dive: The Sun keeps time

The winding and snapping repeats on an eleven-year rhythm. See it in ten years of real CME counts. Seven minutes.

Open →

Check yourself · Module 2

1. Why do sunspots look dark?

A sunspot's field chokes off the heat delivery from below, leaving it roughly 1,500 to 2,000 K cooler. It still glows; it's only dark next to the hotter surface around it.

2. Where does the energy of a flare and CME come from?

Years of differential rotation wind and stress the field like a spring. Reconnection converts that stored energy into the flash and the launched cloud in minutes.

3. Can a CME launch without a big flare?

Flare and CME are two outputs of one process, but the split varies. That's why flare class alone doesn't predict storms.

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