Why the Sun Has Solar Flares

Earth rotates as a whole: the equator and poles complete a turn in the same day. The Sun is hot gas, with no rigid framework, so different latitudes move at different speeds. A rotation at the equator takes about 9 days less than in the polar regions.

The magnetic field is frozen into the plasma: it moves with it and cannot slip through. Over several rotations, the difference in speed winds up the field lines like an elastic band around your finger. The lines rise outward in loops, and where a loop pierces the surface, the field prevents hot gas from rising from below - making that spot look darker. That is how sunspots form.

Energy builds up in these loops until there are too many of them, too close together. Then opposing lines touch and reconnect - they break and join again in a simpler configuration. Tension built up over weeks is released in minutes. That is a flare. This is why large flares almost always happen above groups of sunspots: that is where the field is most tangled.

If you have read that a flare "reached Earth in 3 days," it was probably not the flare itself. News reports usually merge two events into one, even though they differ greatly, including in travel time.

This leads to an unpleasant fact: it is impossible to warn about a flare in advance - its light and news of it arrive at the same time. But instruments can see a plasma cloud after an ejection, and we can say that it is on its way. That is what space weather services rely on.

Flares are unevenly distributed over time. The field's complexity builds up and releases in cycles of about 11 years: in quiet years there are almost no sunspots, while near the maximum, small flares happen several times a day. No one knows in advance how high a particular maximum will be: cycle forecasts still differ.