A solar flare is a sudden, intense burst of radiation from the Sun's surface, caused when magnetic field lines in a sunspot region twist, snap and reconnect, releasing energy equivalent to billions of hydrogen bombs in minutes. The light from a flare reaches Earth in about eight minutes; the charged particles it throws out take hours to days.

Flares are classified by their X-ray brightness into A, B, C, M and X classes, each ten times stronger than the last. C-class flares are common and harmless. M-class flares can cause brief radio blackouts on the sunlit side of Earth. X-class flares are the strongest, and the biggest ones can disrupt high-frequency radio, degrade GPS accuracy and, in extreme cases, stress power grids.

A flare is often accompanied by a coronal mass ejection, a billion-tonne cloud of charged particles hurled into space. When that cloud hits Earth's magnetic field, it can trigger a geomagnetic storm. The storm squeezes and shakes the magnetosphere, induces currents in long power lines and pipelines, and pushes the aurora far beyond its usual polar home. That is why the most spectacular northern lights often follow a big flare by one to three days.

The Sun runs on an roughly 11-year activity cycle, and we are currently near the peak of Solar Cycle 25, which is why auroras have been unusually visible at low latitudes in 2025 and 2026. Sunspot numbers, flare counts and geomagnetic storms all rise near solar maximum, and forecasters at NOAA's Space Weather Prediction Center track active regions daily and issue alerts when a major flare is likely.

For most people a solar flare is nothing to fear, but it is worth knowing what to watch. If you rely on HF radio, aviation communications or precision GPS, check the space weather forecast before a critical operation. If you want to see an aurora, watch the KP index: a KP of 5 means a geomagnetic storm is under way and the lights may be visible far from the poles, while a KP of 7 or higher can push them over the northern United States, much of Europe and even farther south. The Sun is a star, and on a flare day it reminds us that our technology lives inside its weather.