Solar storms

The Sun's activity rises and falls on a cycle of roughly 11 years, and with it the number of sunspots, solar flares and coronal mass ejections.^[1] The largest can disturb radio, satellites and power grids, and they produce the aurora far from the poles. NOAA's Space Weather Prediction Center rates their effects on three scales: R for radio blackouts, S for radiation storms and G for geomagnetic storms.^[2]

Sunspots and the solar cycle

Sunspots are dark areas where intense magnetic fields push up through the Sun's visible surface; they are cooler than their surroundings, about 7,000 °F against 10,000 °F.^[1] Rapid changes in the magnetic fields of sunspot groups are the most likely source of flares and coronal mass ejections.^[1] The peak of sunspot activity is called solar maximum and the low point solar minimum. Solar cycle 25 began in December 2019.^[1]

Solar flares

A solar flare is a sudden burst of radiation from the Sun lasting minutes to hours.^[3] Because the radiation travels at the speed of light, its effects on Earth begin at the moment the flare is seen, and only on the sunlit side of the planet.^[3] X-rays from a strong flare ionize the lower layers of the ionosphere, where they can weaken or completely absorb high-frequency radio signals.^[3]

Flares are classed by their peak X-ray brightness in five letters, A, B, C, M and X, each ten times stronger than the one before.^[3] NOAA's radio blackout scale starts at R1 for an M1 flare, which happens about 2,000 times in an 11-year cycle, and rises to R5 for an X20 flare, which happens less than once a cycle.^[2]

Coronal mass ejections

A coronal mass ejection, or CME, is a vast cloud of plasma and magnetic field thrown out from the Sun's outer atmosphere; it can carry billions of tons of material.^[4] CMEs leave the Sun at speeds from below 250 km per second to nearly 3,000 km per second.^[4] The fastest that head toward Earth arrive in as little as 15 to 18 hours, and slower ones take several days.^[4] A spacecraft stationed between Earth and the Sun, the Deep Space Climate Observatory, detects an arriving CME first and usually gives 15 to 60 minutes' warning before it reaches Earth.^[4]

When a CME's magnetic field points southward, opposite to Earth's, energy passes efficiently into Earth's magnetic environment and drives a geomagnetic storm.^[5] The largest geomagnetic storms come from CMEs.^[5] Storms can cause errors in GPS positioning, extra drag on low satellites and unwanted electric currents in power grids and pipelines, and they also produce the aurora.^[5] How far the aurora reaches in each storm level is covered in Aurora: how far south and when.

Radiation storms

Large eruptions can also accelerate protons to a large fraction of the speed of light, crossing the 150 million km to Earth in tens of minutes or less.^[6] These solar radiation storms, rated S1 to S5, mainly affect satellites, astronauts and high-latitude radio; in the most extreme storms, passengers and crew on high-flying aircraft at high latitudes may be exposed to raised radiation.^[2]

An example: May 2024

In the first full week of May 2024, a series of large flares and CMEs produced the strongest solar storm to reach Earth in two decades.^[7] From 7 to 11 May, at least seven CMEs headed toward Earth and eight X-class flares erupted, the strongest rated X5.8.^[7] The CMEs reached Earth from 10 May and drove a G5 storm, the highest level, for the first time since 2003; on 14 May the same region of the Sun produced an X8.7 flare, the most powerful of solar cycle 25 up to then.^[7]