Space Data API prototype
The Sun throws out light, particles and magnetic field. Some of it reaches Earth, and some of what reaches Earth reaches your phone's GPS. Here's the chain, one link at a time.
Draft: science review pending. Our team's scientists haven't checked this text yet.
A solar flare is a burst of light and X-rays from the Sun, lasting minutes to hours. It travels at the speed of light, so it reaches Earth about 8 minutes later, with no warning.
A coronal mass ejection (CME) is different: billions of tons of plasma and magnetic field thrown off the Sun. It's slower, so the fastest take 15 to 18 hours to reach Earth and slower ones several days. Flares and CMEs often come together, but not always.
CMEs travel at speeds from under 250 km/s to nearly 3,000 km/s. One faster than the solar wind ahead of it can drive a shock wave, which is often the first thing instruments see arrive. Both flares and CMEs usually come from active regions, the areas of strong magnetic field around sunspots.
The Sun also blows a steady stream of particles, the solar wind. Near Earth's orbit it usually moves at about 400 km/s. Spacecraft at L1, 1.5 million km towards the Sun, measure it before it reaches us, which gives us a warning that's often under an hour.
The number to watch is Bz, the north–south direction of the Sun's magnetic field arriving at Earth. When it points south, energy gets into Earth's magnetic field much more easily.
Coronal holes on the Sun produce faster wind, 500 to 800 km/s. The wind carries the Sun's magnetic field with it, and since the Sun turns about every 27 days, fast and slow streams pass Earth in turn.
When the solar wind shakes Earth's magnetic field, instruments on the ground feel it. Kp sums that up as a number from 0 to 9, for each 3 hours. At 5 or more it's a storm, and NOAA's G-scale names the level, from G1 (minor) to G5 (extreme).
Kp is a global index, so it doesn't say where the disturbance is worst. It's also published after the fact, in 3-hour steps. The storms in our library are the extreme end of the scale, at Kp 8.67 or more.
See it: Kp with NOAA's forecast
High above us, sunlight strips electrons off the air and makes a layer of charged gas, the ionosphere. TEC (total electron content) says how many electrons a radio signal passes through on its way down. It's highest by day, and storms change it.
TEC is counted in TECU, and vertical values range from a few to several hundred TECU. It depends on the time of day, place, season, solar cycle and storms. One TECU delays the GPS L1 signal by about 16 cm straight up, and more at low angles.
See it: the TEC map on the globe
Signals slow down in the ionosphere, so a position can be off. Phones and many drones and cars use one frequency and model only part of this; dual-frequency receivers remove most of it. In storms, signals can also flicker (scintillation) and receivers can lose satellites.
A strong flare thickens the lowest layer of the ionosphere on the sunlit side, which absorbs HF radio (roughly 3 to 30 MHz), used by aircraft and radio amateurs among others. That's a radio blackout, and it starts as the flare does.
Storms heat the upper atmosphere, so denser air rises to where low satellites fly. That means more drag, and it pulls them lower. We show a rough draft of this: real height changes, and a level from height, Kp and F10.7, not a measured density.
A shaking magnetic field drives currents through the ground and into long power lines, which can stress transformers. In March 1989 a storm collapsed Hydro-Québec's grid.
Ground electric field (US and Canada, a model, not a measurement) · March 1989
Electrons guided by Earth's magnetic field hit the upper atmosphere, 80 to 500 km up, and it glows. The bigger the storm, the closer to the equator it can be seen. You need darkness and a clear sky.
In May 2024 the strongest geomagnetic storm since 2003 reached Earth. Walk through it step by step, from the first flare to the peak, with the real measurements and the globe.
All from NOAA's Space Weather Prediction Center. Numbers on the linked pages of this site come from our own API.