Solar Flares and Geomagnetic Storms Explained: R, S and G Scales

A solar flare is a sudden flash of light and X-rays from the Sun; a geomagnetic storm is a disturbance of Earth's magnetic field, usually caused by a coronal mass ejection (CME) arriving one to several days later. Flares can black out high-frequency radio on the daylit side of Earth within minutes. Storms can degrade GPS, stress power grids, drag down low satellites and bring the northern lights far south. Neither can hurt you on the ground: NASA says Earth's atmosphere and magnetic field shield us from flare radiation.

See today's space weather

What is a solar flare?

NOAA describes solar flares as large eruptions of electromagnetic radiation from the Sun lasting from minutes to hours (NOAA SWPC). Because that energy travels at the speed of light, the effect on Earth's sunlit side starts the moment the flare is seen. There's no warning time once a flare begins.

A flare is light, not matter. NASA says its harmful radiation "does not physically affect us on the ground" (NASA). What it affects is the ionosphere, the charged upper layer of the atmosphere that radio signals pass through or bounce off.

Flare classes: A, B, C, M and X

Flares are classed by peak X-ray brightness, measured by NOAA's GOES satellites. The scale starts at A (from 10⁻⁸ watts per square meter), and each letter after it is ten times stronger: B, C, M and X (10⁻⁴ W/m² and above). The number is a multiplier (an X2 is twice an X1), and X has no upper limit: the strongest flare ever measured, in 2003, was recorded as an X28 before the sensors were overwhelmed (NASA).

A, B and C flares have no noticeable effect at Earth. Radio blackouts start at M1.

Was there a solar flare today?

When we checked our space weather page just before 01:00 UTC on November 6, 2026 (the evening of November 5 in North America), NOAA's GOES data showed 40 flares in the previous 7 days. Only one was M-class or stronger: an M3.5 that peaked at 18:33 UTC on November 5 and caused an R1 minor radio blackout. The X-ray level had dropped back to the B class, Kp was 2.3 (quiet), and the solar wind was slow at about 318 km/s.

For each of November 6 to 8, NOAA's 3-day forecast gave a 35% chance of R1 to R2 blackouts, 10% for R3 or stronger, 5% for a radiation storm, and a G1 minor storm.

What is a CME, and how does it cause a geomagnetic storm?

A coronal mass ejection is, in NASA's words, "an enormous cloud of electrically charged gas, called plasma, that erupts from the Sun". Flares and CMEs often come together, but the flare is the flash and the CME is the cloud.

NOAA says CMEs leave the Sun at anything from under 250 km/s to nearly 3,000 km/s. The fastest Earth-directed ones arrive in as little as 15 to 18 hours; slower ones take several days (NOAA SWPC). That delay is why storms can be forecast and flare effects can't.

Storm strength depends heavily on the CME's magnetic field. When it points south (a negative Bz) for a long time, it couples with Earth's field and pours energy in; a fast CME with a northward field can pass with little effect.

How is a geomagnetic storm measured? Kp and the G scale

The Kp index is a 0 to 9 measure of how disturbed Earth's magnetic field is in each three-hour period. NOAA's G scale maps onto it: G1 is Kp 5, G2 is Kp 6, G3 is Kp 7, G4 is Kp 8 (including 9-), and G5 is Kp 9. Our guide to what Kp index you need to see the northern lights covers the decimal values apps show.

What do the NOAA R, S and G levels mean?

The measure, main effects and average frequency below come from NOAA's scales explanation. Frequency is per 11-year solar cycle.

pfu = particle flux units, the flux of protons above 10 MeV measured by GOES.

G1 storms and R1 blackouts happen hundreds of times a cycle; R5 and S5 less than once. Solar radiation storms (S) are surges of high-energy protons that matter mostly for astronauts, satellite electronics and, from S2, people on high-flying aircraft at high latitudes.

What do solar storms affect on Earth?

HF radio, and why only the sunlit side

During a flare, extra X-rays thicken the lowest layer of the ionosphere, the D layer, and NOAA explains that HF signals passing through it lose energy or are completely absorbed. This happens only on the side of Earth facing the Sun.

Our London, Ontario page showed why timing matters. The November 5 M3.5 flare peaked at 1:33 pm local time, in daylight, so the page said HF radio there may have faded briefly. By 7:59 pm the Sun was 31° below the horizon and the city was out of reach of any new flare blackout.

Do solar flares affect GPS?

Sometimes, but geomagnetic storms are the bigger issue. NOAA notes that solar radio bursts can interfere with GPS signals (NOAA). The ionosphere bends GPS signals like a lens, and in severe storms a phone that's normally accurate to a meter or better can be off by "tens of meters or more", and centimeter-level systems can lose lock (NOAA SWPC). The worst disturbances are usually at high latitudes.

Deliberate interference is a different problem; see GPS jamming and spoofing explained and the GPS jamming map.

Power grids

A storm induces currents in long conductors such as power lines and pipelines. NOAA says extreme storms can damage high-voltage transformers, with repairs taking days, weeks or longer, and ESA says the worst cases can cause blackouts (ESA). High-latitude grids are most exposed.

Satellites and polar flights

Storms heat and expand the upper atmosphere, adding drag on low-orbit satellites, one reason orbits on the live satellite tracker need fresh data (how satellite tracking works). During significant radiation storms, NOAA says airlines may suspend transpolar flights.

Aurora

The upside is the aurora. NOAA's G scale says it can reach Illinois and Oregon at G3, and Florida at G5. Check the aurora forecast or a city such as Oslo when a storm is due. Our London, Ontario page said the city needs about Kp 6, so the forecast Kp 5 peak would leave the aurora's visible edge about 131 km north of it.

What were the biggest solar storms?

• September 1859, the Carrington Event. The storm after a flare Richard Carrington saw from Redhill, England is the strongest on record; NOAA says telegraph sparks set paper alight and the aurora reached the Caribbean (NOAA Research).
• March 13, 1989, Quebec. At 2:44 a.m., Hydro-Québec's grid protection tripped; the province went dark in under a minute and stayed dark for more than nine hours (Hydro-Québec). USGS rates it more severe than May 2024.
• February 2022, Starlink. A storm the day after a launch into a low orbit (perigee about 210 km) raised drag up to 50 percent above previous launches, SpaceX said, and up to 40 of 49 new satellites re-entered (Spaceflight Now).
• May 10, 2024, G5. The first G5 since October 31, 2003 disrupted satellite operations, GPS and radio, with aurora across much of the continental US (USGS).

Where is solar cycle 25 now?

Solar Cycle 25 began in December 2019, NASA says, and on October 15, 2024, NASA and NOAA announced the solar maximum period (NASA). Its biggest flare so far was an X9.0 on October 3, 2024. In 2025, the Royal Observatory of Belgium's SILSO center confirmed the smoothed sunspot number peaked at 161 in October 2024 and the cycle is now declining (STCE). Declining doesn't mean quiet: strong flares still happen in the years after a maximum.

How do you check solar flares and storms for your city?

The free GodsView space weather page shows NOAA's R, S and G levels, the past week's flares, solar wind, Bz and the 3-day forecast, all from NOAA SWPC. City pages, such as London, Ontario or Tokyo, add whether the city is on the sunlit side, how exposed it is to GPS, HF and grid effects by geomagnetic latitude, and its aurora chance tonight. Sources are on the methodology page, and the same views are in the free GodsView AI iPhone app.

Check space weather for your city