Look at the Scales panel on the Aurora Watch dashboard and you'll see three letters — G, S, R — each with a number from 0 to 5. When all three show 0, it means nothing unusual is happening in any of these three categories right now. When one goes active, it tells you something specific is occurring — and depending on which letter it is, the implications are completely different.

The short version

G = Geomagnetic. This is the aurora and power grid scale. It's what aurora chasers watch.
S = Solar Radiation. This is the invisible particle hazard scale. Astronauts and polar flight crews watch this.
R = Radio Blackout. This is the HF communications scale. Ham radio operators and aviation watch this.

The three happen for different reasons and can occur independently of each other.

Why Three Separate Scales?

Space weather isn't a single type of event — it's a family of related phenomena that can accompany each other but don't have to. A solar flare might produce an R-scale radio blackout without any associated CME, meaning no G-scale storm follows. A fast CME with the wrong Bz orientation might drive G0 despite raising every other alarm. An S-scale radiation storm might occur from a CME that completely misses Earth's magnetosphere (G0) but whose shock still accelerated protons in Earth's direction.

Reading all three scales together gives you the complete picture of what's happening.

The G Scale: Geomagnetic Storms

The G scale measures how hard Earth's magnetic field is being disturbed by the solar wind — most commonly by a CME with southward-pointing Bz. It maps directly to the Kp index: G1 = Kp 5, G2 = Kp 6, G3 = Kp 7, G4 = Kp 8, G5 = Kp 9. This is the scale that determines aurora visibility latitudes.

In plain terms

Think of the G scale as "how badly is Earth's magnetic shield being rattled right now?" G0 is calm water. G5 is a hurricane. The higher it goes, the further south the aurora oval expands, and the more real-world infrastructure starts to feel the effects.

G

Geomagnetic Storm Scale

Driven by solar wind / CME interaction · Kp index correlation · Affects magnetosphere

Level
Real-world effects
Aurora & cause
G1
Weak power grid fluctuations. Minor satellite orientation issues. HF radio degraded at high latitudes. Migratory animals may be affected.
Aurora at high latitudes — Alaska, northern Canada, Scandinavia. Kp 5.
G2
Voltage alarms on high-latitude power systems. Long-duration storms may cause transformer damage. Satellites require corrective manoeuvres. HF blackout at high latitudes.
Aurora to New York, Oregon, northern Germany. Kp 6.
G3
Voltage corrections required across power systems. Surface charging on satellites. GPS degraded. HF radio intermittent. Low-frequency navigation disrupted.
Aurora to Illinois, Oregon, UK, central Europe. Kp 7.
G4
Widespread voltage control problems. Some grid elements may trip. Significant satellite surface charging and increased drag. Pipeline engineers take protective action. GPS inaccurate.
Aurora as far south as Alabama, California, southern Europe. Kp 8.
G5
Complete HF blackout globally. GPS navigation failure. Widespread power outages possible. Transformer damage risk. Extreme satellite disruption. Rare — fewer than 4 events per solar cycle on average.
Aurora visible near equatorial latitudes. Kp 9. Historic extreme events only.

The S Scale: Solar Radiation Storms

The S scale measures something completely different from the G scale: the flux of high-energy protons accelerated by a CME's shock wave or a solar flare. These particles travel at near-light speed and can arrive at Earth in as little as 30–60 minutes after a major eruption.

An S-scale event is largely invisible to people on Earth's surface — the atmosphere blocks these particles completely. But they are a serious concern for anyone above the atmosphere or at high altitude on polar routes, where the protection is thinner.

In plain terms

The G scale is about what the solar wind does to Earth's magnetic field. The S scale is about invisible high-speed particles — like a burst of cosmic radiation — that can damage electronics, harm astronauts, and raise radiation doses for polar flight passengers. You feel nothing. Your phone feels nothing. A satellite's computer chips feel it directly.

S

Solar Radiation Storm Scale

High-energy solar protons · Affects space assets, polar aviation · Arrives in minutes

Level
Real-world effects
Cause / threshold
S1
Minor HF radio degradation at polar regions. Low-level radiation hazard on polar flights.
10 particle flux units (>10 MeV protons). Approximately 50 events per solar cycle.
S2
Increased radiation dose for polar flight passengers and crew. Satellite anomalies possible. Infrequent single-event upsets in satellites.
100 pfu. Roughly 25 events per solar cycle.
S3
Biological risk to astronauts on spacewalks. High-altitude aircraft passengers at elevated radiation risk. Satellite anomalies common. HF blackout at polar caps.
1,000 pfu. Roughly 10 events per solar cycle.
S4
Definite radiation hazard to aircraft passengers on polar routes. Satellite computer failures possible. Navigation failure on polar routes. High-inclination spacecraft at risk.
10,000 pfu. Roughly 3 events per solar cycle.
S5
Unavoidable high radiation dose to astronauts. Potential radiation sickness for high-altitude crew. Satellite systems may fail completely. Total HF blackout at polar caps for days.
100,000 pfu. Fewer than 1 per solar cycle on average. Extremely rare.

The R Scale: Radio Blackouts

The R scale is the fastest-acting of the three — and the only one that operates entirely independently of CMEs. It's driven by solar flares alone.

When a solar flare erupts, it emits intense X-ray and extreme ultraviolet radiation. This travels at the speed of light and reaches Earth in about 8 minutes. It ionises the ionosphere — the layer of Earth's upper atmosphere that normally reflects high-frequency (HF) radio waves, allowing them to bounce over the horizon for long-distance communications. When the ionosphere is over-ionised by a flare, it absorbs HF radio instead of reflecting it. The result is a blackout on all HF radio on the sunlit side of Earth.

What the ionosphere is, in plain terms

The ionosphere is a layer of the atmosphere, roughly 60–1,000 km up, where ultraviolet light from the Sun strips electrons off gas molecules, creating a layer of charged particles. This layer acts like a mirror for certain radio frequencies — HF (high frequency) radio waves bounce off it, allowing shortwave broadcasts and aviation communications to travel far beyond line of sight. When a flare hits, this mirror briefly becomes a sponge, absorbing the signal instead.

R

Radio Blackout Scale

Solar X-ray flares · Sunlit hemisphere only · Begins within 8 minutes of flare

Level
HF radio effects
Flare class
R1
Minor HF degradation on sunlit side. Occasional signal loss at lower frequencies. Low-frequency navigation degraded slightly.
M1-class flare. ~2,000 events per solar cycle.
R2
Limited HF blackout on sunlit side. Loss of contact at low frequencies for tens of minutes. Degraded navigation signals for hours.
M5-class flare. ~350 events per solar cycle.
R3
Wide-area HF blackout. Loss of radio contact for ~1 hour across sunlit side. Low-frequency navigation outage for ~1 hour.
X1-class flare. ~175 events per solar cycle.
R4
HF blackout on most of sunlit side for 1–2 hours. Navigation outages for hours. Aviation and maritime HF severely affected.
X10-class flare. ~8 events per solar cycle.
R5
Complete HF blackout on entire sunlit side for hours. No HF contact possible. Navigation outages for many hours. Extreme disruption to aviation and shortwave.
X20+ class flare. Fewer than 1 per solar cycle. Extreme rarity.

Scale thresholds and event frequency estimates are based on NOAA's published documentation at swpc.noaa.gov/noaa-scales-explanation. Event frequency figures are approximate. Verify from primary sources for operational decisions.

THREE NOAA SCALES · ONE SOLAR EVENT · THREE DIFFERENT ARRIVAL TIMES SOLAR ERUPTION (flare + CME) R SCALE X-rays · speed of light 8 min HF blackout S SCALE protons · near-light speed 30–120 min Radiation storm G SCALE CME · 1–4 days 1–4 days Geomag storm Aurora ✦ T+8 min T+30–120 min T+1–4 days T+0
A single solar eruption can trigger all three NOAA scales at different times. X-rays from the flare reach Earth in 8 minutes (R scale). Accelerated protons arrive in 30–120 minutes (S scale). The physical CME plasma cloud takes 1–4 days to arrive and drives the geomagnetic storm (G scale) — and with it, aurora. Diagram: Aurora Watch.

How the Three Scales Relate to Each Other

The same solar event can trigger all three scales — or just one. Here's how they connect:

Scale
Cause
Arrival at Earth
Who it affects
G
CME / solar wind Bz
1–4 days, then 15–60 min from L1
Aurora chasers, power grids, GPS, pipelines
S
CME/flare shock protons
30 min – 2 hours
Astronauts, polar aviation, satellites
R
Solar flare X-rays
8 minutes (speed of light)
HF radio, aviation comms, shortwave
Reading the Aurora Watch Scales Panel

For aurora forecasting, focus on G. G1 = first storm threshold, aurora possible at high latitudes. G3+ = mid-latitude aurora likely.

S and R being active alongside G tells you the event was caused by a significant eruption — both a flare and a CME — and that the Sun is in an active phase. An active S combined with rising G is a sign that conditions may continue escalating.

"An X-class flare produces an R3 radio blackout in 8 minutes. The CME that launched with it might not arrive for two days — or might miss entirely. Always read all three scales."

Scale descriptions are based on NOAA SWPC's published scales explanation at swpc.noaa.gov/noaa-scales-explanation. Aurora Watch live scales data is sourced directly from NOAA SWPC APIs. Not affiliated with NOAA.