Space Weather
Sunspot Number — explained
The classic index of solar activity, tracking the Sun's 11-year cycle between quiet minimums and stormy maximums.
Latest Monthly Sunspot Number
Source: NOAA SWPC / SIDC Brussels
Why it matters
Sunspots mark active regions with tangled, energetic magnetic fields — exactly the regions that produce flares and CMEs. The sunspot number is used as a broad, months-to-years proxy for how active the Sun is likely to be overall, even though it says nothing about any single event or day.
Normal range
During solar minimum, monthly values can sit near 0 for extended stretches. During a strong solar maximum, they regularly exceed 100-150; Solar Cycle 25 (the current cycle) peaked in the 150-160 range around its maximum.
What today's value means
Waiting for live data to interpret…
What is the sunspot number?
Sunspots are temporary dark patches on the Sun's surface caused by concentrated magnetic field lines. The sunspot number is a standardized count — using the Wolf Sunspot Number formula, R = k(10g + s), where g is the number of sunspot groups and s is the total count of individual spots — that's been tracked continuously since the mid-1700s, making it one of the longest-running scientific records in existence. It rises and falls with the Sun's roughly 11-year activity cycle: during solar minimum it can sit near zero for weeks at a time, while during solar maximum it regularly exceeds 100.
Why sunspots matter for space weather
Sunspots mark active regions with tangled, energetic magnetic fields — and it's exactly these regions that produce solar flares and coronal mass ejections. More sunspots generally means more solar activity overall, which is why the sunspot number is used as a broad proxy for how "stormy" the Sun is likely to be over months and years, even though it says nothing about any single event.
FAQ
What's a high sunspot number?
During solar minimum, monthly values can be near 0. During a strong solar maximum, they regularly exceed 100-150, with the most active months of Solar Cycle 25 (the current cycle) peaking around 150-160.
Does a high sunspot number mean aurora tonight?
Not directly — it's a monthly average reflecting overall solar activity, not a forecast for any specific night. For that, check the live Kp index and aurora forecast.