Solar Physics
Sunspot Cycle
11-year cycle of solar magnetic activity — from few spots to many and back
The sunspot cycle (also called the solar cycle) is an ~11-year periodic variation in the Sun's magnetic activity, marked by changing numbers of sunspots — dark cool regions on the photosphere where strong magnetic fields suppress convection. Cycle features: solar minimum (few spots), then increasing activity to solar maximum, then declining. Discovered ~1844 by Schwabe. Underlying mechanism: solar dynamo and field reversal every cycle. Cycle 25 began Dec 2019; expected max ~2025.
- Cycle length~11 years (avg; range 9-14 years)
- Sunspot temperature~3500 K (cooler than 5778 K photosphere)
- Magnetic field strength~3000 Gauss (sunspot) vs 1 G (avg solar)
- Field reversalPolarity flips every 11 years (22-year magnetic cycle)
- DiscoveredHeinrich Schwabe, 1843
- Current cycleCycle 25 (started Dec 2019)
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Why the solar cycle matters
- Space weather. Cycle phase determines CME frequency.
- Climate. Total solar irradiance varies subtly over cycle.
- Stellar physics. Sun is paradigm for stellar activity cycles.
- Solar dynamo. Mechanism of magnetic field generation in the Sun.
- Carbon dating. Cosmic ray flux varies with cycle — affects ¹⁴C production.
- Aurora prediction. Solar max means more displays.
- Satellite operations. Drag depends on cycle phase.
Common misconceptions
- Sunspots cause climate change. Some influence, but small compared to greenhouse forcing.
- 11-year cycle is exact. Average; ranges 9-14 years.
- Cycle is predictable. Improving but still significant uncertainty.
- Sunspots are holes. Just cooler; still photosphere-level.
- Cycle stops at minimum. Continues — different phase of dynamo activity.
- Same field reversal every cycle. Yes — flips each ~11 years.
Frequently asked questions
What causes sunspots?
Strong magnetic fields. Sun has differential rotation — equator rotates faster than poles. This twists internal magnetic field. Field rises buoyantly through convection zone, breaks through surface in pairs of opposite polarity. Magnetic field suppresses convection in those regions → less hot plasma supplied → cooler temperature → appears dark against hotter surroundings.
What's solar maximum vs minimum?
Maximum — peak sunspot count; most flares, CMEs, prominences, auroras. Minimum — few/no sunspots; quiet Sun; less space weather. Cycle 24 max in 2014; min ~2019; Cycle 25 max ~2025. Total solar irradiance varies only ~0.1% — most variability is in UV and active features.
What's the magnetic field reversal?
Each ~11-year cycle, the Sun's polar magnetic field reverses polarity. North becomes south; south becomes north. Together with the sunspot cycle, this means the Sun has a 22-year "magnetic cycle" (Hale cycle). Field reversal happens around solar max.
How are sunspots counted?
Wolf number (R) — formula based on individual spots and groups. Daily counts averaged. Continuous record since 1700s, semi-quantitative since 1610 (Galileo's observations). Modern observation with sunspot-monitoring telescopes plus space-based observatories (SDO, etc.).
What's the Maunder minimum?
~1645-1715 — period with very few sunspots. Coincided with cooler global temperatures (Little Ice Age). Suggests solar variability affects climate. Modern solar-min cycles less pronounced. Long-term solar variability (centuries) studied through tree rings, ice cores.
How does activity affect Earth?
Solar maximum brings more CMEs, geomagnetic storms, brighter auroras, satellite damage, GPS errors. Power grid disruption possible (Carrington Event 1859). Higher UV/X-ray output affects upper atmosphere; satellites in low Earth orbit experience more drag. Solar irradiance variation small but climatically important.
How is the cycle predicted?
Multiple models — physical (dynamo), statistical (cycle history), observational (polar field strength near min). Predictions for cycle peaks gain accuracy over years. Current Cycle 25 prediction was for moderate activity; observations track with that.