In brief
The Sun alternates periods of calm and intense activity on an ~11-year cycle. We are currently near the peak of cycle 25, which has already produced the largest storm since 2003 in May 2024 and should remain very active through 2027-2028.
A stellar heartbeat
The Sun's activity is not constant. Its internal magnetic field oscillates on an ~11-year cycle: it grows more complex until reaching a maximum, then simplifies back to a minimum where almost no sunspots are visible. At each cycle, the Sun's global magnetic polarity flips: the true fundamental cycle is therefore 22 years, called the Hale cycle.
Cycles have been numbered since 1755. We are currently in cycle 25, which started in December 2019, with maximum expected between 2024 and 2026.
How we measure it
Historically, we count sunspots visible on the surface: the more spots, the more active the cycle. Today, the SIDC in Brussels maintains the international reference Wolf number. Other indicators complement the measure:
- F10.7 cm flux: 2.8 GHz radio emission from the Sun, available daily.
- Flare rate by class (M, X).
- Number of geoeffective active regions per solar rotation.
Cycle 25, initially predicted weak by models, turned out more intense than forecast. Its maximum is already above predictions, and 2024-2025 were the most active months since 2001-2003.
Why 2026 is a good year
During solar maximum:
- The number of X-class flares doubles or triples vs. minimum.
- CMEs become more frequent and faster.
- G3 to G5 geomagnetic storms become regular.
- Auroras descend more often to mid-latitudes, including France.
Concretely: between 2022 and 2026, France has seen more auroras than in the preceding 10 years combined. The events of 24 March 2024, 10-11 May 2024, 11 October 2024 and 1 January 2025 all produced auroras visible from at least half the country. The maximum won't last: the cycle will naturally decline from 2027-2028, with the next minimum likely around 2030.
What we don't yet understand
The internal mechanics of the solar cycle remain one of the great unsolved problems in stellar physics. We know the solar dynamo couples differential rotation and convection, but we struggle to predict precisely the amplitude and duration of each cycle. Cycle 24 (2008-2019) was the weakest in a century; cycle 25 was initially predicted similar, before surprising with its vigour.
Very long cycles, like the Maunder minimum (1645-1715) during which almost no sunspots were observed, remind us that century-scale variability exists. Whether such a minimum could recur is unknown.