solsyklus

Solsyklus, or the solar cycle, is the roughly 11-year cycle of waxing and waning magnetic activity on the Sun. It is most visibly tracked by variations in sunspot numbers on the solar surface, which rise from a solar minimum to a solar maximum and then decline again. Because the Sun’s magnetic field reverses polarity with each cycle, a complete magnetic cycle lasts about 22 years, known as the Hale cycle.

The cycle is driven by a solar dynamo in the Sun’s convection zone, where differential rotation stretches magnetic fields (the omega effect) and turbulent convection regenerates fields (the alpha effect). The result is a periodically changing magnetic field that governs the appearance and disappearance of sunspots and the release of energetic solar activity.

Key observables include sunspot number and area, the 10.7 cm solar radio flux, and changes in total solar irradiance. The cycle also modulates the solar wind, coronal mass ejections, and solar flares, which in turn influence space weather. These phenomena can affect radio communications, satellite operations and orbits, GPS accuracy, and power grids on Earth.

Phases of the cycle are the minimum, ascending phase, maximum, and descending phase. The length and strength of cycles vary, with typical durations around 9 to 14 years and amplitudes that differ from one cycle to another.

History and current knowledge: the 11-year periodicity was identified by Samuel Heinrich Schwabe in 1843, with the sunspot number index formalized later. As of the 2020s, Solar Cycle 25 has begun (around 2019–2020), with a maximum expected in the mid-2020s.