Astrophysics — Space Weather & Solar Physics

The Sun's "Switch-Off" Moment That Predicts the Next Space-Weather Storm

Counting sunspots at one sharp instant inside each solar cycle lets scientists forecast the strength of the next space-weather peak six to seven years before it arrives.

• Astronomers have pinpointed a single cutoff moment in every solar cycle where the most violent space weather shuts off almost overnight.
• The sunspot count at that instant forecasts how strong the next cycle will be up to seven years in advance.
• The method correctly predicted that Solar Cycle 25 would be stronger than earlier forecasts suggested.

The Sun follows a roughly 11-year rhythm driven by its magnetic field. Sunspots rise and fall as the magnetic poles flip, and with them come solar flares and coronal mass ejections — the eruptions that can knock out satellites, disrupt radio links, and drive auroras as far south as Cornwall and Puerto Rico. The standard way to predict the next cycle is to watch the Sun's polar magnetic fields as they recover, but that signal only becomes readable a year or two before the next peak.

Physicists at the University of Warwick have found a much earlier tell. Rather than fading slowly, the Sun's most extreme space weather snaps off at a precise moment near the end of each cycle. Sandra Chapman's group showed that the remaining number of sunspots at this "switch-off" point carries information about the cycle that is still building up seven years away. In a dramatic case, their method flagged Solar Cycle 25 as stronger than the consensus forecast — a call vindicated by the May 2024 "Gannon" storm, which sent auroras south of their usual latitude across the UK and worldwide.

The mechanism is tied to how the solar dynamo rebuilds itself. After a cycle ends, the polar magnetic field takes time to recharge before it can seed the next round of activity. By reading the leftover sunspot count at the abrupt switch-off, researchers get a cleaner snapshot of that recharge state much earlier than polar-field measurements allow. Early application of the method suggests Solar Cycle 26 will be a moderate one, with a firmer estimate expected in about two years.

For anyone managing power grids, satellite fleets, or aviation routes, this is a shift from reacting to a year or two out to planning seven years out. The Sun does not gently go to sleep and gently wake up — it flips a switch, and the count on that switch is the calendar for the next storm season.

Knowledge takeaway: the Sun's most violent space weather ends almost overnight at a specific moment, not gradually; the sunspot count at that moment forecasts the next cycle six to seven years ahead; the approach is already producing firmer early estimates of Solar Cycle 26.