Solar Cycle 25 and the aurora: a dated timeline

The Sun runs an activity cycle averaging about 11 years, and it changes what aurora forecasting can promise. This page sets out what happened in Solar Cycle 25 with dates, because a great deal of aurora advice on the internet was written at one point in the cycle and never marked with when.

A solar cycle is the roughly 11-year oscillation in the Sun’s magnetic activity, measured most commonly by sunspot number, running from one minimum through a maximum to the next minimum. Cycles are numbered from the first one systematically recorded, which began in 1755.

The dated timeline

Why the forecast was wrong, and why that is not a scandal

In 2019 the NOAA and NASA Solar Cycle 25 Prediction Panel forecast a weak cycle with a peak smoothed sunspot number near 115 — comparable to Cycle 24, which had itself been the weakest in about a century. A good deal of the aurora advice published in 2020 and 2021 was written on that basis, and some of it said, in effect, that this would be a poor decade for the northern lights.

Observed activity ran well above the prediction from 2022 onward, and the cycle went on to produce the strongest geomagnetic storm since 2003.

The reason for the miss is worth understanding, because it generalises. Solar cycle prediction depends on modelling the Sun’s internal magnetic dynamo from indirect surface observations, and different methods have historically disagreed with each other by more than the difference between a weak and a strong cycle. The panel’s forecast was a considered consensus with stated uncertainty; the outcome fell outside the central estimate.

The practical lesson is not that forecasters are unreliable. It is that a statement about solar activity more than a year or two ahead carries error bars wide enough to change a trip decision, and it should be read as such. This is the long-range version of the same limitation that caps short-range aurora forecasting at about an hour, described in how accurate the aurora forecast is.

What solar maximum actually changes

Solar maximum raises the background rate at which eruptions are possible. It does not schedule them, and it does not improve the odds on any particular night.

Sunspot number counts active regions on the visible solar disc. More active regions means more opportunities for a coronal mass ejection, which means more geomagnetic storms per year. That is a statement about rates over months, not about tonight.

Three things stay exactly the same regardless of where the cycle is:

• Your geomagnetic latitude. Solar maximum does not move you closer to the auroral oval. The threshold for your location, computed with the geomagnetic latitude calculator, is the same at maximum as at minimum. What changes is how often that threshold is exceeded.
• Darkness. Astronomical twilight is solar geometry, not solar activity. Above the Arctic Circle, midsummer remains unusable at any point in the cycle — see why the aurora has a season.
• Cloud. Unaffected, and still the most common single reason a night fails.

A quiet week during solar maximum is entirely ordinary. So is a G3 storm three years after it.

The declining phase is not the end of the show

The most consequential misconception about solar cycles, for anyone planning a trip, is that the years after maximum are a write-off. They are not, and the reason is that the two things that cause geomagnetic storms peak at different times.

Coronal mass ejections track sunspot number fairly closely and are most frequent around maximum. They produce the largest, most sudden storms — the kind that put aurora over unusual latitudes with a day’s notice.

Coronal holes are regions of open magnetic field from which fast solar wind escapes, and they become large, stable and well-placed during the declining phase. The fast streams they produce arrive predictably, and because the Sun rotates in about 27 days as seen from Earth, a long-lived coronal hole can produce recurring disturbances at roughly that interval.

That recurrence is the entire basis of NOAA’s 27-day outlook, and it is genuinely useful for planning during the declining phase in a way it is not near maximum. It identifies dates worth watching rather than predicting a specific night — a distinction covered in the forecast-accuracy article.

Historically, some of the strongest geomagnetic disturbance in a cycle has come a year or more after sunspot maximum rather than at it. Planning a trip for two years after a maximum is a defensible decision, not a consolation prize.

How to read cycle claims with a date attached

Aurora content ages badly and rarely says when it was written. Three specific claims should trigger a check of the publication date.

“The best aurora in twenty years.” True of the May 2024 storm, which was the first G5 since October 2003. Not a statement about any subsequent night, and repeated far beyond its shelf life.

“Solar maximum is coming in [year].” Check whether the year has passed. Maximum was announced on 15 October 2024, so anything published later still forecasting it forward is recycled.

“This will be a weak cycle.” Written against the 2019 forecast, and superseded by observation from 2022 onward.

For a current figure, the right move is to read NOAA’s solar cycle progression product, which publishes observed and predicted smoothed sunspot numbers, rather than trusting a number typed into an article at an unknown date. That is why this page gives dated events and not a current sunspot count: a number printed here would be stale within weeks, and a stale number with no date is worse than none.

What none of this tells you about tonight

Cycle phase sets the background rate. It is the weakest of the inputs to a decision about a particular evening, and it is frequently the only one that general news coverage mentions.

The order of usefulness for tonight runs roughly opposite to the order in which these things get reported. The interplanetary magnetic field Bz measured at L1, available 20 to 60 minutes ahead, matters most. Then the current activity level and whether a substorm is loading. Then your geomagnetic latitude and whether it is dark. Then cloud, which overrides everything. Solar cycle phase comes last, because it is a statement about the year.

News coverage inverts this almost exactly, leading with sunspot numbers and solar maximum because they are the parts that hold still long enough to write about. What an aurora alert can and cannot promise covers the same gap from the product side.

Bottom line

Solar Cycle 25 began at the December 2019 minimum. The NOAA and NASA prediction panel forecast in 2019 that it would be weak, with a peak smoothed sunspot number near 115; observed activity ran well above that from 2022 onward. The storm of 10 to 11 May 2024 reached G5, the extreme level on NOAA’s scale and the first since October 2003, with aurora reported at very low geomagnetic latitudes worldwide; a second severe storm followed on 10 October 2024. On 15 October 2024, NOAA, NASA and the international panel announced the Sun had entered its solar maximum period.

Solar maximum raises the rate at which storms are possible over months. It does not change your geomagnetic latitude, whether your sky is dark, or whether it is cloudy — and those decide the night. The declining phase remains productive because long-lived coronal holes generate recurring fast solar wind streams at roughly 27-day intervals, and some of the strongest disturbance in a cycle has historically arrived a year or more after sunspot maximum.

Treat any undated claim about “the best aurora in years” as expired, and read the current cycle position from NOAA’s solar cycle progression product rather than from an article.