Solar cycle 25: why the descending phase is still great news for auroras

Solar cycle 25 has passed its maximum, placed around late 2024 by forecasting centres. Does that mean the aurora window over France is closing? No - if anything, the opposite holds for the next few years. The history of solar cycles shows that the most violent geomagnetic storms often strike after the peak, during the descending phase. This article explains why 2026-2028 remains one of the best stretches to catch an aurora from France, and what to actually watch for.

The 11-year solar cycle in a nutshell

The Sun follows an activity cycle of roughly 11 years, tracked by the number of sunspots visible on its surface. Sunspots are regions where concentrated magnetic field pierces through from the interior; they are where solar flares and coronal mass ejections (CMEs) originate. Over a cycle, the sunspot count climbs from a nearly blank minimum to a maximum, then falls back. At each maximum, the Sun's global magnetic field flips: magnetic north becomes magnetic south and vice versa. That reversal is not instantaneous - it plays out over months and leaves behind a tangled magnetic configuration that takes years to settle.

Cycle 25 began in late 2019 and turned out noticeably stronger than predicted. Early forecasts called for a weak cycle, comparable to cycle 24; in reality, sunspot numbers overtook those projections as early as 2022, and the maximum came in stronger and slightly earlier than announced. The concrete result: the May 2024 storm, the most intense in over two decades, with auroras photographed as far south as Spain and North Africa.

The misconception: "the maximum is over, so is the show"

Intuitively, more sunspots should mean more auroras, so the aurora curve should track the solar cycle exactly. That intuition fails for a simple reason: what triggers a geomagnetic storm visible from France is not the sunspot count, it is the arrival at Earth of fast, magnetically well-oriented solar wind. And the sources of that wind - major flares, fast CMEs, coronal holes - do not all peak at the same point of the cycle.

Geomagnetic climatologies built on more than a century of records show that geomagnetic activity often has two peaks: one near solar maximum, and a second, frequently stronger one, two to three years later, on the way down. Some of the most famous storms of the modern era belong to that second category.

Three historical examples

The Halloween storms, October-November 2003. Cycle 23 peaked in 2000-2001. Three years later, a series of flares among the most powerful ever recorded (one so intense it saturated the sensors on the GOES satellite) drove G5-level storms, with auroras seen across southern Europe and down to Texas. It is the textbook case of the Sun unleashing itself deep into the declining phase.
March 1989, the Quebec blackout. This extreme storm, which cut power to six million people for nine hours, hit during the late rising phase of cycle 22, a few months before its maximum. It is a reminder that no phase of the cycle is truly quiet outside minimum - and above all that major events spread widely around the peak.
September 2017. Three years after cycle 24's maximum (2014), when the Sun was supposedly winding down, one active region produced the strongest flares of the entire cycle, along with severe geomagnetic storms and auroras at unusually low latitudes.

And of course the recent benchmark: the May 2024 storm, a G5 event with Kp reaching 9, which struck near cycle 25's maximum. If the historical pattern repeats, cycle 25 most likely has not had its final word.

Why the descending phase stays explosive

Coronal holes: the fast-wind machine

A coronal hole is a region of the solar corona where the magnetic field opens out into space: solar wind escapes freely, at 600 to 800 km/s instead of the 300 to 400 km/s of the slow wind. These holes grow larger, more frequent and more persistent during the descending phase, as they migrate from the poles towards the Sun's mid-latitudes, facing Earth.

Their biggest asset for aurora chasers is recurrence. A coronal hole can survive several 27-day solar rotations. Each time it swings back to face Earth, it sends out a high speed stream (HSS) which, ploughing into the slow wind ahead of it, builds a co-rotating interaction region (CIR): a compressed, dense, magnetically turbulent zone. The result: moderate storms (G1-G2, occasionally G3) returning on a near-fixed schedule, every 27 days. Less spectacular than an extreme CME, but far more predictable - and enough for auroras visible from northern France when conditions line up.

Fewer active regions, but nastier ones

Late in a cycle, active regions become scarcer, but the ones that do emerge are often magnetically complex: large structures where opposite polarities intertwine, able to store enormous energy before releasing it abruptly. The Halloween 2003 and September 2017 flares both came from regions of exactly that kind. Fewer events, but potentially more violent ones: that is the signature of the decline.

The equinox effect: the Russell-McPherron mechanism

Geomagnetic storms have a well-established seasonality: they are statistically more frequent in March-April and September-October than around the solstices. The leading explanation is the Russell-McPherron mechanism, described in 1973: around the equinoxes, the geometry between Earth's magnetic axis and the interplanetary magnetic field carried by the solar wind favours a southward Bz component - precisely the orientation that opens the magnetosphere and lets solar wind energy pour in.

In practice: the same fast-wind stream will, on average, drive a stronger storm in October than in June. Combine that with the recurrent coronal holes of the descending phase and you get the best windows of the period: the weeks around the 2026 and 2027 equinoxes. A welcome bonus for observers in France: at those dates the nights are already long (or long again), unlike midsummer, when permanent twilight ruins the show.

What this means for an observer in France

As a reminder, mainland France needs a serious storm to see auroras: on the order of Kp 6-7 for glows across the north of the country, Kp 8-9 for clear colours down to the south (the city-by-city detail is in our Kp index guide). The descending phase guarantees nothing, but it keeps a real probability of major events alive for several more years. Here is how to make the most of it:

Favour the equinox windows. March-April and September-October 2026-2027 statistically concentrate the best odds. Plan your outings, high-altitude nights or dark-sky weekends around those periods.
Learn the 27-day rhythm. If a coronal-hole storm occurs, note the date: the same source can become geoeffective again one solar rotation later. Forecasting centres track these structures from rotation to rotation.
Do not switch off flare watch. Late-cycle active regions can produce major CMEs with 1 to 3 days of warning between eruption and arrival at Earth. A May 2024-style alert remains possible at any point of the decline.
Watch Bz more than the forecast Kp. For coronal-hole storms, often moderate, it is the southward orientation of the magnetic field in the preceding hours that separates "nothing" from "photographable glow on the northern horizon".

Key takeaway: solar maximum is a statistical marker, not a switch. Carrington 1859, Halloween 2003, September 2017: a notable share of history's greatest storms happened away from their cycle's official peak. Our comparison Carrington 1859 vs May 2024 puts those events on a single scale.

One last practical point: a descending-phase storm driven by a coronal hole tends to build more gradually than a CME impact, sometimes staying geoeffective for a whole night or two. That changes how you chase it. Rather than a single frantic dash outside at the moment of impact, treat these events as an evening-long watch: check the real-time data every half hour, and be ready for the sub-storm intensifications that come in waves. Our tonight page is built exactly for that use case, with the live Kp translated into an observation probability for your city.

And after 2028?

Cycle 25's descending phase should stretch to the next minimum, expected towards the end of the decade. Activity will taper off gradually: major CMEs will grow rarer, then the mid-latitude coronal holes will fade in turn. The lean years of minimum (probably around 2029-2031) will offer little at French latitudes. Which is exactly why 2026-2028 is the window not to waste: it is the final stretch before several quiet years, and the experience of past cycles suggests it can still deliver a headline event.