The solar cycle is the roughly 11-year rhythm in which the Sun’s magnetic activity rises and falls, most visibly tracked by the number of dark sunspots on its surface. At solar minimum the Sun can go days without a single spot; at solar maximum hundreds may appear in a month, accompanied by flares, coronal mass ejections, and geomagnetic storms that ripple outward through the entire solar system. The cycle shapes everything from satellite orbits to airline radiation exposure, yet its underlying engine remains one of the harder problems in astrophysics. Current Cycle 25, which began around December 2019, has already exceeded most early forecasts and is expected to peak around 2024–2025.
How the Cycle Shows Up on the Sun
The most intuitive way to watch the solar cycle is through sunspots. These cooler, darker patches mark regions where intense magnetic field lines poke through the photosphere and inhibit convective heat flow. Early in a cycle, sunspots tend to appear at mid-latitudes, around 25–30 degrees north and south of the equator. As the cycle progresses, the zones of emergence drift steadily toward the equator, producing a pattern called the butterfly diagram because a time-latitude plot of sunspot positions over many cycles looks like a series of nested butterfly wings. Research has confirmed that regardless of whether a given cycle is strong or weak, the equatorward drift of sunspot zones follows a remarkably consistent path, decaying exponentially from about 28 degrees latitude with an e-folding time near 90 months.1Astronomy & Astrophysics. The mean solar butterfly diagram and poloidal field generation rate at the surface of the Sun The butterfly diagram is essentially the equatorward migration of sunspot emergence latitudes as each cycle evolves.2The Astrophysical Journal Letters. A Potential New Mechanism for the Butterfly Diagram of the Solar Cycle: Latitude-dependent Radial Flux Transport
Besides changing in number, the eruptions that accompany sunspots also change character over the cycle. Coronal mass ejections, the enormous blobs of magnetized plasma hurled into space, are both faster and wider during solar maximum. During solar minimum, CMEs cluster near the equator and average about 300 km/s; by the time maximum arrives they appear at all latitudes and average roughly 500 km/s.3Journal of Geophysical Research: Space Physics. A catalog of white light coronal mass ejections observed by the SOHO spacecraft That acceleration matters because faster CMEs are more likely to slam into Earth’s magnetosphere hard enough to cause problems on the ground.
The 22-Year Magnetic Cycle
The 11-year sunspot count is only half the story. Each sunspot cycle ends with the Sun’s global magnetic polarity flipped: what was a magnetic north pole becomes south, and vice versa. It takes two consecutive 11-year sunspot cycles for the polarity to return to its starting configuration, making the full magnetic cycle about 22 years. This longer period is often called the Hale cycle, after George Ellery Hale, who first detected the magnetic nature of sunspots in 1908. The polarity reversal typically occurs around solar maximum, when the accumulation of decayed sunspot magnetic flux at the poles overwhelms and replaces the existing polar field.
The Hale cycle is not just a curiosity of magnetic bookkeeping. It leaves fingerprints in hemispheric activity patterns: analyses of radio flux and sunspot data show that odd-numbered solar cycles (19, 21, 23) tend to have higher peak activity in the northern hemisphere, while even-numbered cycles (18, 20, 22, 24) favor the southern hemisphere, alternating in step with the 22-year magnetic polarity pattern.4Astronomy & Astrophysics. Hale cycle in solar hemispheric radio flux and sunspots: Evidence for a northward-shifted relic field This systematic alternation hints that a persistent, deeply rooted asymmetry in the Sun’s interior magnetic field may be modulating how each hemisphere responds to the dynamo.
North Versus South
Even within a single 11-year cycle, the two hemispheres rarely behave identically. The northern hemisphere tends to dominate during the ascending (rising) phase of most cycles, suggesting that magnetic flux preferentially emerges there first. The asymmetry weakens during the polarity reversal near maximum, when a global magnetic reconfiguration appears to bring the hemispheres temporarily into balance.5The Astrophysical Journal Supplement Series. Hemispheric Asymmetry of Relative Sunspot Numbers during Solar Cycles 17–25
Looking ahead, predictions using spectral analysis and machine-learning methods suggest the southern hemisphere will outpace the north during both Cycle 25 and Cycle 26, continuing a trend of southern dominance that began around 1980.6Solar Physics. Hemispheric Sunspot Number Prediction for Solar Cycles 25 and 26 Using Spectral Analysis and Machine Learning Techniques For the current cycle, the predicted southern-hemisphere peak sunspot number of about 77 sits well above the northern peak near 59, with the southern peak arriving a year or so later. That difference is not cosmetic: hemispheric asymmetry affects the timing and latitude of geomagnetic storms, the accuracy of space-weather forecasts, and even where auroras are most likely to appear.
Predicting the Next Cycle
Forecasting the strength of a future solar cycle remains frustratingly imprecise. The most trusted approach is the polar precursor method, which looks at the strength of the Sun’s polar magnetic field around sunspot minimum and uses it to predict the amplitude of the coming maximum. The physical logic is straightforward: the polar field at minimum acts as the seed for the toroidal magnetic field that drives the next cycle’s sunspots. An extensive analysis of various polar predictors, using both direct observations and outputs from dynamo models, found that the earliest point at which the polar predictor becomes reliable is about four years after the polar field reverses. That turns out to be roughly two to three years before sunspot minimum and about seven years before the predicted maximum, giving forecasters a useful but not enormous lead time.7The Astrophysical Journal. The Polar Precursor Method for Solar Cycle Prediction: Comparison of Predictors and Their Temporal Range
Physics-based dynamo models, which simulate the generation and transport of the Sun’s magnetic field, reinforce this picture. A review of these models notes that the most successful prediction approaches are built on the Babcock-Leighton mechanism, in which the tilt of decaying sunspot pairs feeds magnetic flux to the poles, setting up the seed field for the next cycle.8Space Science Reviews. Physical Models for Solar Cycle Predictions But there is a catch: when a simple flux-transport model is fed the actually observed latitudes at which sunspots emerge, rather than an idealized smooth drift, its predictive skill collapses.9The Astrophysical Journal. Solar Cycle Prediction Using Precursors and Flux Transport Models Real sunspot emergence is messier than any tidy mathematical belt can capture, and that messiness limits how far ahead anyone can reliably project.
Tracing Solar Cycles Deep into the Past
Telescope-based sunspot records reach back only to about 1610. To peer further, researchers turn to cosmogenic isotopes, especially carbon-14 preserved in tree rings. When the Sun is active, its stronger magnetic field deflects more galactic cosmic rays away from Earth, reducing the production of radiocarbon in the upper atmosphere. During quiet periods the cosmic-ray flux rises and more carbon-14 is created. By measuring annual radiocarbon concentrations in precisely dated tree-ring sequences, scientists can reconstruct past solar activity. A high-precision tree-ring dataset spanning parts of the first millennium BCE detected a clear signal with a mean period of about 10.5 years and an amplitude of roughly 0.4 per mil in radiocarbon variation, consistent with the Schwabe sunspot cycle.10Nature Communications. Tracing ancient solar cycles with tree rings and radiocarbon in the first millennium BCE This means the roughly 11-year rhythm has been humming along for at least three thousand years.
These records also reveal periods when the cycle nearly vanished. The best-known is the Maunder Minimum, from about 1645 to 1715, when sunspots were extremely scarce. The Maunder Minimum coincided with some of the coldest decades of the Little Ice Age, and for a long time this was taken as strong evidence that the Sun drove the cooling. More recent work, however, argues that the timing is not as suggestive of causation as it first appears. Climate model simulations point to volcanic eruptions as the primary driver of Northern Hemisphere cooling during the Little Ice Age, with reduced solar output playing a supporting role comparable to changes in land use.11J. Space Weather Space Clim.. The Maunder minimum and the Little Ice Age: an update from recent reconstructions and climate simulations Estimates of the total solar irradiance drop during the Maunder Minimum suggest it was around 0.24 percent, enough to cool Earth’s equilibrium temperature by somewhere between 0.2 and 0.6 degrees Celsius, which accounts for only a fraction of the roughly 1-degree cooling the Little Ice Age actually produced.12Geophysical Research Letters. Estimating the Sun’s radiative output during the Maunder Minimum
What Happens When a Storm Hits Earth
The practical side of the solar cycle becomes vivid during intense geomagnetic storms. When a fast CME strikes Earth’s magnetosphere, it compresses and distorts the planet’s magnetic field, driving currents through the ionosphere and, by induction, through long conductors on the ground. The most famous historical example is the Carrington Event of 1859, widely regarded as one of the most extreme space weather events in the observational record. The storm was driven by extreme CMEs from a large, complex active region and produced auroras visible at tropical latitudes. Comparisons with similar storms in 1872, 1909, 1921, and 1989 suggest the Carrington Event was among the most severe but was likely not unique in Earth’s history.13Space Weather. Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms Around the Carrington Event in 1859
During the main phase of an intense geomagnetic storm, only about 3.5 percent of the solar wind’s kinetic energy gets funneled into the magnetosphere, and of that, roughly 20 percent ends up in the inner magnetosphere and the auroral zones of both hemispheres.14Journal of Geophysical Research: Space Physics. Some characteristics of intense geomagnetic storms and their energy budget That sounds small, but the total energy involved is enormous. The energy dissipated in the auroral ionosphere alone during an intense storm can reach several times 10^15 joules, enough to light entire countries for hours.
Power Grids, Satellites, and GPS
Modern infrastructure is threaded with long conductors that are vulnerable to geomagnetically induced currents. High-voltage power transformers are the chief concern: GICs can push them into magnetic core saturation, causing overheating and, in a worst case, catastrophic failure.15arXiv. Global transformer overheating from geomagnetic storms Mitigation options exist but remain imperfect. GIC-blocking devices can be installed at transformer neutral points, though they require extensive deployment and their system-wide reliability effects are still being studied. A more practical near-term strategy is to combine early-warning data from solar observatories with load-management plans, reducing transformer operating levels so that a GIC-induced DC offset does not push the core into saturation.16Life Cycle Reliability and Safety Engineering. Impact of geomagnetic disturbances on power transformers: risk assessment of extreme events and data availability
Satellites in low Earth orbit face a different problem. During solar maximum the upper atmosphere absorbs more solar radiation, heats up, and expands, increasing atmospheric density at orbital altitudes. That extra drag pulls satellites lower faster, shortening their useful lifetimes and forcing more frequent orbit-raising maneuvers.17Advances in Space Research. Effects of plasma drag on low Earth orbiting satellites due to solar forcing induced perturbations and heating SpaceX learned this the hard way in February 2022, when a moderate geomagnetic storm caused enough atmospheric expansion to doom dozens of newly launched Starlink satellites before they could reach their operational altitude.
Navigation and communications also suffer. When satellite signals pass through an ionosphere disturbed by solar activity, they pick up random fluctuations in amplitude and phase known as scintillation. This scintillation degrades GPS accuracy and can cause receiver lock to drop out entirely. The effect is strongest at equatorial and high latitudes and peaks during solar maxima.18Radio Science. Comparison of the effect of high‐latitude and equatorial ionospheric scintillation on GPS signals during the maximum of solar cycle 24 For precision applications such as aircraft instrument approaches, centimeter-accurate surveying, or autonomous vehicle positioning, solar-cycle-driven scintillation is a real operational headache.
Radiation at Flight Altitudes
Airline passengers and crews are exposed to cosmic radiation that increases with altitude and latitude. During solar energetic particle events, radiation levels at cruising altitude can spike dramatically. The January 20, 2005 event, one of the strongest ground-level enhancements on record, pushed dose rates at about 12 km altitude to nearly 2 mSv per hour briefly in the Antarctic region and around 0.1 mSv per hour at high northern latitudes. After 12 hours, dose rates above 60 degrees latitude were still elevated by 50 percent, while latitudes below 40 degrees saw no increase at all.19Journal of Geophysical Research: Space Physics. Temporal and spatial evolution of the solar energetic particle event on 20 January 2005 and resulting radiation doses in aviation
Extreme ancient events make the modern ones look modest. A study modeling an event comparable to the 774 AD particle storm, identified through a sharp carbon-14 spike in tree rings, found that radiation doses at flight altitudes in polar regions could reach levels associated with acute radiation syndrome.20Journal of Space Weather and Space Climate. Assessment of the radiation risk at flight altitudes for an extreme solar particle storm of 774 AD Probabilistic assessments suggest that an event strong enough to require changing flight conditions, such as descending or rerouting away from high latitudes, occurs roughly once every 17 to 47 years depending on the threshold used.21Scientific Reports. Probabilistic risk assessment of solar particle events considering the cost of countermeasures to reduce the aviation radiation dose Airlines operating long-haul polar routes already monitor space weather advisories, but the protocols are still evolving.
Cosmic Rays and the Heliosphere
The solar cycle modulates not just what leaves the Sun but what reaches us from outside the solar system. Galactic cosmic rays, high-energy particles originating from supernovae and other violent astrophysical sources, are partially deflected by the Sun’s magnetic field and the solar wind. When the Sun is active, its expanded and more turbulent magnetic field shields the inner solar system more effectively, and cosmic-ray counts at Earth drop. At solar minimum the shield weakens and cosmic-ray intensity climbs.
Data from the Voyager probes have shown that more than half of the total reduction in galactic cosmic-ray flux caused by solar modulation actually occurs in the heliosheath, the region beyond the termination shock where the solar wind slows to subsonic speeds.22PubMed Central. Galactic Cosmic Rays Throughout the Heliosphere and in the Very Local Interstellar Medium This was a surprise: it means the outermost, least-studied boundary layers of the heliosphere are doing the heavy lifting in cosmic-ray shielding, and accurate models of solar modulation cannot treat those boundaries as simple parameters. For anyone planning long-duration space missions beyond Earth’s magnetosphere, understanding how the solar cycle modulates this shield is critical for estimating crew radiation exposure.
Solar Cycles on Other Stars
The Sun is not the only star with a magnetic cycle. Decades of monitoring chromospheric emission in solar-type stars have shown that activity cycles with periods near 10 years are common among stars with similar internal structures. The Sun’s roughly 11-year Schwabe cycle fits comfortably within the average dominant cycle of about 10 years found among a sample of older, slowly rotating stars with simple cyclic behavior.23Astronomy & Astrophysics. Magnetic cycles at different ages of stars This is reassuring in one sense: it means the solar cycle is not some quirk of our particular star but a generic outcome of the dynamo process in Sun-like convective interiors.
Global turbulent simulations of stellar dynamos have found that the magnetic cycle period scales inversely with the Rossby number, a quantity that measures how strongly rotation influences turbulent convection. Stars that rotate faster relative to their convective turnover time tend to have shorter cycles.24PubMed. Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations The Sun, rotating somewhat faster than the most quiescent cyclic stars but slower than the rapidly spinning young ones, sits in a sweet spot that produces a moderate and fairly regular cycle.25Space Science Reviews. Stellar Activity Cycles For exoplanet habitability, the host star’s cycle behavior matters because flares and particle storms driven by stellar activity can erode planetary atmospheres and bombard surfaces with radiation, much as the Sun’s cycle affects conditions across our own solar system.
The Stratospheric Ozone Connection
One of the subtler ways the solar cycle reaches into Earth’s atmosphere is through ozone chemistry. Ultraviolet radiation from the Sun is the main energy source that creates and destroys ozone in the stratosphere, and because UV output varies more sharply over the solar cycle than total irradiance does, stratospheric ozone concentrations track the cycle. Satellite observations have confirmed a solar-cycle ozone response in the tropical upper stratosphere, though the precise magnitude has been debated. Updated analyses using the latest satellite data processing (SAGE II version 7.0) find a smaller solar-cycle ozone signal in the tropical upper stratosphere, about 1 percent, compared to the roughly 4 percent seen with the older data version, largely because the newer analysis uses a different stratospheric temperature dataset to convert ozone measurements.26Copernicus Publications (Atmospheric Chemistry and Physics). The representation of solar cycle signals in stratospheric ozone – Part 1: A comparison of recently updated satellite observations The ozone signal feeds back into stratospheric temperature and circulation patterns, with implications for weather at the surface through so-called top-down coupling. It is a small effect, but persistent and global.