Solar Cycle 25, the current roughly eleven-year cycle of rising and falling solar activity, officially began in December 2019 and has turned out to be considerably more active than most forecasters anticipated. Early predictions from NOAA’s Solar Cycle Prediction Panel called for a modest cycle, but sunspot counts have consistently outpaced those forecasts, and the cycle has already produced the second most intense geomagnetic storm of the space age. The story of Cycle 25 is still unfolding, but it has already reshaped how scientists and engineers think about the risks of space weather in a world that depends heavily on satellites, GPS, and interconnected power grids.
A Cycle That Outran Its Forecast
Before Cycle 25 ramped up, the consensus prediction was for a cycle about as weak as its predecessor. Cycle 24 peaked at a sunspot number of about 116, making it one of the weaker cycles on record. Analysis of precursor indicators such as polar magnetic fields and curve-fitting to early sunspot data pointed to a Cycle 25 maximum around 135, give or take 10, slightly larger than Cycle 24 but still well below the average maximum of about 179 across all numbered cycles since the 1700s.1Journal of Geophysical Research: Space Physics. Solar Cycle Precursors and the Outlook for Cycle 25 That average spans a wide range: the weakest cycle on record peaked near 81, and the strongest topped out around 285.
What actually happened surprised many solar physicists. Monthly sunspot numbers began exceeding the predicted curve in 2023 and continued climbing through 2024, with some months running far above the upper confidence bounds of the original forecast. By mid-2024, it was clear that Cycle 25 was tracking as a moderately strong cycle rather than the subdued one experts had expected. The prediction miss has reinvigorated debate about what drives cycle strength and whether current forecasting methods are missing something fundamental.
How the Solar Dynamo Drives the Cycle
The eleven-year rise and fall of sunspots is one expression of a deeper magnetic engine inside the Sun. In broad terms, the Sun generates magnetic fields through the motion of electrically conducting plasma in its interior. One widely discussed scenario holds that the initial magnetic field is generated by turbulent, twisting motions of plasma at high latitudes near the base of the convection zone, roughly around 60° from the equator. That field migrates quickly to the surface in a year or two at high latitudes, but at low latitudes the journey takes much longer because the Sun’s differential rotation stretches and wraps the field, converting it from one orientation to another. Those twisted fields eventually become buoyant enough to punch through the surface, forming sunspots. As sunspots decay, their remnant magnetic fields drift poleward, carried by large-scale flows, and eventually sink back down to seed the next cycle.2The Astrophysical Journal. Helioseismic Properties of Dynamo Waves in the Variation of Solar Differential Rotation
This whole process actually takes closer to 22 years to complete a full magnetic cycle, because the Sun’s magnetic poles flip once per sunspot cycle. The familiar eleven-year sunspot cycle is really half of the full magnetic story. Researchers studying 140 years of solar data have mapped this broader pattern using diagnostics from the corona down through the interior, linking activity migration across the whole Sun to the 22-year Hale magnetic cycle.3Solar Physics. Deciphering Solar Magnetic Activity: 140 Years of the ‘Extended Solar Cycle’ – Mapping the Hale Cycle Understanding this longer cycle matters because the magnetic state the Sun carries from one eleven-year cycle into the next affects how strong the following cycle will be, which is one of the key inputs forecasters use when trying to predict a cycle’s peak.
The Polar Field Reversal
One of the milestone events of any solar cycle is the reversal of the Sun’s polar magnetic fields, which typically happens around the time of solar maximum. During Cycle 25, researchers predicted that the northern hemisphere would reverse polarity around August 2024, with the southern hemisphere following roughly five months later, around February 2025. That lag between hemispheres is normal and consistent with the typical pattern seen in previous cycles.4The Astrophysical Journal Letters. Predicting the Timing of the Solar Cycle 25 Polar Field Reversal
The reversal itself is not a sudden event. It plays out over months as the accumulated magnetic flux from decaying sunspot regions gradually overwhelms the existing polar field and replaces it with opposite-polarity flux. Tracking the progress of the reversal gives solar physicists a real-time indicator of where the cycle stands relative to its maximum. It also has practical implications: the polar field strength that emerges after the reversal settles down becomes one of the best predictors of how strong the next cycle will be.
The May 2024 Superstorm
The defining space weather event of Cycle 25 so far struck on May 10–11, 2024. A series of coronal mass ejections, huge clouds of magnetized plasma hurled from the Sun, arrived at Earth in quick succession. The resulting geomagnetic storm drove the Sym-H index below −500 nanotesla, making it the second most intense geomagnetic storm of the entire space era. The dayside boundary of Earth’s magnetic shield, the magnetopause, was compressed inside the orbit of geostationary satellites for about six continuous hours. Modeling suggested it may have been pushed inward to just 3.3 Earth radii from the planet’s center, which is remarkably close.5PubMed Central. Super-Intense Geomagnetic Storm on 10-11 May 2024: Possible Mechanisms and Impacts
What made this storm so extreme was a sustained combination of very high solar wind pressure and an intense electric field within the incoming plasma cloud. That combination persisted for nearly seven hours, driving ring current buildup much closer to Earth than usual. The storm caused GPS positioning errors through a dramatic increase of more than 100 percent in dayside ionospheric electron content, and a prior solar flare had already triggered a radio blackout across the 2–12 MHz frequency band used by long-distance aviation and maritime communications.6PubMed Central. Super-Intense Geomagnetic Storm on 10-11 May 2024: Possible Mechanisms and Impacts For many people, though, the most memorable consequence was visible: vivid auroras at latitudes where most residents had never seen them.
Auroras at Unusual Latitudes
During major geomagnetic storms in Cycle 25, people across large swaths of the midlatitudes, and in some cases well into the subtropics, reported seeing auroras. Social media lit up with photos from places like the southern United States, southern Europe, and parts of India. A natural question is whether the auroral oval, the ring of aurora that normally sits over the Arctic and Antarctic, physically expanded all the way down to those latitudes. Research on an April 2023 storm, a precursor to the larger May 2024 event, found that the answer is more subtle. The red aurora observed from lower latitudes resulted from two factors: a modest equatorward push of the auroral oval’s boundary combined with high-altitude red emissions visible from far away. The auroral oval did not actually reach those low latitudes.7The Astrophysical Journal. Low-latitude Auroras: Insights from 2023 April 23 Solar Storm
The red color is key. The familiar green and purple curtains of aurora come from oxygen and nitrogen being excited at relatively low altitudes, typically 100 to 300 kilometers up, directly beneath the auroral oval. The deep red glow that low-latitude observers see is produced by oxygen atoms at much higher altitudes, above 300 kilometers, where the atmosphere is extremely thin. Because the emission happens so high up, it can be seen over the horizon from locations thousands of kilometers equatorward of where the actual particle precipitation is happening. During especially strong storms with enhanced low-energy electron precipitation, this high-altitude red glow becomes bright enough to photograph easily from places where auroras are otherwise a once-in-a-lifetime sight.
Satellites and Atmospheric Drag
One of the most tangible consequences of increased solar activity is what it does to the thin upper atmosphere where satellites orbit. Solar radiation in the extreme ultraviolet range heats the thermosphere, causing it to expand and become denser at satellite altitudes.8Advances in Space Research. Thermosphere and satellite drag Denser air at orbital altitudes means more drag, which slows satellites and causes them to lose altitude faster. During solar maximum, this effect is dramatically amplified, and geomagnetic storms can cause short-term spikes in density that catch satellite operators off guard.
Cycle 25 delivered an early and costly lesson on this front. In February 2022, SpaceX launched a batch of 49 Starlink satellites into a low initial orbit at about 210 kilometers altitude. A geomagnetic storm hit the next day. Thermospheric density at that altitude jumped by at least 20 to 30 percent compared to the nine days before launch, creating far more drag than the satellites’ onboard propulsion could overcome. Up to 38 of the 49 satellites reentered the atmosphere and were lost.9Space Weather. The Thermosphere Is a Drag: The 2022 Starlink Incident and the Threat of Geomagnetic Storms to Low Earth Orbit Space Operations The incident was not caused by a particularly extreme storm. It highlighted how routine geomagnetic activity during an ascending solar cycle can threaten spacecraft in very low orbits, especially during the vulnerable period right after launch when satellites have not yet raised their altitude.
With thousands of new satellites being deployed in low Earth orbit, the drag problem matters more now than it did during previous cycles. Operators must factor in both the baseline rise in atmospheric density across solar maximum and the sudden spikes from individual storms. Failing to account for either can mean lost hardware, gaps in service, or uncontrolled reentries.
Power Grids and Geomagnetically Induced Currents
When a geomagnetic storm rapidly changes Earth’s magnetic field, it induces electric currents in the ground. Those currents find their way into long conductors like power transmission lines, pipelines, and undersea cables, creating what are called geomagnetically induced currents, or GICs. In transformers, GICs can cause half-cycle saturation, which drives up reactive power consumption, increases heating, and in extreme cases can permanently damage the transformer core.
This is not just a concern for high-latitude countries. A study of the Iranian power grid found that even at mid-latitudes, transformer performance degraded measurably during geomagnetic disturbances. High-performance transformers showed at least a 55 percent correlation between drops in their power factor and periods when the geomagnetic index indicated storm conditions.10Advances in Space Research. Effect of geomagnetic storms on a power network at mid latitudes The effect was most visible in transformers with certain winding configurations that make them more susceptible to ground currents.
The May 2024 storm pushed the issue even further. Measurements from the Mexican power grid recorded significant GICs at multiple substations during that event, confirming that even low-latitude grids are vulnerable during extreme storms.11Geophysical Research Letters. The Impact of Geomagnetically Induced Currents (GIC) on the Mexican Power Grid: Numerical Modeling and Observations From the 10 May 2024, Geomagnetic Storm Modeling of the 500 kilovolt grid in Guangxi, China found that some substations experienced GIC peaks exceeding 300 amperes during the same storm, with six high-risk substations identified in the southeastern coastal region and three recommended for priority installation of GIC monitoring equipment.12Space Weather. Modeling and Risk Assessment of Geomagnetically Induced Currents During Geomagnetic Storm in the 500 kV Power Grid of Guangxi, China The finding that large GICs can occur at subtropical latitudes has pushed grid operators in countries that previously considered themselves safe to begin reassessing their exposure.
Navigation, Communication, and Transportation
Space weather disrupts the systems that modern transportation depends on. Solar flares can black out high-frequency radio communication across an entire hemisphere within minutes. Ionospheric disturbances degrade GPS accuracy, sometimes by tens of meters or more, which matters for precision agriculture, autonomous vehicles, and aviation instrument approaches. And the radiation environment at flight altitudes rises during solar particle events, potentially affecting crew dose limits on polar routes.13Space Weather. Space Weather Effects on Transportation Systems: A Review of Current Understanding and Future Outlook
The practical impact during a major storm is real but not apocalyptic. Analysis of extreme space weather scenarios has found that if electrical power stays on, the consequences are manageable: intermittent loss of long-distance radio, modest direct satellite losses as a fraction of the total fleet, interference with GPS timing and satellite communications, and no direct damage to consumer electronics.14Space Weather. Extreme Space Weather Impact: An Emergency Management Perspective The big “if” in that sentence is the power grid. Most of the catastrophic scenarios for extreme space weather ultimately trace back to the possibility of widespread, long-duration power outages that cascade through other infrastructure.
An economic angle reinforces this. A statistical analysis of more than 11,000 insurance claims for industrial electrical equipment damage across North America over an eleven-year span found that claim rates rose by about 20 percent on the most geomagnetically active days, those in the top 5 percent by daily magnetic variability. Claims specifically attributed to electrical surges, which made up more than half the sample, tracked closely with major disturbances in the high-voltage grid.15Space Weather. Assessing the impact of space weather on the electric power grid based on insurance claims for industrial electrical equipment Those costs add up across a whole solar cycle, even in the absence of a single catastrophic event.
Radiation Exposure in Aviation and Space
Solar energetic particle events send protons and heavier ions streaming through the solar system at a large fraction of the speed of light. At commercial flight altitudes, especially on polar routes, Earth’s magnetic field provides less shielding, and the additional radiation from a strong solar particle event adds to the background dose from galactic cosmic rays. A detailed reconstruction of the September 2017 solar particle event, one of the strongest of the previous cycle, found that the event was of little significance compared to total exposure from galactic cosmic radiation for most aviation and low-Earth-orbit scenarios. However, for a lightly shielded astronaut in interplanetary space, the skin dose could have reached about 30 to 60 percent of NASA’s 30-day dose limit.16Space Weather. The Solar Particle Event on 10–13 September 2017: Spectral Reconstruction and Calculation of the Radiation Exposure in Aviation and Space
For airline passengers and crews on a single flight, the added dose from even a large event is tiny compared to medical imaging or other common exposures. The concern is cumulative: flight crews who accumulate hundreds of hours per year on high-latitude routes across the peak of a solar cycle face a measurably higher annual dose. Airlines in some countries are required to track crew radiation exposure and reroute flights away from polar corridors during major particle events. As crewed missions beyond low Earth orbit become more common, the radiation risk from solar particle events during solar maximum becomes a serious design constraint for spacecraft shielding and mission timing.
How the Solar Cycle Reaches Earth’s Lower Atmosphere
The Sun’s influence on Earth extends beyond the upper atmosphere. Changes in ultraviolet radiation across the solar cycle alter ozone concentrations in the stratosphere, which in turn affect temperature and wind patterns that can propagate downward into the troposphere where weather happens. Climate model simulations show that the atmospheric response to solar-cycle UV variations has a visible regional character, with temperature changes on the order of a few degrees in the troposphere during winter.17Journal of Geophysical Research: Space Physics. Solar cycle influence on troposphere and middle atmosphere via ozone layer in the presence of planetary waves: Simulation with ARM Global climate modeling with interactive ozone chemistry confirms that upper stratospheric ozone changes can amplify the relatively small variations in total solar brightness across the eleven-year cycle into larger regional climate signals.18PubMed. Solar cycle variability, ozone, and climate
These effects are small compared to greenhouse-gas-driven warming and do not meaningfully contribute to long-term climate trends, but they are detectable in the climate record and can modulate seasonal patterns. Understanding how solar cycles affect stratospheric dynamics is also relevant for predicting the recovery of the ozone layer, since UV flux variations interact with the chemistry of ozone-depleting substances in ways that vary across the cycle.
Forecasting and Why It Remains Difficult
Predicting the strength and timing of a solar cycle remains one of the harder problems in solar physics. Dozens of methods exist, ranging from statistical extrapolation of past cycles to physics-based dynamo models, and they routinely disagree with each other by large margins. One promising approach uses torsional oscillations, subtle patterns of faster and slower rotation bands in the Sun’s interior detectable through helioseismology. These oscillation parameters, including a 22-year mode tied to the full Hale cycle, show a strong correlation with the strength of subsequent cycles when the data is shifted forward by 11 to 20 years.19The Astrophysical Journal. Torsional Oscillations in Dynamo Models with Fluctuations and Potential for Helioseismic Predictions of the Solar Cycles If that correlation holds, it could offer a genuinely physics-based way to forecast solar cycles a decade or more in advance.
The difficulty is that the solar dynamo involves turbulent processes with inherent randomness. Even a perfect model of the average dynamo would not capture the stochastic fluctuations that cause one cycle to be twice as strong as another. Cycle 25 itself is an object lesson: the consensus forecast missed on the low side, and the methods that happened to predict a stronger cycle did so for different and sometimes contradictory reasons. The field is candid about these limitations, and improving predictions is one of the major motivations for current observational missions.
Parker Solar Probe and the View From Close In
Cycle 25 is the first solar cycle observed with a spacecraft flying through the Sun’s outer atmosphere. NASA’s Parker Solar Probe, launched in 2018, has been spiraling closer to the Sun with each orbit, sampling the solar wind and energetic particles at distances no previous mission has reached. In late November 2020, while the probe was at about 0.81 astronomical units from the Sun, two coronal mass ejections swept past, producing the largest energetic particle enhancement seen by the spacecraft during its first eight orbits and the first spatially widespread solar energetic particle event of Cycle 25.20The Astrophysical Journal. Energetic Electron Observations by Parker Solar Probe/IS⊙IS during the First Widespread SEP Event of Solar Cycle 25 on 2020 November 29
Having a probe inside the heliosphere during solar maximum is providing data that was simply unavailable during previous cycles. Researchers can now measure the properties of coronal mass ejections and solar energetic particles much closer to their source, before the solar wind has had time to mix and dilute the signatures. Combined with observations from the European Space Agency’s Solar Orbiter and the constellation of near-Earth spacecraft, this gives scientists a three-dimensional view of how eruptions propagate through the solar system. That perspective is expected to sharpen space weather forecasting in coming years, even if the fundamental unpredictability of the dynamo sets a ceiling on how far ahead useful predictions can reach.
Citizen Science and the Aurora Chasers
The dramatic auroral displays of Cycle 25 have driven a surge in public engagement with space weather. Apps that push alerts when geomagnetic storms are forecast have millions of users, and social media communities share real-time photographs that sometimes provide useful data for researchers, especially from geographic locations that lack professional monitoring stations. This kind of participation is part of a broader trend in citizen science across atmosphere and space physics, where projects range from collecting observational data to hosting instruments in non-scientific structures to contributing to data analysis.21Surveys in Geophysics. Citizen Science in Space and Atmospheric Sciences: Opportunities and Challenges
For anyone hoping to see an aurora during the remainder of Cycle 25, the odds are reasonable. Solar maximum conditions can persist for a year or more around the peak, and individual storms can drive auroras to lower latitudes at any point during that window. The key is monitoring real-time solar wind data and geomagnetic indices, which are freely available from agencies like NOAA’s Space Weather Prediction Center. Even as the cycle begins to decline, isolated strong storms can occur well past the statistical peak, so the opportunity does not vanish overnight.