A solar maximum is the peak of the Sun’s roughly 11-year activity cycle, when sunspots, solar flares, and massive eruptions of charged particles are at their most frequent and intense. The current cycle, Solar Cycle 25, is predicted to reach its maximum around July 2025, with a peak sunspot number of about 115.
The 11-Year Solar Cycle
The Sun isn’t a steady, unchanging ball of light. It cycles between periods of calm and periods of intense magnetic activity over approximately 11 years. At the quiet end of this cycle, called solar minimum, the Sun’s surface is relatively smooth and featureless. At the active end, solar maximum, it’s covered in dark sunspots and prone to violent outbursts.
What drives this cycle is the Sun’s magnetic field. The Sun is a churning ball of electrically charged gas, and that motion generates powerful magnetic fields that twist, tangle, and build up over time. At solar maximum, those tangled fields break through the surface as sunspots and release stored energy as flares and coronal mass ejections (CMEs), which are billion-ton clouds of magnetized plasma hurled into space. At the height of each cycle, the Sun’s magnetic poles actually flip, the equivalent of Earth’s North and South poles swapping places. After the flip, activity gradually winds down toward the next minimum, and the whole process starts again.
How Scientists Track It
The simplest measure of where the Sun is in its cycle is the sunspot number. Sunspots are cooler, darker patches on the Sun’s surface where intense magnetic field lines poke through. During solar minimum, the Sun can go days or weeks with no visible sunspots at all. During solar maximum, the monthly average sunspot number typically climbs above 100, though this varies from cycle to cycle. Some maximums are fierce, with sunspot counts well above 200, while others are relatively mild.
Scientists at NOAA’s Space Weather Prediction Center track sunspot numbers, solar flare counts, and radio emissions to gauge the cycle’s progression. For Solar Cycle 25, the official prediction panel expects a peak sunspot number between 105 and 125, with the peak window falling somewhere between November 2024 and March 2026. That prediction was updated in February 2025 based on new observational data.
What Happens During Solar Maximum
The Sun becomes far more prone to explosive events. Solar flares are sudden bursts of light and radiation that travel to Earth in about eight minutes. CMEs are slower but more physically disruptive: massive clouds of charged particles that take one to three days to reach Earth. When a CME slams into Earth’s magnetic field, it triggers a geomagnetic storm.
These storms are what produce most of the effects people notice. The most visible one is the aurora. During strong geomagnetic storms at solar maximum, the northern and southern lights can appear far from their usual polar latitudes, sometimes visible in the southern United States or northern Australia. The September 1859 Carrington Event, the most powerful geomagnetic storm on record, produced auroral displays visible near the tropics. That event followed a solar flare so intense that it was visible to the naked eye on the Sun’s surface, something that has never been observed again since.
Effects on Technology
Solar maximum matters to modern life because our technology is vulnerable in ways it wasn’t in 1859. The main risks fall into three categories: navigation systems, power grids, and satellites.
GPS and other satellite navigation systems rely on radio signals passing through the ionosphere, a layer of electrically charged particles high in the atmosphere. Solar flares flood the ionosphere with extra energy, causing the signals to scatter or degrade. Solar radio bursts can also swamp the receivers directly by increasing background noise. During strong events, GPS accuracy can drop significantly or fail entirely for minutes to hours.
Power grids face a different problem. When a CME distorts Earth’s magnetic field, it induces electric currents along the planet’s surface. These geomagnetically induced currents flow into the grid through the grounded connections of large transformers. The currents are essentially direct current (DC) flowing through equipment designed for alternating current (AC), and even small amounts of DC can saturate a transformer’s core, causing overheating and potential damage. Surface voltages during severe storms can reach hundreds of volts, driving dangerously high currents into the grid. Regions with resistive bedrock, like parts of the northeastern United States and Scandinavia, are especially vulnerable because the currents concentrate more easily there.
Satellites in orbit face increased drag from the expanding upper atmosphere, which heats up and puffs outward during solar storms. This can alter satellite orbits unpredictably. The electronics on board are also exposed to higher radiation levels, which can cause glitches or permanent damage.
Radiation Risks in Space
For astronauts, solar maximum creates a complicated tradeoff. The intense solar activity produces dangerous bursts of radiation from flares and CMEs that can deliver a high dose in a short time. A single powerful event could force crew members on the International Space Station into shielded areas for protection.
Counterintuitively, though, the overall background radiation exposure from deep-space cosmic rays actually drops during solar maximum. The Sun’s stronger magnetic field during this phase deflects more of the high-energy particles streaming in from outside our solar system. Behind typical spacecraft shielding, cosmic ray exposure at solar maximum is roughly half what it is at solar minimum. For planning long missions to the Moon or Mars, this means solar maximum offers lower baseline radiation, but with the added risk of sudden, intense solar particle events that require quick shelter.
The Carrington Event Benchmark
The 1859 Carrington Event remains the standard reference point for worst-case solar storms. Over a span of about ten days in late August and early September of that year, the Sun produced sunspots, flares, and magnetic storms daily. The most dramatic moment came on September 1, when astronomer Richard Carrington observed a white-light flare on the Sun’s surface. About 18 hours later, a massive geomagnetic storm struck Earth, a transit time that implies the CME was traveling at extraordinary speed.
At the time, the telegraph was the only electrical infrastructure, and operators reported sparks flying from their equipment. The storm caused no measurable economic or health effects because there simply wasn’t much technology to disrupt. A comparable event today would be a different story. The Halloween storms of November 2003, which damaged satellites and caused power grid problems in Sweden, were estimated to be significantly weaker than the Carrington Event. A true Carrington-scale storm hitting modern infrastructure remains one of the most studied scenarios in space weather risk assessment.
Where We Are Now
Solar Cycle 25 has been more active than initially predicted. The original 2019 forecast called for a relatively weak cycle, but the Sun has consistently outperformed those expectations. NOAA’s updated prediction, published in February 2025, reflects this stronger-than-expected activity. With the predicted peak window of November 2024 through March 2026, we are currently in or very near the maximum phase of this cycle.
In practical terms, this means the next year or two will see the highest likelihood of strong solar flares, geomagnetic storms, and visible auroras at lower latitudes. Activity doesn’t shut off like a switch after the peak. Solar maximum is more of a broad plateau, and significant storms can occur a year or two on either side of the official peak. The Sun will gradually settle back toward minimum over the following several years before the cycle begins again.