Solar Cycle Twenty-Five, the May Twenty-Twenty-Four G Five Geomagnetic Storm, and What Space Weather Means for Every Pilot Who Trusts a GPS Approach

On May 10, 2024, Earth experienced its first G5 geomagnetic storm since October 2003. GPS accuracy degraded across large portions of the United States, HF radio blacked out completely on the sunlit hemisphere three separate times, and a significant share of transoceanic polar flights were rerouted to lower latitudes - adding up to two to three hours of flight time per crossing. The pilots and dispatchers who handled it smoothly were the ones who had been watching the NOAA forecasts before the storm arrived.

Solar Cycle 25 Is Running Hotter Than Predicted

The sun operates on a roughly eleven-year cycle of magnetic activity. Solar Cycle 25 began at solar minimum in December 2019. Early forecasts predicted a moderate cycle. It has not been moderate. Cycle 25 has produced more frequent and more intense solar events than the models anticipated, and its peak - solar maximum - is happening now, in the mid-2020s.

More sunspots mean more solar flares and more coronal mass ejections (CMEs). More CMEs mean more events that interact with Earth’s magnetic field. And those interactions have direct operational consequences for every pilot who relies on GPS or HF radio.

The next solar minimum is not expected until around 2030 or 2031. The elevated activity is not a temporary anomaly. It is the current operating environment.

The Three NOAA Scales Every Pilot Should Know

NOAA rates space weather events on three separate scales, each running from 1 (minor) to 5 (extreme). Each one affects aviation differently.

R scale - radio blackouts
S scale - solar radiation storms
G scale - geomagnetic storms

Understanding all three matters because a major solar event can produce significant readings across all three simultaneously.

R Scale: HF Radio Blackouts Over the Oceans

When the sun fires an X-class solar flare, X-ray radiation floods the sunlit side of Earth almost instantaneously. That radiation ionizes the ionosphere - the atmospheric layer that makes long-distance high-frequency (HF) radio communication possible.

Over the oceans, HF radio is the primary communications backbone between aircraft and air traffic control. The North Atlantic Track system, Pacific crossing routes, and South Atlantic routes all depend on HF voice for position reports, oceanic clearances, and SELCAL alerts. An R3 event or higher degrades HF radio dramatically on the sunlit hemisphere. An R5 event means complete blackout for an hour or more.

During the week of the May 2024 storm, complete HF blackout occurred three separate times. SATCOM-equipped aircraft have an alternative, but not every aircraft carries it - and even crews with SATCOM need to understand the HF environment for contingency planning. HF is the infrastructure oceanic ATC is built around. When it goes dark, the operational picture changes fundamentally.

S Scale: Radiation Risk on Polar Routes

When the sun accelerates energetic particles toward Earth, those particles funnel into polar regions where Earth’s magnetic field is weakest. Aircraft flying polar routes at altitude receive substantially higher particle radiation doses than aircraft flying comparable altitudes at mid-latitudes.

At 35,000 feet over the pole during an S3 event or higher, crew and passenger radiation exposure becomes a meaningful operational variable. Aviation authorities in Europe and the United States maintain regulatory limits on flight crew radiation exposure, and airlines operating polar routes run active dosimetry programs because of this. An S5 event can deliver radiation doses during a single polar crossing that regulators consider significant.

Rerouting during major S events is not a preference. It is a safety and regulatory obligation.

G Scale: GPS, WAAS, and RAIM During a Geomagnetic Storm

The G scale carries the most complex set of effects on systems pilots use every day. When a CME arrives and its embedded magnetic field interacts with Earth’s own, it drives massive electrical currents through the ionosphere - and those currents create disturbances that cascade across multiple cockpit systems simultaneously.

GPS signals travel through the ionosphere on their way to your receiver. Under normal conditions, your GPS compensates for predictable ionospheric delay. Under a G3 event or higher, the ionosphere becomes turbulent and unpredictable. GPS accuracy degrades. RAIM (Receiver Autonomous Integrity Monitoring) failures become more frequent. WAAS (Wide Area Augmentation System), the system that enables precision GPS approaches in the United States, can become unavailable across large portions of the country.

This matters directly for approach planning. LPV approaches (Localizer Performance with Vertical guidance) and LNAV+V approaches both depend on WAAS availability. WAAS availability is not guaranteed during a major geomagnetic storm. The FAA issues NOTAMs referencing GPS degradation during significant solar events, but those NOTAMs can arrive with limited lead time - space weather forecasting, while meaningfully better than it was fifteen years ago, still carries uncertainty measured in plus or minus six to twelve hours for a CME arrival forecast.

A secondary G-scale effect: compass deviation. During extreme geomagnetic disturbance, magnetic north wanders measurably. During the May 2024 storm, magnetometer stations across the northern United States and Canada recorded deviations of several degrees. Modern cockpits using GPS-aligned attitude and heading reference systems manage this reasonably well. Older aircraft relying primarily on a magnetic compass at high latitudes face a genuinely degraded navigation picture during a major event.

What the May 2024 G5 Storm Actually Did to Aviation

Active Region 3664 - a large, complex sunspot cluster - produced a series of X-class solar flares and multiple CMEs between May 8 and May 11, 2024. Those ejections arrived at Earth in close succession, compounding the total geomagnetic effect. From the evening of May 10 through the following day, Earth sustained a G5 geomagnetic storm - the most severe on the scale, and the first since the October 2003 Halloween storms.

The Northern Lights were visible from Florida, Texas, and parts of Mexico. That geography illustrates the scale of the disturbance: aurora that far south means the magnetosphere was being compressed at a level most people alive today had never witnessed. European power grid operators went on high alert for geomagnetically induced currents in transmission infrastructure.

For aviation, the primary operational consequence was polar routing. A significant portion of transoceanic flights between North America and Europe, and between North America and Asia, were rerouted to lower latitudes. Depending on specific routing, some flights added two to three hours of flight time - an enormous operational cost, accepted to protect crews, passengers, and navigation integrity.

The carriers that handled it smoothly had been monitoring NOAA forecasts in the days before the CME arrived. Their flight operations teams had assessed R, S, and G forecasts before route release. When those forecasts crossed defined thresholds, rerouting was a straightforward decision. Preparation made it operational, not reactive.

Why This Also Affected Domestic Flights - And Mostly Went Unnoticed

The broader story of the May 2024 storm is what happened to domestic GPS operations - effects that were largely invisible to pilots who were not looking for them. WAAS availability across portions of the United States was degraded during the peak of the storm. RAIM availability was reduced over large areas.

Most flights landed without incident because other approach options were available. But a pilot who had planned a trip assuming LPV minimums at an alternate with no ILS - and who arrived during a period of WAAS outage - would have been in a genuinely difficult position. Not illegal. Not negligent. Just in a situation that a brief preflight check might have reshaped.

The fix is straightforward. During periods of elevated solar activity, build non-GPS approaches into your alternate planning. Verify that your planned alternates have ground-based approaches available. Checking the current R, S, and G forecasts before departure takes about thirty seconds on the NOAA Space Weather Prediction Center website. That thirty seconds can meaningfully change your alternate planning. It warrants the same habit as checking the convective outlook.

Where Space Weather Forecasting Is Headed

Two developments are worth tracking.

The first is avionics integration. Some avionics developers are beginning to explore space weather data feeds that would appear alongside weather and terrain data in flight planning and cockpit displays. Space weather data should eventually sit alongside convective data and TFRs in an integrated preflight picture. That integration does not yet exist in most cockpits, which is why the burden currently falls on pilots to seek the data out themselves.

The second is the regulatory infrastructure. Since 2019, ICAO has designated four Global Space Weather Centers to provide forecasting products to aviation. These centers coordinate to issue space weather advisories and SIGMET-equivalent products across the international aviation system. The data products exist. What lags is pilot familiarity with them.

The analogy to meteorological forecasting is instructive. Forty years ago, pilots worked from surface analysis charts and basic telephone weather briefings. Today, real-time radar, satellite-derived turbulence products, convective SIGMETs, and integrated EFB weather are standard. Space weather is on a similar development trajectory, running roughly ten to fifteen years behind meteorological integration. The data infrastructure is being built now, during Solar Cycle 25.

The pilots entering the profession today will likely have space weather integrated into their preflight workflow the same way radar is now.

Key Takeaways

Solar Cycle 25 is more active than forecast, and solar maximum is happening now. Elevated activity will continue through the mid-2020s, with the next solar minimum not expected until around 2030–2031.
The May 10–11, 2024 G5 storm - the first since October 2003 - disrupted HF radio three times, degraded WAAS across the United States, and forced transoceanic polar reroutes adding up to two to three hours per flight.
The three NOAA scales (R, S, G) each affect aviation differently: R affects HF radio, S affects radiation exposure on polar routes, and G affects GPS/WAAS/RAIM accuracy and compass reliability.
Checking the NOAA Space Weather Prediction Center before departure takes thirty seconds and should carry the same weight as checking the convective outlook during periods of elevated solar activity.
ICAO’s four Global Space Weather Centers have been issuing SIGMET-equivalent space weather advisories since 2019. The data infrastructure exists - pilot familiarity with it is the gap.