The May Twenty-Twenty-Four Geomagnetic Storm, the G5 Event That Rerouted Transatlantic Flights, and What Solar Weather Does to the GPS Signal Every Pilot Depends On

On May 10, 2024, the most severe geomagnetic storm since 2003 hit Earth’s magnetic field - a G5 event, the top of NOAA’s geomagnetic storm scale - and its effects reached directly into the cockpit. GPS precision approach accuracy degraded across the continental United States, HF radio blacked out over the North Atlantic for hours, and at least five major carriers rerouted transatlantic flights to avoid elevated polar radiation exposure. Solar Cycle 25 remains near its peak, and the conditions that produced that storm have not gone away.

What Made the May 2024 Storm a G5

The event originated at Active Region 3664, a sunspot cluster with a combined area roughly 17 times the size of Earth. Between May 8 and May 12, 2024, that region fired off a series of X-class solar flares - the highest classification on the solar flare scale - each generating a coronal mass ejection: an enormous cloud of magnetized plasma flung toward Earth at millions of miles per hour.

When those clouds arrived, Earth’s geomagnetic field absorbed the impact. The planetary K-index peaked at 9, the maximum value on the scale. NOAA classified the storm as G5, its most severe category. Aurora borealis was visible as far south as Texas, Alabama, and parts of Mexico.

The last G5 before this was the Halloween storms of October 2003. Before that, March 1989 - the storm that knocked out Quebec’s power grid for nine hours and left six million people without electricity.

How Solar Flares Black Out HF Radio

HF radio works by bouncing signals off the ionosphere - the electrically charged atmospheric layer extending from roughly 60 to 600 miles above Earth’s surface. Oceanic routes depend on it entirely. North Atlantic track crossings, North Pacific routes, and in-flight position reports all run on HF. When it’s gone, so is that communication layer.

X-class flares produce intense X-ray bursts that travel at the speed of light and reach Earth approximately eight minutes after the flare. Those X-rays dramatically increase ionization in the lower ionospheric layers, absorbing HF signals before they can bounce anywhere useful.

During the May 2024 event, NOAA logged a major HF radio blackout at storm peak. Signals degraded or blacked out entirely across multiple frequency bands for hours. Aircraft over the North Atlantic during that window shifted entirely to satellite communications and SELCAL to maintain contact with oceanic control centers.

Most modern widebody aircraft have satellite communications capability and can work around an HF blackout. But the degradation was real and documented. Any pilot whose emergency procedure calls for transmitting on guard should understand that during a major geomagnetic event, that HF guard frequency may not reach anyone.

What the Storm Actually Did to GPS Approach Minimums

GPS signals travel through the ionosphere from satellites at approximately 12,500 miles altitude. Under normal conditions, the ionospheric delay those signals accumulate introduces small, predictable timing errors. The Wide Area Augmentation System (WAAS) - the FAA’s satellite augmentation network - continuously monitors those errors from ground stations across North America and broadcasts real-time corrections back to your receiver.

That correction is what makes Localizer Performance with Vertical guidance (LPV) approaches possible - precision GPS approaches with 200-foot decision altitudes and visibility requirements comparable to a Category I ILS. Thousands of airports have LPV approaches and no ILS at all.

During a G5 event, the ionosphere becomes turbulent. Total electron content spikes and shifts in ways that are extremely difficult to model in real time. Position errors normally under 3 meters can jump to 20, 50, or over 100 meters. WAAS can correct moderate ionospheric disturbance. A G5 can overwhelm its ability to certify signal integrity.

When that happens, WAAS does exactly what it’s engineered to do: it declares it cannot guarantee the signal. Navigation displays may drop from LPV to LNAV+V, then to LNAV only. Some approaches become unavailable entirely. The system is protecting you by telling you honestly that it cannot make a safety guarantee at that moment.

The FAA issued space weather NOTAMs for the continental United States during the May 2024 event - not advisory language, a formal notice that GPS accuracy and WAAS availability could not be guaranteed during the storm window. Ground-based ILS approaches were completely unaffected. Any pilot planning a GPS-only approach to minimums during those hours was working with reduced assurance they may not have been fully aware of.

Radiation Exposure and Why Airlines Rerouted Polar Flights

Radiation dose rates at cruise altitude are already substantially higher than at sea level. Many aviation authorities classify flight crews as occupationally exposed radiation workers and require tracking of cumulative dose. That is the baseline before a solar storm.

During a major geomagnetic storm, the sun ejects high-energy protons accelerating close to the speed of light. These Solar Energetic Particle (SEP) events can reach Earth within 30 minutes of the initiating flare. They penetrate far deeper into the atmosphere than normal cosmic radiation, particularly over the polar regions where magnetic field lines converge and provide the least natural shielding.

A crew flying a polar route - New York to London over the polar cap - during the peak of the May 2024 event could have received radiation exposure several times the normal rate for that flight duration.

Several major carriers had been monitoring Active Region 3664 for days before the peak. NOAA’s Space Weather Prediction Center had been issuing elevated storm probability forecasts well in advance, with multiple X-class flares and CMEs anticipated before the event hit maximum. At least five major international operators preemptively rerouted polar flights to lower-latitude tracks around May 10 and 11, 2024. Those reroutes added flight time and fuel burn. They also kept crews below radiation exposure thresholds established as operational limits.

That is operational space weather monitoring translated into dispatch decisions - the same logic as checking convective SIGMETs before release, applied to a threat originating 150 million kilometers away and requiring a longer planning horizon.

The Technology Closing the Vulnerability Gap

The GPS Block III satellites - the newest generation in the constellation - carry the L5 signal, a second civilian GPS frequency added with aviation safety specifically in mind. A receiver using both L1 and L5 simultaneously can directly measure ionospheric delay from the difference between those two signals. That removes ionospheric uncertainty from the position calculation in a way that WAAS corrections alone cannot fully replicate.

Dual-frequency GPS receivers are significantly more resistant to geomagnetic disruption than single-frequency units. Aviation-certified dual-frequency avionics are starting to appear in the market, though they are not yet widespread in the general aviation fleet. The FAA and ICAO are actively developing the regulatory framework for eventually requiring dual-frequency capability in certain operations.

NOAA’s forecasting capability is also improving. The Space Weather Follow-On instrument package, positioned at the L1 Lagrange point between Earth and the Sun, provides continuous real-time solar wind measurements before a storm front reaches Earth’s magnetic field. The stated goal is reliable 48-hour forecasting for major geomagnetic events - the difference between a planned operational reroute and a reactive scramble.

Practical Steps for IFR Pilots During Active Solar Periods

Solar Cycle 25 remains near its peak. Activity has consistently exceeded initial forecasts. The probability of additional G3, G4, and G5 events in the next 12 to 18 months remains elevated before the cycle begins declining toward solar minimum.

Space weather NOTAMs in your preflight briefing are real operational information. When you see one, understand what it means before planning to an approach that relies solely on WAAS. Know whether your destination and alternate have ILS or VOR approaches that don’t depend on satellite augmentation. That knowledge is your contingency.

NOAA’s Space Weather Prediction Center at swpc.noaa.gov shows real-time geomagnetic conditions. During periods of elevated solar activity, checking the current K-index before filing takes 30 seconds. A G3 event is manageable in most operational contexts. A G5 is a different conversation - one that begins before engine start.

For oceanic routes, know your SELCAL and satellite communications procedures for the HF blackout scenario. The May 2024 event was a documented, real-world demonstration of exactly what those procedures exist for.

The Larger Risk Picture

The Carrington Event of 1859 was the most powerful geomagnetic storm in recorded history. It induced currents that melted telegraph wires. Aurora was visible in the Caribbean. What an event of that magnitude would do to the GPS constellation, to satellite communications infrastructure, and to the ground-based systems supporting air traffic control and instrument approach procedures is genuinely uncertain. Estimates range from significant disruption to recovery measured in weeks.

Aviation has built its navigation and communication infrastructure on space-based systems. The star powering that infrastructure operates on its own timeline and is not fully predictable.

The May 2024 storm was a rehearsal at manageable scale. The monitoring systems worked. Airlines made informed dispatch decisions. The procedures held. It also produced a clear and documented picture of exactly where the vulnerabilities live - and that picture is worth understanding before the next G5 arrives.

Key Takeaways

• The May 2024 G5 geomagnetic storm - the strongest since October 2003 - caused documented HF radio blackouts over the North Atlantic, GPS/WAAS degradation across the continental US, and elevated radiation on polar routes.
• WAAS-dependent LPV approaches can degrade or become entirely unavailable during major geomagnetic events; ground-based ILS and VOR approaches are unaffected.
• At least five major carriers proactively rerouted polar flights around May 10–11, 2024 based on advance forecasts from NOAA’s Space Weather Prediction Center.
• Dual-frequency GPS receivers (L1 + L5) are significantly more resistant to ionospheric disruption than single-frequency units; certified avionics are entering the market as the FAA and ICAO develop supporting regulations.
• Solar Cycle 25 remains near peak - check swpc.noaa.gov and review space weather NOTAMs as a standard part of IFR preflight during active solar periods.