A coronal mass ejection, or CME, is a massive burst of magnetized plasma launched from the sun’s outer atmosphere into space. Each eruption can release billions of tons of solar material at speeds ranging from a few tens of kilometers per second to more than 2,000 km/s. When one of these clouds of charged particles is aimed at Earth, it can trigger geomagnetic storms that disrupt power grids, GPS systems, and radio communications.
How a CME Forms on the Sun
CMEs don’t happen suddenly. They build over days or even weeks as the sun’s magnetic field gradually becomes stressed and twisted, storing energy like a rubber band being wound tighter and tighter. This stored energy accumulates in structures called magnetic flux ropes, loops of magnetic field lines loaded with hot plasma in the sun’s corona.
At some point, the structure becomes unstable. The flux rope begins to rise, stretching the magnetic field lines above it. As it does, opposing magnetic field lines are forced together and “reconnect,” a process that converts stored magnetic energy into explosive motion and heat. This reconnection has two major effects: it releases an intense burst of radiation (a solar flare) beneath the rising structure, and it severs the magnetic “tethers” holding the flux rope down. Freed from restraint, the flux rope accelerates outward, plowing through the corona and dragging billions of tons of charged particles along with it. The leading edge of the CME is formed by overlying magnetic field lines that get pushed ahead and pile up like a snowplow.
CMEs vs. Solar Flares
People often use “solar flare” and “CME” interchangeably, but they are distinct events. A solar flare is a flash of electromagnetic radiation, light, X-rays, and ultraviolet energy, that travels at the speed of light and reaches Earth in about eight minutes. A CME is a physical cloud of magnetized plasma that moves far more slowly, taking anywhere from 15 hours to several days to cross the roughly 150 million kilometers between the sun and Earth.
Flares and CMEs frequently occur together because both are powered by the same magnetic reconnection process, but not always. A flare can fire without launching material into space, and a CME can erupt without producing a notable flare. When both happen in tandem, the flare arrives first as a burst of radiation, followed hours or days later by the CME’s wall of plasma.
Speed, Size, and Travel Time
A single CME can carry between 100 million and 10 billion metric tons of plasma. The average speed of a CME is roughly 350 km/s (about 780,000 mph), but the range is enormous. Slow CMEs drift outward at just a few tens of kilometers per second, while the fastest exceed 2,000 km/s. The fastest Earth-directed CMEs can reach our planet in as little as 15 to 18 hours. Slower ones take several days.
Not every CME heads toward Earth. The sun launches them in all directions, and only a fraction follow a path that intersects our planet’s orbit. When a CME appears as a halo of expanding material surrounding the sun in coronagraph images (instruments that block the sun’s bright disk to reveal faint structures around it), that’s a strong sign it’s heading straight toward or directly away from Earth. These “halo CMEs” are the ones forecasters watch most closely.
What Happens When a CME Hits Earth
Earth’s magnetic field normally acts as a shield, deflecting the solar wind and most incoming charged particles. A CME can overwhelm that shield, but only under specific conditions. The key factor is the orientation of the CME’s own magnetic field. If the magnetic field embedded in the CME points southward, opposite to Earth’s northward-pointing field, the two fields link up and merge. This opens a pathway for the CME’s energy and particles to pour into Earth’s magnetosphere, triggering a geomagnetic storm.
When the CME’s magnetic field points northward instead, Earth’s shield holds firm and the storm effects are minimal. This directional dependence is why predicting the severity of a geomagnetic storm is difficult: forecasters can see a CME coming, but determining the precise orientation of its magnetic field before it arrives remains a challenge.
Effects on Power Grids
The most serious infrastructure risk from a geomagnetic storm is damage to electrical transformers. As the storm disturbs Earth’s magnetic field, that changing field induces electric currents in any long conductor connected to the ground. High-voltage transmission lines, which can stretch for hundreds of kilometers and are grounded at both ends, are especially vulnerable. These geomagnetically induced currents (GICs) can reach 100 amps or more.
The problem is that transformers run on alternating current (AC), and GICs are essentially direct current (DC). Even a relatively small DC offset pushes the transformer’s magnetic core toward saturation, a state it was never designed to operate in. Once saturated, the transformer begins producing distorted electrical waveforms full of harmonic frequencies. These harmonics generate heat in the transformer windings, and that heat degrades insulation over time. In severe cases, the overheating can cause catastrophic failure. The harmonics can also resonate with other components in the grid, producing voltage spikes that cascade outward and potentially trip protective relays, causing blackouts across wide areas.
Effects on GPS and Radio
GPS signals travel from satellites through Earth’s ionosphere, a layer of charged particles in the upper atmosphere. Under normal conditions, GPS receivers use models of the “quiet” ionosphere to correct for the signal delay this layer introduces, delivering accuracy within a meter for standard receivers and down to a few centimeters for dual-frequency systems.
A geomagnetic storm injects energy and currents into the ionosphere, dramatically increasing the number of free electrons the signal must pass through. The standard correction models can’t keep up with this rapid change, and positioning errors balloon. During a severe storm, single-frequency GPS accuracy can degrade to tens of meters or worse. In extreme cases, receivers lose their lock on satellite signals entirely. High-frequency (HF) radio communications, which bounce signals off the ionosphere, suffer similar disruptions as the ionosphere’s structure becomes chaotic and unpredictable.
How CMEs Are Detected and Tracked
Forecasters rely on space-based coronagraphs to spot CMEs shortly after they leave the sun. These instruments use a disk to block the sun’s blinding light, revealing the faint plasma clouds expanding outward through the corona. Once a CME is detected, analysts estimate its speed, direction, and likely arrival time at Earth.
The warning window depends entirely on speed. A fast CME traveling at 2,000 km/s might give only 15 to 18 hours of lead time. A slower one could provide two to three days. NOAA’s Space Weather Prediction Center issues alerts and watches when an Earth-directed CME is detected, giving power grid operators, satellite controllers, and airlines time to take protective measures like adjusting satellite orientations, rerouting polar flights, or reducing load on vulnerable transformers.