What are Coronal Mass Ejections (CMEs)?

While commonly lumped in with electro-magnetic pulses (EMPs), the coronal-mass ejection is a separate event that is worth understanding.

The Sun Burps

Yes, the sun burps. Regularly. And these burps actually have two parts:

• Solar Flares
• Coronal Mass Ejections (CMEs)

A solar flare is a sudden, intense flash of electromagnetic radiation. A CME is a massive physical ejection of physical material (plasma) from the sun. I’ve heard the two compared to a gunshot, and it’s a good analogy. The flash is the solar flare, while the projectile is the CME. It should be noted that solar flares and coronal mass ejections can occur independently and do not require each other even though they frequently occur together.

How often the sun has these “burps” varies. The sun actually goes through a repeating cycle where the sun becomes stronger and weaker over time. One full cycle sees both the highest strength (solar maximum) as well as the lowest strength (solar minimum) and this entire cycle takes an average of 11 years. During a solar minimum, we may see fewer than one flare per week. During a solar maximum we can have multiple flares per day. As I write this, we are nearing the end of a solar maximum (2024-2026) and we will expect to see solar activity decrease in the coming years with the next minimum expected to occur around 2030.

That said, the sun doesn’t always stick to our schedule. Researchers posted in mid-2025 that they were already seeing potential signs of the next cycle beginning even though we’re only mid-way through the current cycle. As a result, don’t discount this threat as not applying for another decade as we could be in for a surprise much earlier.

Behavior & Effects

As you would assume, solar flares and coronal mass ejections (CMEs) behave very differently. Solar flares travel at the speed of light as they are only electromagnetic waves. This means they reach earth about 8 minutes after they occur. Flares aren’t harmless unto themselves. They ionize Earth’s upper atmosphere, which can affect radio communication. Typically, they cause temporary blackouts for radio communication, GPS navigation, etc. As you would imagine, there are different “severities” to solar flares, and there is a rating system for them:

• Small (C-class): These are minor and often occur multiple times per day at solar maximum. These have essentially no effect on Earth.
• Medium (M-class): These are moderate flares that trigger brief radio blackouts. Their frequency peaks at a few a day at solar maximum.
• Severe (X-class): These are the most powerful eruptions, triggering widespread communication issues and intense auroras. The Sun produces about 10 to 20 X-class flares per year on average, with most occurring during solar maximums.

CMEs are made of plasma and therefore have physical mass, so they move much slower. They normally travel between 150 and 1,900 miles per second. This means it can take between a few hours and a couple days to reach earth, which is good - it means we have warning. What’s generated doesn’t move like a bullet either. A shotgun is a better analogy as while the shot is directional, it does have some “spread” as it travels outward from the sun.

When a CME reaches Earth is when things get interesting. When the plasma hits Earth’s magnetosphere (magnetic field), the plasma causes the magnetosphere to compress and then snap-back, similar to a balloon being poked. This snap back creates powerful geomagnetic storms within our atmosphere, which can induce massive currents into long conductors like power lines, antennas, etc. This can cause regional blackouts and even damage infrastructure in severe events. The plasma can also directly affect satellites, degrading their electronics. CMEs have two different ratings. The first is the initial ejection rating, using the S.C.O.R.E. system:

• S-type: Slow CMEs, traveling under 500 km/s.
• C-type: Average CMEs, traveling between 500 and 999 km/s.
• O-type: Fast CMEs, ranging from 1000 to 1999 km/s.
• R-type: Very fast CMEs, moving between 2000 and 2999 km/s.
• E-type: Extremely fast CMEs, traveling at speeds of 3000 km/s or greater.

These ratings don’t consider the size or density of an event, only speed. The biggest risk to Earth comes from large, dense, fast-moving events as they have the largest effect on our magnetosphere. When a CME hits Earth, its severity is rated according to the following scale:

• G1 (Minor) - Kp=5: Weak fluctuations in power grids, minor impacts on satellite operations, and aurora visible at high latitudes.
• G2 (Moderate) - Kp=6: Voltage alarm systems at high latitudes may require corrective actions; high-frequency radio propagation can fade at high latitudes.
• G3 (Strong) - Kp=7: Power systems may require voltage corrections; satellite navigation and low-frequency radio navigation can be degraded. Aurora can occasionally be seen further south (e.g., in Illinois or Oregon).
• G4 (Severe) - Kp=8: Possible widespread voltage control problems, satellite surface charging, and aurora visible as far south as mid-latitudes (e.g., down to Missouri and other lower states).
• G5 (Extreme) - Kp=9: Widespread power grid and transformer damage possible, complete HF radio blackouts over large areas, and aurora visible very close to the equator.

A unique aspect of CMEs is that they can last for days in severe instances. This long timeframe is part of the reason they can be so damaging to power transformers and other key components within power grids. Another unique aspect is that the effects vary by geography. How far north you are is the key factor. The closer to the north pole, the greater the effects. This means that those in the southern United States and Hawaii are relatively safe from this threat directly. That said, consider the implications to your location if all those in the north no longer have power - even if you’re spared direct effects, you will most certainly suffer from secondary effects.

The Odds

While we think of Earth being huge, in celestial terms it’s tiny. Earth has a diameter of just under 8,000 miles and sits about 93 million miles away from the sun. Unfortunately, most CMEs are located along the sun’s equator, which is in line with earth’s orbit, greatly increasing the chances of impact. This means the odds of a given CME impacting Earth is about 0.0014%. These are still low odds, but when you consider the number of events, over time our number will come up. Here are a few notable events:

• May 2024 - multiple CMEs created large geomagnetic storms that created auroras that were seen as far south as the Bahamas, Yucatan Peninsula and Hawaii. The storms caused degradation in GPS performance as well as interference with high-frequency radio communications. This was the largest storm since March of 1989.
• July 2012 - A near-miss that is considered the most powerful event in 150 years. If it had hit Earth, it would have caused trillions of dollars in damage to power grids globally.
• October 2003 - A series of intense X-class flares and CMEs triggered G5 geomagnetic storms, causing worldwide aviation disruptions, satellite issues and power grid fluctuations.
• July 2000 - Bastille Day event caused by a massive X5.7 solar flare and associated halo CME. Caused major satellite damage, radio blackouts and GPS disruption.
• March 1989 - substantial communications blackouts as well as significant interference with the U.S. power grid. The Canadian power grid was impacted even more severely and a large portion around Quebec completely failed for a period of about 9 hours. Many power transformers in Canada were irreparably damaged and had to be replaced.
• September 1859 - The largest known event is commonly called the Carrington Event. Due to the limited technology in use at the time, the effects were contained. But, it was able to set fire to telegraph wires and cause other physical damage to communication and power systems. Should a similar sized CME hit earth today, it’s expected it would cause significant and long-lasting damage to the power grid.

In 2019 researchers used a Weibull distribution to try and estimate the chance of Earth being hit by a Carrington-level event in the next decade. They determined the chances were between 0.46% and 1.88%. While seemingly low, you need to understand that these percentages compound over time. So, the chance in the next 50 years is 1.88⁵ which equates to up to 23.5%. Given it’s been over 150 years since the last major event, some scientists say we’re overdue.

Bottom Line

So, why should you as a prepper care about CMEs? The simple answer is power. The modern world is extremely reliant upon electricity. A Carrington-level event today would likely cause a near-complete destruction of the power grid in North America. Much smaller events can create regional blackouts that persist for days or weeks.

Even if you’re at a southern latitude, I recommend you consider the effects if this did occur and affect those to your north. At a minimum it could mean major increased demand from your power grid. In a worst-case scenario, it could mean refugees fleeing south.

How can you prepare?

This isn’t something you can prevent. The only real thing that preppers can do is mitigate the effects and prepare for the aftermath. Specifically, you can:

• Be aware as this event offers a warning and preppers should be ready to take advantage of this warning. One resource is NASA’s Space Weather Site.
• Keep critical preparations unplugged when not in use. Due to the signals involved, no faraday cages are required here to protect electrical devices (unlike for EMP). As long as the device isn’t connected to long wires, it should be safe.

Beyond this, you need to prepare for this like you would for any long grid-down event.

• Backup power to keep critical electronics functional.
• Non-electric heat, light, and cooking.
• Comms that work without the network
• Water and food that don’t depend on refrigeration.
• Prepare for refugees fleeing more severely affected areas.