The strongest category of solar flares, known to potentially cause worldwide transmission problems and blackouts, could be emitted this week, scientists say. On Sunday, radio blackouts were already detected, though scientists did not say where.

The warning comes from scientists from both the U.S. and Russia. The latter, from Moscow's Fedorov Institute of Applied Geophysics, said on Sunday that they had observed three solar flares that day and that they believed X-class flares are possible on Monday, according to Reuters. 

X-class flares are the biggest category of solar flare activity, and are essentially "explosions on the surface of the sun ranging from minutes to hours in length," according to NASA, which calls X-class flares "the real juggernauts." 

"Large flares can release enough energy to power the entire United States for a million years," NASA says, adding that the most powerful X-class flare ever recorded was in 2003. That event "was so powerful that it overloaded the sensors measuring it," NASA says. 

"A powerful X-class flare like that can create long-lasting radiation storms, which can harm satellites, and even give airline passengers flying near the poles small radiation doses," said the agency. "X flares also have the potential to create global transmission problems and worldwide blackouts."

Unlike geomagnetic storms, which are known for causing electrical power outages and driving intense viewings of the northern lights, solar flares directly affect Earth's radio communications and release energetic particles into space, the European Space Agency says. Strong flares affect the ionosphere, which is the layer of the atmosphere that conducts electricity. The ionosphere is the atmospheric level that interacts with radio waves, and such impacts cause radio signals to "become degraded or completely absorbed," NASA says, resulting in a radio blackout. High-frequency radio between 3 and 30 megahertz — such as GPS — is primarily what's affected.

NOAA's Space Weather Prediction Center has also said in its latest forecast that there is a "chance" of a strong X-class event on Monday or Tuesday, with another "slight chance" of them appearing on Wednesday. The events on Monday or Tuesday could be an R3 on its radio blackout scale of R1-R5, NOAA said, meaning they have the potential to cause a "wide area blackout of HF radio communication" with a loss of radio contact for roughly an hour in some parts of Earth. 

Radio blackouts have already been observed within the past 24 hours, NOAA said in its Monday forecast. There's at least a 50% chance for smaller radio blackouts through Wednesday, the agency said, with a 25% chance for the R3 blackout on Monday and Tuesday, a likelihood that decreases to 15% on Wednesday. 

Are solar flares dangerous? 

Just a few weeks ago, fears of an "internet apocalypse" that could happen within the decade due to activity on the sun went viral. The term seems to have come from a  2021 paper about solar storm impacts, in which a researcher described a "solar superstorm" that could cause global internet outages for months. 

While extreme geomagnetic storms can cause blackouts and grid systems to collapse, such events are only expected to happen once every 500 years. The last time such an event happened was 164 years ago.

NASA explains that solar flares become "bigger and more common" every 11 years, when the sun reaches its maximum activity in its cycle. This cycle has "ramped up much faster" than what scientists originally predicted, but it's still expected to be an "average" cycle overall compared. 

Most solar flares aren't dangerous to humans on Earth.  

"Earth's atmosphere absorbs most of the Sun's intense radiation, so flares are not directly harmful to humans on the ground," NASA says. "However, the radiation from a flare can be harmful to astronauts outside of Earth's atmosphere, and they can affect the technology we rely on."

Solar flares are ranked from A-class, which are essentially "background levels," to X, which are the strongest flares, with the rankings of B, C and M in between. Each of those classification levels represents a 10-fold increase in energy output, NASA says, meaning that an X-class flare, for example, is 10 times stronger than an M. Each of those classes is then broken down to a number, from 1 to 9. 

C-class and weaker flares don't noticeably affect the planet, while strong flares — those rated at an M5 or higher — can impact technology as it affects the planet's ionosphere, which is used by navigation and GPS. If the light from the flare hits Earth, it can also cause electrical surges or light flashes in the ionosphere that creates radio signal blackouts that last, in the worst case, up to "hours at a time," NASA says, which could impact radios used for emergency communications. 

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Li Cohen

Li Cohen is a social media producer and trending content writer for CBS News.