Strong M-class and X-class flares and can trigger coronal mass ejections — a large release of plasma and magnetic field from the sun. This behavior can disrupt Earth's magnetosphere and result in geomagnetic storms. Such geomagnetic storms can lead to auroras closer to the equator than is possible during calm conditions.

In 1989, a large solar flare accompanied a coronal mass ejection and hit Earth, plunging the entire province of Quebec, Canada, into an electrical blackout that lasted 12 hours, according to a NASA statement. The solar eruption triggered a geomagnetic storm on Earth, resulting in aurora borealis, or northern lights, that could be seen as far south as Florida and Cuba.

During an eruption, M-class and X-class flares can also cause minor to extensive radio blackouts on the side of Earth facing the sun. Radio blackouts mostly affect High Frequency (HF) (3-30 MHz) radio communications though sometimes Very High Frequency (VHF) (30-300 MHz) and higher frequencies can be affected, according to SpaceWeatherLive…

Powerful flares can significantly affect spacecraft, satellites and ground-based technologies, and don't give us a whole lot of notice before striking. Numerous organizations — including NASA, NOAA and the U.S. Air Force Weather Agency (AFWA) — are keeping a watchful eye on the sun and monitoring it closely for powerful flares and associated magnetic storms. These organizations can send out warnings to technology sectors vulnerable to solar flare activity so that appropriate precautions can be taken.

"We can't ignore space weather, but we can take appropriate measures to protect ourselves," NASA says.

In general, solar flares are nothing to worry about. So-called "killer flares" do not exist and although solar flares can significantly disrupt the technological world, they don't contain enough energy to do any lasting damage to Earth itself.

"Even at their worst, the sun's flares are not physically capable of destroying Earth," NASA says.

https://www.space.com/solar-flares-effects-classification-formation