CHICAGO—A fireworks show that has nothing to do with the Fourth of July and everything to do with the cosmos is poised to be visible across the northern United States and Europe Sunday night. On Thursday, the sun launched what is called an “X-class solar flare” that was strong enough to spark a high-frequency radio blackout across parts of South America. The energy from that flare is trailed by a cluster of solar plasma and other material called a coronal mass ejection, or CME for short. That’s heading toward Earth, prompting the National Oceanic and Atmospheric Administration to issue a warning about a potentially strong geomagnetic storm. It might sound like something from a science fiction movie. But really it just means that a good chunk of the northern part of the country may get treated to a light show called the aurora borealis, or Northern Lights. Geomagnetic storms as …