SpaceX launches the SAOCOM 1A satellite for Argentina's Space Agency on October 7 from Vandenberg Air Force Base in California.

It happened again: SpaceX launched a rocket from the coast of California at dusk on Sunday, and the atmospheric conditions created a stunning visual display. Social media lit up in response, as people across Los Angeles took in the rocket's glowing plume, which was visible from hundreds of miles away.

The U.S. Air Force released sonic boom warnings to California residents ahead of the launch, as this was also the first time SpaceX landed its rocket back near the West Coast launch site. On its return a few minutes after launch, the first stage of the Falcon 9 rocket created a sonic boom that could be heard in several California counties.

Despite the advance warnings, Angelenos took to social media to speculate over the glowing cloud that appeared above the city.

@NathanBoucaud: UFO... only in LA

@ezass: I just nearly had a heart attack watching the SpaceX rocket separate from its booster right above the LA skyline

LA Mayor Eric Garcetti explained in a tweet that the glow was "definitely not aliens."

"What you're looking at is the first launch and landing of the SpaceX Falcon 9 rocket on the West Coast," Garcetti said.

@MayorOfLA: Nope, definitely not aliens. What you're looking at is the first launch and landing of the @SpaceX Falcon 9 rocket on the West Coast. The rocket took off from Vandenberg Air Force Base at 7:21 p.m. and landed safely back on Earth.

This phenomenon has happened before. In December, SpaceX launched the Iridium-4 mission just after sunset, which also created a highly visible plume in the sky. That launch caused similar reactions of shock and confusion across LA.