Northern lights filled the night sky Sunday but probably will not be as intense tonight, though they may return Wednesday.

“Because of the solar flares erupting on the sun, we have an especially good chance of seeing northern lights Wednesday,” NIU Meteorologist Gilbert Sebenste said.

Sunday night’s aurora was a surprise, mainly because the heavy activity from the sun wasn’t expected, NIU observatory manager Andrew Morrison said.

An extreme geomagnetic storm that began Sunday helped produce the northern lights that night and may cause the lights to appear on Wednesday night.

The lights occur when a coronal mass ejection from the sun hits the earth’s magnetic field, causing the night sky to change colors.

Spaceweather.com tracks what the sun has been doing and is a good source for finding out if conditions are right for northern lights to occur, Morrison said.

The observatory will be open from 8 to 10 p.m. Wednesday, and Morrison said he may try to take pictures of the northern lights if they are visible.

“If the aurora is visible in the sky, we will definitely look at it; if not, we will try to look at something different; we are really at the mercy of the weather,” Morrison said. “It’s hard to tell how intense it will be Wednesday night.”

The best chance to see the northern lights will be on Wednesday night, although there is a high chance of cloud cover, Sebenste said.