Among the many signs of fall in the Twin Cities: leaves turn crimson and gold, pumpkin spice lattes abound, and the enchanting lights atop Target Plaza South flip on not just right before sunset, but also at 4:30 a.m. every morning because of the later sunrise.

As the city stirs to life and commuters hit the road, they are often greeted with the undulating purples, blues and red lights that are meant to mimic Northern Lights.

Sometimes, the upper reaches of ­Target Corp.’s headquarters transform into an underwater scene with tropical fish darting around, including a playful puffer fish that blows itself up. This time of year, they might be treated to one of many seasonal scenes such as a Halloween one — and coming soon, a snow globe and ­Christmas lights.

Over the years, Target’s in-house creative team has designed about 30 such motifs that rotate throughout the year. Ranging in length from one to three minutes, one video is selected to play every night and morning in a continuous loop.

The boxy rectangular building along Nicollet Mall is not the tallest nor as distinctive in shape as the IDS or Capella towers in downtown Minneapolis. But it’s become a recognizable skyline feature nonetheless because of the artistic graphics it displays up top.

One thing you never see up there is the words “Target” or the retailer’s trademark bull’s-eye. Part of the reason is that the city tightly regulates that kind of branding atop ­buildings.

In any case, Target executives say it’s not meant to be a living billboard. Rather, it’s a public art display — an extension of the company’s design ethos that is part of its DNA.

“It’s about an artistic expression,” said Todd Waterbury, Target’s chief creative officer who, among other duties, oversees the small team in charge of the lights display. “It’s designed to celebrate the moments that are important to the Twin Cities.”

His team updated its patriotic red, white and blue display for the Ryder Cup when it was held in town last month. Its rainbow-colored lights have become a staple of the city’s Pride celebration every June. They also have the flexibility to change the colors of designs for big championship games — for example, changing the colors to green and blue for the Minnesota Lynx.

In April, when the city reeled from the shocking news of Prince’s death, Waterbury’s team debated how to mark the moment. Should they make the lights go black for the first time? Should they project purple raindrops?

In the end, they settled on beaming out a sea of purple — one of many Minneapolis landmarks to do so.

“One of the ways I like to talk about it is as a digital mood ring,” said Waterbury. “It reflects how we’re feeling and how the community is feeling. That’s really important to us. It gets to the emotion — the humanity — and the fact that we are part of this community.”

Of course, building lights have always been a defining part of skylines and cities.

“We’re all familiar with lighted signs and lights atop the Empire State Building that change colors,” said Larry ­Millett, a St. Paul-based architectural historian and author. “But I think this is fairly unique. … It’s a form of high-altitude public art — a way to enliven the night sky. I like that.”

Without that overt branding, some observers probably don’t even know that the lights are from Target’s headquarters, he said. Still, he added, there is surely a more subtle form of advertising at play in terms of drumming up goodwill.

Another peculiarity of the Target lights is that they can only be seen from certain vantage points because they are blocked by taller buildings from some directions such as the east. Loring Park is a prime viewing spot — as is driving from the west into the city along Interstate 394.

The lights were built into the building’s design when it was erected in 2001. It was an idea that came from Target’s then-CEO Bob Ulrich who wanted to replicate some of the beauty and wonder of the Aurora Borealis.

Target initially installed 130 long tube lights with 575-watt bulbs around the top perimeter of the building. They were first turned on a couple of months earlier than planned to show solidarity after the Sept. 11 attacks that year.

A decade later, in 2011, the lights were overhauled to LED lights, which are brighter and allow for more sophisticated graphics. They also are more efficient and last longer than the old tube lights, which had to be replaced every year, leading to considerable cost ­savings.

One of largest displays

With 25,000 square feet of surface area lit from 687,000 individual LED lights, Target’s tower is apparently one of the largest displays of its kind in the Midwest.

So who else comes close?

“Somebody’s front lawn Christmas display that took it way too far?” joked Brian LaShomb, a senior Target engineer who occasionally gives employees and schoolchildren tours.

To get to the lights, he takes groups on a freight elevator and pushes a button that says “PH,” the highest level of the building above the 32nd floor.

A door leads to a walkway along the perimeter of the building from which visitors can see the back of the multistory lights. It’s a popular place for selfies, LaShomb said.

He points out how the 850 panels of LED lights, stacked six high, were custom built so they can swing out, making it easier to access them to replace broken lights.

Visitors often have lots of questions about the lights. One is how long they stay on. They are turned on every day a half-hour before sunset and are turned off at 2 a.m., he said. In addition, the lights are turned on a few hours every morning in the fall and winter months.

“I’ve had kids ask me if they can hook their Xbox up to this,” he said. “I just playfully answer, ‘I don’t know. I’ll have to find out.’ ”

From time to time, Target hears from other people in the community making appeals for things they’d like to put up there such as wedding proposals. But Target doesn’t give in.

“People are always requesting things — ‘Put my name up there!’ ” said Waterbury. “But no kiss cam — sorry.”