SUPERIOR, Wis. – "Welcome aboard," Katherine York chirped, greeting the newest arrivals onto the SunSpot. "We ain't your mama. We ain't your teacher. But we will care about you!"

The passengers laughed, grabbing a seat inside the bus, which was parked outside the student union at the University of Wisconsin-Superior one recent afternoon.

It was finals week, and the stressed-out students closed their eyes and basked in the warm glow surrounding them. A row of light therapy boxes hung from the bus ceiling, producing a brightness often missing from the skies Up North during these dark and cold winter months.

This lack of sunlight took some getting used to for Katherine and her twin sister, Sue York. They grew up in Florida, the Sunshine State. But fate brought them both to live in the Duluth area, where they've embarked on a mission to lighten up Minnesota winters.

The bubbly duo who call themselves the Sol Sisters operate what is believed to be the only "light-therapy bus" around.

The sisters refer to the SunSpot as a mobile rescue unit. For them, the business is a labor of love.

"We want to do something worthwhile in the world, making a living helping people," said Sue York, who has struggled for years with seasonal affective disorder (SAD). "We call it spreading our rays of sunshine."

She came up with the idea for a bus equipped with light boxes to help herself and others combat the effects of SAD, a type of depression brought on by a change of seasons. Many people with SAD use light boxes to help treat their symptoms.

Katherine joined her sister in launching SunSpot last year, moving from San Francisco.

"Sue has been dealing with this for so long," she said. "I started doing research on it. I said, 'Gee willikers, Sue. This is amazing! This is a real problem and we'll be the first ones offering a solution.' I missed my sister and thought, why not help her dream manifest into a reality?"

The reason for a bus, the sisters say, is because it allows them to reach out to people who could benefit from light therapy but don't have the energy to venture far. Besides, sitting in front of a light box at home is less social than being on the bus with others, they said.

"Depression can make you isolate yourself," Katherine said. "We're promoting socializing."

The sisters, 56, admitted that persuading people to pay $5 for a 30-minute session of sitting on a brightly lit bus with strangers hasn't been easy. Passers-by don't always understand what the bus is all about.

"We're not a tanning salon," Sue often tells onlookers.

"Yeah, people wonder," Katherine said.

Party on a bus

Painted fire-engine red with the word "SunSpot" splashed in yellow on the front, the bus is hard to miss as it cruises up and down the steep streets of Duluth and surrounding towns.

The Yorks paid $5,000 to buy the 16-person bus. It was a former city bus with no rust on it. Someone had already converted it so that it came with bunk beds and a table.

The Yorks transformed the bus into something that looks like a cross between a living room and a spa. Going for a summer vibe, they installed blue and white vinyl benches from an old Bridgeman's ice cream parlor and added orange mini-tray tables and a colorful rug.

They added 12 medical-grade light boxes from a company called Aurora Light Solutions. The boxes emit 10,000 lux (unit of illumination), Katherine said, which is optimal.

At the front of the bus is a "golden curtain," separating the steps leading onto the bus from the cabin, to help keep the cold air out. The cabin stays at a steady 70 degrees.

They're licensed as a food truck, but the sisters argue that city officials should categorize them as something else: a peddler of light.

Neither sister has any education or formal training in the health sciences. What they've learned, they say, comes from their own experiences and from reading extensively about light therapy, vitamin D deficiency and SAD.

Although the two have never run a business before, they say it runs in their blood.

Their father was a typewriter and computer salesman who received many sales awards over the years. Katherine said she still remembers the best advice he gave her: "He said, 'It takes 10 calls to get a meeting. And it takes 10 meetings to get a sale.' "

As teenagers in Florida, they were sun worshipers. "We were baby oil babes on the beaches," Katherine said.

In high school, they worked at Disney World in Orlando, playing the chipmunk duo Chip 'n' Dale. They're still entertainers, always looking for ways to liven up things on the bus.

"We tried to do Dance Party Tuesdays," Katherine said, recalling the times they parked the SunSpot outside a local nightclub. "People brought their tap shoes and hula-hoops."

They've had musicians playing guitar serenades. They've served healthy snacks. And they've invited people to play Jenga and other games.

A place to unwind

During the SunSpot's recent stop at the university, the guests included students as well as faculty members.

"Hello, come on in, honey-do," Katherine York cooed at a young woman. "Wanna cup of tea?"

The woman, Jazmin Wong, said "sure" and boarded the SunSpot.

Katherine handed her a cup of orange pekoe tea, asking her how her exams were going and about her studies. Wong, a senior, was about to graduate, with only her final essays and a presentation standing between her and the next chapter of her life.

"I'm just really tired today," Wong said. "I woke up at 6 a.m. to prepare for a presentation. But it's all over for me today."

She had been on the SunSpot a few times before when she needed a little pick-me-up.

"It's a really good idea," she said of the bus.

Across from Wong sat two freshmen, Evan Hungerbuhler and James Crawford. They jumped on between their exams to relax.

"Everyone has stress in their lives," Hungerbuhler said, shedding his jacket as he soaked up the light box rays and sank into his seat.

A half-hour later, the three were gone and a new group paused outside the SunSpot, waiting to board.

"Hey, you guys," Katherine beckoned, waving at them. "Come on in!"

Allie Shah • 612-673-4488

@allieshah