One week ago, the northern lights stunned America. When will it happen again?

One week ago, a spectacular northern lights display wowed millions of Americans. Experts and aurora chasers say that was likely just the beginning of the spectacles as our sun heads into what's known as the "solar maximum."

But when exactly will the northern lights appear again? Even though experts say conditions are primed for more aurora shows over the next few years, predicting exactly when and where they occur will continue to be a challenge. Even the best predictions can only accurately be made a few days or even hours in advance.

For those who caught the forecast in time last week and headed outside, the view was "epic," aurora chaser Melissa F. Kaelin told USA TODAY via e-mail days after the event. Last week's dramatic display of the northern lights near Dexter, Michigan, "rivals anything I've seen in 12 years of aurora chasing. The excitement still gives me chills," she said.

Amazingly, aurora were seen in all 50 states of the U.S. last week and across Europe and even Australia.

Experts say it's a great time to be an aurora chaser.

"Solar max has definitely arrived," astronomer Tony Phillips, of SpaceWeather.com, told USA TODAY this week via e-mail. Phillips said the solar maximum could last for 2 or 3 years.

"The May 10th superstorm may have been just the first of several magnificent displays we experience between now and 2026," he said.

So what's causing all of the excitement? Why is the aurora so visible, all of a sudden? Experts say it all has to do with our sun, solar cycles and the solar maximum. Here's the details:

What are solar cycles? What is the solar maximum?

Solar cycles track the activity level of the sun, our nearest star. A cycle is traditionally measured by the rise and fall in the number of sunspots, but it also coincides with increases in solar flares, coronal mass ejections, radio emissions and other forms of space weather.

"The sun has an 11-year cycle where it goes through maximum and minimum," Shannon Schmoll, the director of the Abrams Planetarium at Michigan State University told USA TODAY in an interview last year. "This results in the number of sunspots seen on the sun. Sunspots result from areas of the sun that have stronger magnetic fields."

The number of sunspots on the sun's surface changes on a fairly regular cycle, which scientists refer to as the sun's 11-year solar cycle. Sunspot activity, and hence auroral activity, tends to peak every 11 years.

Sunspots produce solar flares and coronal mass ejections, which create the geomagnetic storms here on Earth that cause the aurora to appear.

"We are entering the peak of Solar Cycle 25," Erica Grow Cei, a spokesperson for the National Weather Service, told USA TODAY this week in an e-mail. "This period of heightened activity is expected to last into the first half of 2025," she added, meaning that additional chances for seeing the aurora will continue for at least the next year.

Kaelin said the 11-year solar maximum "holds enormous potential for aurora chasers, but until Friday (May 10), we were worried it was going to be a quiet year." She said that although there had been a number of impressive solar flares, "the ejections of solar wind that these eruptions create have been mostly directly away from Earth. Until now!"

One solar flare that blasted off the sun this week was the strongest in seven years, but it was pointed away from the Earth, scientists said.

When will the next northern lights show be? We won't know until days – or hours – before.

What's the northern lights forecast for the days and months ahead?

"More chances may wait for us in the days ahead, as we watch the number of sunspots on the sun explode," Kaelin said this week via e-mail. "But we can only predict these impacts with any certainty about three days in advance, and it's an emerging science.

"Humans still have a lot to learn about space weather," she admitted.

Indeed, the aurora can be fickle to forecast. Unlike terrestrial weather, scientists who forecast space weather – which includes the aurora – must rely on observations of the sun, 93-million-miles-away, to make their predictions.

"There are so many uncertainties, it makes it difficult to predict," Bill Murtagh, the program coordinator at the Space Weather Prediction Center, told USA TODAY last year. And as hard as it is to forecast weather here on Earth, "we are decades behind the forecast capabilities of our colleagues in meteorology," he admitted, referring to space weather.

Solar cycles tracked back to Colonial era

How are solar cycles measured? "The timing of the maximum and minimum of the solar cycle is predicted based largely on historical values and is verified through observations," Cei told USA TODAY.

"The cycle is about 11 years in length, and in that regard it is very predictable. This solar cycle is known as #25 because it has been tracked for that long – back to the Colonial era," she said.

"Here is the chart for the current cycle, which you can see is running hotter than originally predicted," Cei said.

Auroras also bring danger for electrical grid

But with the exciting news about the uptick in aurora sightings also comes the worrisome side of heightened solar activity: It can interfere with the electrical grid, degrade GPS signals, increase orbital drag on satellites, and pose radiation hazards to airline crews and astronauts, the Space Weather Prediction Center warned.

Stronger solar cycles produce more solar storms with greater intensity and therefore pose a larger hazard for these critical technologies and services, the prediction center said.

Minor disruptions from last week's geomagnetic storm

The solar storm, which prompted NOAA to issue a watch alert for the first time in 19 years, caused some power grid irregularities and interfered with GPS signals – even farming equipment.

However, "based solely on the impacts that were reported to us, the disruptions were minor," Cei said this week. "We may be able to provide specifics in the future, but not at this time, as many of our stakeholders have requested that we share this information in generalities."

In general, she added that preparation pays off: "From what we've gathered so far (and this will be an ongoing process), it was a combination of infrastructure improvements over the past decade or so, plus excellent forecast information and decision support from the Space Weather Prediction Center that led to Americans experiencing a low-impact event."

More: NOAA detects another solar flare following sun-produced geomagnetic storm: 'Not done yet'

What are the northern lights?

The auroras are a natural light display in Earth's sky that are famously best seen in high-latitude regions.

The aurora forms when the particles flowing from the sun get caught up in the Earth's magnetic field. The particles interact with molecules of atmospheric gases to cause the famed glowing green and reddish colors of the aurora, according to NASA.

The composition and density of the atmosphere and the altitude of the collisions determine the colors of the lights, the University of Alaska, Fairbanks, said. "The aurora is most often seen as a striking green, but it also occasionally shows off other colors, ranging from red to pink or blue to purple," the University said on its website.

This article originally appeared on USA TODAY: Northern lights forecast: When will they appear again?