Two bursts of charged matter that spewed from the sun's surface Sunday were forecast to sideswipe the Earth with minor solar winds late Tuesday night. Instead, they arrived in the morning as a severe solar storm that created dramatic shows of the Northern Lights expected to continue into early Wednesday, potentially as far south as Baltimore.
A solar storm rated a four on a scale of five arrived at 10 a.m. eastern time Tuesday, lighting up the pre-dawn skies in the northern United States with the aurora borealis and creating concerns of power grid disruptions and GPS interference, though no major problems were reported. The auroras were spotted around the globe, and if the solar storm's strength held up, were expected to be visible unusually far south across the United States.
The colorful phenomenon occurs when the energized particles bombard Earth's atmosphere, exciting the oxygen and nitrogen atoms so much that they give off energy in the form of brilliantly hued light.
But what surprised scientists is how much they underestimated the storm even though they acknowledge they know little about what happens between when a solar flare bursts from the sun's surface and when the energized matter is detected by a satellite about an hour before it washes over the planet.
"Our models showed we were just going to receive a glancing blow from this cloud coming off the sun," said Thomas Berger, director of the Space Weather Prediction Center, a branch of the National Weather Service. "We are now thinking we caught more than just a glancing blow."
Scientists had based their forecasts on observations Sunday of two coronal mass ejections from the right edge of the sun as seen from Earth. The events spew plasma from the sun's corona, a halo of speeding protons and electrons that is hundreds of times hotter than the sun's surface, into space.
But since then, the two clouds of charged particles appear to have merged and intensified, arriving about 15 hours earlier than expected, scientists at the space weather center said. The storm was the strongest to affect Earth since late 2013 and one of two storms rated "severe" in the current cycle of solar activity that started in 2008, they said.
The storm was slow to weaken Tuesday, and space weather forecasters said they expected it to continue for 12 hours, with lingering effects for 24 hours to 36 hours.
Because such storms surround Earth with a massive amount of energy, they can overwhelm power grids and wreak havoc on GPS communications, but no major disruptions were reported. Berger said it's possible that electricity disruptions are less common than in decades past because power companies are better prepared for the storms.
"This is still an inexact science and its difficult to gauge what the impacts will be," Berger said in an interview.
A NASA satellite being built and managed by scientists at the Johns Hopkins Applied Physics Laboratory and another that launched in February and is managed by the NASA Goddard Space Flight Center in Greenbelt seek to improve on the ability to track the flares and forecast their scale and impact.
While difficult to predict, this storm did produce dramatic auroras, a harmless display of light created as the solar particles' energy is transferred into Earth's atmosphere.
The connection between this solar storm and Earth's magnetosphere was strong because the Earth's magnetic field flows north and the storm was oriented southward — and, with magnets, opposites attract, explained Nicky Fox, project scientist at the Hopkins lab for a satellite known as the Solar Probe Plus.
When the solar particles energize oxygen atoms, they emit green light, or red if they are especially excited. And nitrogen atoms give off blue light. In combination, they can produce all colors of the rainbow, Fox said. But the storms have no lasting impact on the atmosphere, she said.
"It's a bit like giving a 2-year-old sugar," Fox said. "They run around getting rid of their energy and then they're the same 2-year-old as before you gave them all the sugar."
The strongest solar storms have the potential to be catastrophic on Earth. There hasn't been a storm rated a 5 on the five-step scale for geomagnetic storms since 2005, Berger said. A massive solar storm in 1859, known as the Carrington event, caused telegraphs to fail and gave some operators electric shocks. The associated aurora was seen as far south as Cuba and Hawaii.
To prepare for such events, scientists are working to learn more about why some coronal mass ejections move through space faster than others and how to better forecast whether one will hit Earth directly. In 2013, cientists said the Earth missed being struck by a potentially disastrous solar storm the previous year by a matter of weeks.
NASA is scheduled to launch the Solar Probe Plus satellite in 2018, and send it closer to the sun than any previous spacecraft. Engineers at the Hopkins lab are designing it to withstand temperatures of up to 2,500 degrees as it passes through the sun's corona, studying the charged solar particles.
"The big mystery for us is why solar winds accelerate," Fox said. "There are future missions that are going to provide us a lot of data that will help these [space weather forecasters] improve their models."
By the end of this year, a National Oceanic and Atmospheric Administration satellite known as DSCOVR will replace the aged spacecraft that rings the alarm bell for approaching solar storms. The current satellite, the Advanced Composition Explorer, or ACE, launched in 1997, and is not ideally designed for observing strong solar storms, said Adam Szabo, a project scientist at Goddard working on DSCOVR, which stands for Deep Space Climate Observatory.
In extreme solar storms, the energy can overwhelm electronic components on the ACE satellite, but DSCOVR is intentionally designed to be simpler, with just a gold-coated aluminum plate to detect the solar particles, Szabo said. DSCOVR will reach its destination 1 million miles from Earth in June and is expected to become operational a month or two after that.
Then scientists will be better prepared for the next Carrington event, he said.
"The fair weather forecast is not that interesting," Szabo said. "You want to get the bad weather forecast accurately."
An earlier version of this article incorrectly stated the federal agency overseeing the DSCOVR satellite. The Sun regrets the error.