Why the Northern Lights Are Visible over U.S.

Spectacular ripples of green, purple and red lights were seen across the U.S. and Canada on Sunday night, much further south than normal.

Photos on social media captured the gorgeous light shows in the night sky as far south as California, as well as in Illinois, Wyoming and Nevada.

These auroras were a result of a "severe" G4-class geomagnetic storm, caused by coronal mass ejection (CME) from the sun hitting the Earth's atmosphere, according to the National Oceanic and Atmospheric Administration (NOAA) Space Weather Predictions Center (SWPC).

The reason for these unusually south-reaching northern lights (and north-reaching southern lights in the southern hemisphere) is the strength of the geomagnetic storm.

What are geomagnetic storms?

"A geomagnetic storm is essentially caused by the interaction of a disturbance in the solar wind and the Earth's magnetosphere-ionosphere system," Brett Carter, an associate professor in space science at RMIT University in Australia, told Newsweek. "Significant disturbances in the solar wind can be caused by Coronal Mass Ejections (CMEs), like the big one we've just had)."

CMEs are huge chunks of solar plasma and solar magnetic field flung into space during periods of magnetic rearrangement on the sun's surface. CMEs interact with the Earth's magnetic field and atmosphere when they collide with the planet, causing geomagnetic storms.

What makes some geomagnetic storms stronger?

"A geomagnetic storm is the alteration of the Earth's magnetic environment, this means when the magnetic fields that usually surround our Earth start to be distorted," Daniel Brown, an associate professor in astronomy and science communication at Nottingham Trent University in the U.K, told Newsweek.

The strength of the geomagnetic storm depends on the degree to which the Earth's magnetic field is affected by the magnetic field of the CME.

"The amount of matter ejected, its speed, the associated magnetic fields, as well as how they interact with other already emitted particles from the sun, all add up to a bumpy environment moving outwards from the sun for our Earth's magnetic field to travel through," Brown explained. "The more prolonged, the stronger the interaction will be and the higher the likelihood of a strong geomagnetic storm."

Geomagnetic storm scale

Geomagnetic storms are classed on a scale of G1 (minor) to G5 (extreme), according to NOAA's G scale, with G4 (which the storm on Sunday night was) being the second strongest.

"NOAA's G scale is used to quantify the impacts of geomagnetic disturbances, and it is informed by the Kp index, which is effectively a measure of the disturbance in the Earth's magnetic field as measured by magnetometer stations around the world," Carter said. "The more disturbed the Earth's magnetic field is, the larger the Kp value and subsequently the larger the G value."

More powerful storms cause brighter and more spectacular aurora borealis and aurora australis displays, as well as causing the lights to creep further away from the Earth's poles.

Northern lights

The auroras appear brighter because the atoms of gas in our atmosphere are given more energy from the CME.

"Stronger storms will impart more energy on the electrons in our Earth's magnetic environment or magnetosphere," Brown said. "These electrons are then going to be the source of the light seen in southern/northern lights, as they crash into oxygen or nitrogen in our high atmosphere, making them glow. The more energetic the electrons are, the brighter the display."

The auroras are able to stretch further towards the equator due to distortions to the Earth's magnetic field caused by the CME.

"What takes place within the magnetosphere during a geomagnetic storm is quite complicated, but in essence, the appearance of auroras at lower latitudes during storms is due to electric fields in the magnetosphere," Carter said.

Under quiet conditions, charged particles (both from the solar wind and the ionosphere) can become trapped in the magnetosphere and collide with upper atmospheric particles, causing the normal level of auroras in the polar regions.

"During geomagnetic storms, electric fields within the magnetosphere drive charged particles closer to the Earth, which effectively causes them to collide with Earth's atmosphere at lower latitudes," Carter said.

Impacts of geomagnetic storms

Other than gorgeous light shows, these geomagnetic storms can also cause a number of other impacts.

"The largest storms not only create individual particle impacts associated with the aurora, but also rivers or currents of charged particles in the upper atmosphere," Dolores Knipp, a space weather research professor at the University of Colorado Boulder, told Newsweek. "These currents create new magnetic field variations that can cause distant currents at the ground (induced currents) that flow not only in the ground, but will seek a path of least resistance in long power lines that form our power grid. Too much current or current at unusual frequencies disturbs the power systems (especially big power transformers)."

This can cause fluctuations and outages in the power grid. Space-borne electronics and systems can also gather unusual amounts of charged particles during geomagnetic storms and then unexpectedly discharge them to a different, possibly sensitive, component on spacecraft, or back into space.

"Currents that flow in the upper atmosphere not only induce ground currents, but they can heat the upper atmosphere, causing satellite drag," Knipp said. "The disturbances in the upper atmosphere can also produce bubbles of cool plasma in the upper regions where the atmosphere meets space. These bubbles can interfere with or slightly redirect Global Navigation Satellite System (GNSS) signals. GPS is one such system. These signals need to be very precisely known and measured to assure the good position navigation and timing on which we rely. Geomagnetic storms can cause a myriad of problems with GNSS. The system is more robust than it used to be, nonetheless we often hear of GPS/GNSS signal 'scintillation' during geomagnetic storms."

Effects on humans

Other than the impact on our power grid and electrical equipment, geomagnetic storms have very little effect on humans.

"As far as we know there is no harm caused to life on Earth by such events, since our own magnetic field and atmosphere protects us against the incoming CME," Brown said.

"We might experience some higher doses of radiation while flying during such areas of high activity, but this is not life-threatening. We might, however, have to consider how such exposure to solar radiation storms and CMEs could impact us while considering prolonged interplanetary travel or living on planets or moons without a substantial magnetic field such as or Moon or Mars to protect us."

Solar radiation storms, although a different phenomenon from geomagnetic storms, often coincide with them, and can lead to issues for astronauts in orbit—and even airplane passengers.

"The G4 storm we've just had did have a minor solar radiation storm (S1) accompany it," Carter said. "So, even though this was a big event, the impacts on humans in orbit (and high-altitude flight over the polar regions, for that matter) was effectively nil. However, when such space weather events cause lots of high-energy protons to reach Earth, they can have biological impacts. For S3 to S5 events, astronauts are advised to avoid undertaking any extra-vehicular activity to protect them. People on cross-polar flights during S2 to S5 events will experience increased radiation levels."

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