Green auroras illuminate the sky over Whitehorse, in Canada's Yukon Territory, on Monday.
Such auroral displays are triggered when clouds of charged particles from the sun—known as coronal mass ejections (CMEs)—slam into Earth's magnetic field.
A "severe" CME hit September 26, sparking auroras at both Poles and inducing light shows visible in five U.S. states, including Michigan, New York, South Dakota, Maine, and Minnesota, according to NASA.
As solar particles get funneled along Earth's field lines toward the Poles, they collide with molecules in the atmosphere, infusing them with extra energy. The molecules in turn release the energy as light.
Capturing the above aurora required "a long night of waiting-but the activity picked up," photographer Jonathan Tucker wrote on SpaceWeather.com.
Many auroras appear green, but sometimes other colors such as red show up—as in this picture taken from the International Space Station on September 26.
An aurora's colors depend on which types of atoms cause the splash of light. In most cases, auroral lights appear when charged particles from the solar wind collide with oxygen atoms in Earth's atmosphere, according to a NASA statement.
"This produces a green photon, so most auroras appear green. However, lower-energy oxygen collisions—as well as collisions with nitrogen atoms—can produce red photons, so sometimes auroras also show a red band, as seen here."
The northern lights reflect brightly in a lake just outside Yellowknife in Canada's Northwest Territories in a picture taken September 28.
Strong solar storms—such as the September 26 CME that triggered this week's auroras—can also hit low-Earth orbiting satellites with potentially damaging solar particles and magnetic fields, according to NASA. (See pictures of solar eruptions.)
A vibrant spectrum of color fills the sky above Tromsø, Norway, on September 26.
"We were lucky that we were able to find a place with clear sky around," photographer Thilo Bubek told National Geographic News. "First we saw some green auroras, but later we saw colors we had not seen this strong [in] the last years."