An extreme (G5) geomagnetic storm hit Earth last weekend, delighting viewers as far south as Florida with green and red curtains of light.

aurora
This photo captures stunning colors from Bow, New Hampshire.
Rob Mack / S&T Online Photo Gallery

An "extreme" G5 geomagnetic storm hit Earth this past weekend, delighting viewers as far south as Florida with green and red curtains of light.

These were not blazing, rainbow displays of light to the eye. At mid- and southerly latitudes, even extreme aurorae are faint, and skies where most people live are bright. Most viewers saw curtains of white with just hints of color. However, photos revealed the true extent of those hues, thanks to multi-second exposures: The eye's "shutter speed" is 1/50 of a second, and the cones in our eyes need a minimum amount of light to perceive color — that iPhone in your pocket, on the other hand, can easily take 3-second exposures, capturing a good many more photons and showing their color.

The event was initially predicted to be a G4 (severe) geomagnetic storm, but the National Oceanic and Atmospheric Administration (NOAA) ultimately pegged it as G5 (extreme). Such an event hasn't crossed Earth's path since October 2003.

Last weekend's geomagnetic storm arose after one of the largest sunspot groups to emerge on the Sun over the past two decades spat out six coronal mass ejections (gobs of ionized solar gas) in our direction. Those gobs traveled with (and even outpaced) the solar wind that constantly spirals out from the Sun and across the solar system. Some of the later CMEs caught up to the earlier ones, producing a more powerful event when they finally reached Earth. You can see the CMEs in this video from ESA's Solar and Heliospheric Observatory:

SOHO blocks out the view of the Sun using a coronagraph (dark disc in the middle). The coronagraph blocks an area larger than the Sun itself, whose size is depicted by the white circle. The "snow" later in the video appears because charged particles that are part of the CME hit SOHO's detectors directly.

The magnetic fields that travel with those charged and fast-moving particles reconnected with Earth's own magnetic field, allowing the solar particles entry to our upper atmosphere, where they collided with oxygen atoms to produce the green and red colors. Green is a more typical color for aurorae, produced by oxygen at middling altitudes. But red aurorae are more likely when there's a geomagnetic storm like this one, which dumps energetic particles into the atmosphere. Red comes from rarefied oxygen higher up, which means it can be seen from farther away, making red the most easily seen color at low latitudes.

Spectators across the contiguous United States enjoyed the sight, from the rural Pacific Northwest to Boston suburbs and down to Florida and Puerto Rico. Reader Larry Black wrote in from Cedar Rapids, Iowa: "This is the most intense auroral display I have observed since August 1972!" S&T's Sean Walker agrees: "It was perhaps the best aurora display I've seen in more than a decade, and certainly the best one I've seen from my own yard."

Below are some of the images and videos captured during the event. We welcome you to share your own experiences in the comments below as well as your photos in our photo gallery!

Purple aurora
S&T's Sean Walker reports from southern New Hampshire: "As twilight faded, most of the sky was filled with clouds, with the exception of a thinning towards the south. I aimed my camera toward that spot and started snapping five-second exposures though a wide-angle lens. To my surprise, a distinct purplish glow appeared in the clearing — to the south!"
Green and red aurorae
Walker continues: "Later in the evening, gaps in the cloud cover became apparent toward the north. I watched as tall rays peeked through the cloud cover, rising and then fading within a minute or less."
Reddish hues
"As the night grew late and the cloud cover continued to thin, the display became dominated by red aurora, punctuated with white rays rising over my backyard observatory," Walker says.
Sheets of aurorae
S&T's Gary Seronik reports from southern British Columbia, Canada: "Although the photo exaggerates the color, reds and greens were distinctly visible to the unaided eye. The display went on all night, although it seemed to lose structure as the night went on. The aurora gradually became a sort of luminous overcast that overwhelmed all but the brightest stars." (Seronik notes that auroral displays are a rarity for his location, even though it's in Canada, because it lies to the west and thus not as close to Earth's geomagnetic pole.)
Wide-field view of aurorae
Contributing editor Bob King captured this wide-field view of aurorae dancing over a field in northern Minnesota. "It was quite a spectacle here," he says. "At one point the aurora left the northern sky and moved south with rays over Ophiuchus and deep into Scorpius."
Aurorae from Massachusetts
S&T's Monica Young reports from Concord, Massachusetts: "To the unaided eye, the aurorae appeared as white curtains with mere hints of pink and green. A three-second smartphone exposure revealed the deeper hues. (The photo is unfiltered, except for whatever the smartphone automatically applies.) At times, the white-green curtains crossed the sky, though this photograph captures them nearer the horizon."

Readers of Sky & Telescope have shared photos with us from southerly locations:

"This was a one time chance of a lifetime to see the Aurora Borealis as far south as southern California," writes Bruce Gottlieb, who took this photo in the Joshua Tree National Park.
astrorx01 / S&T Online Photo Gallery
Subtler colors appeared under a dark sky in Mississippi.
dbrannan / S&T Online Photo Gallery

Visit the Aurora and Atmospheric category of the S&T Online Photo Gallery for more beautiful images.
Share your shots, too!

To close, we'll leave you with this time-lapse view, brought to you by Gianluca Masi of the Virtual Telescope Project from Rome, Italy:

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