One of the most intense displays of northern lights in years delivered a superb sky show to the northern United States, Canada and other high-latitude locations across the Northern Hemisphere on Wednesday night into Thursday.

Arriving on the heels of a disappointing display of auroras over Halloween weekend, this second event more than compensated.

Unique circumstances helped multiple coronal mass ejections (CMEs), blasts of plasma from the surface of the sun, merge to unleash a major geomagnetic storm. Auroras were seen as far south as Southern New England and Nebraska, in addition to typical locations farther north through Canada, Alaska and Northern Europe.

An aurora chaser in Northern California was even able to witness the phenomenon, according to SpaceWeather.com, while in Sweden, the lights were bright enough to be seen at sunrise Thursday.

When one CME eats another

The catalyst for the magnificent display was what SpaceWeather.com dubbed a “cannibal” CME.

The sun unleashed at least one CME on Monday and then shed a second, faster-moving burst Tuesday. “As it [the second] approached our planet, it overtook at least one other CME and swallowed it,” SpaceWeather.com wrote. As many as three CMEs may have been involved, according to Space.com.

Satellites observing solar winds detected a “stairstep structure” Wednesday night, an indication of several CMEs bunched together.

NOAA’s Space Weather Prediction Center determined the geomagnetic storm reached level G3, or strong, on its five-level scale.

The effects of the geomagnetic storm were waning Thursday, but lingering solar winds may be strong enough for another night of spectacular northern lights in parts of Europe and North America.

Making up for the weekend

Interest in aurora activity spiked over Halloween weekend as a top-tier X-class flare erupted on the sun, launching a CME toward Earth.

Alerts for G3 geomagnetic storming were up for Saturday night, when the CME was expected to collide with our atmosphere. Portions of the flare eventually reached Earth but with weaker effects than had been expected. The northern lights were still fantastic in the high latitudes but did not shine much south of Canada.

As the workweek began, word of the new CMEs emerged as their protons spewed earthward. Given the underwhelming display over the weekend, skywatchers were skeptical.

Skepticism is often warranted with aurora forecasts. The reasons are many and largely relate to limited capabilities to observe CMEs as they hurl through space toward Earth.

Once the Wednesday’s outburst arrived, it quickly became apparent that this episode was different from the weekend’s. Auroras displayed farther south than usual, arriving in multiple waves. The show climaxed late at night, rewarding those who stayed up to wait.

“The best aurora display I have seen in a couple years!!!,” tweeted storm chaser Justin Phillips in Wisconsin.

Wednesday night lights

The lights were seen Wednesday night into Thursday morning from Europe to North America. With the sun up over these areas Thursday, the auroral oval, or zone where northern lights are likely to be visible, migrated to parts of Russia and Asia.

In the United States, northern lights were seen as far south as parts of southern New England, Wisconsin, Iowa, Nebraska, Idaho, and then toward a cloudier Pacific Northwest.

Multicolored and full of shimmering pillars, the show was quite striking in addition to its unusual southward excursion. The diverse color palette emerged as the solar particles interacted with various gases of our atmosphere.

“When the charged particles from the sun hit the atmosphere, different gases emit different colors: Oxygen gives off greens and yellows, while nitrogen emits blues and reds,” Kathryn Prociv wrote in 2017 in a Capital Weather Gang article after one of the last major aurora events to have been visible so far south.

Although most auroras are green, colors such as red and blue often show up during intense solar storming. Blue and/or purple auroras are particularly uncommon and are indicative of interactions of the solar plasma with Earth’s atmosphere at relatively low altitudes (60 miles or less).