Last week's northern lights—which lasted a few days—were products of a large burst of plasma, or charged gas, from the sun known as a coronal mass ejection. A NASA orbiter called the Solar Dynamics Observatory saw last Sunday's eruption, which was aimed directly at Earth and sparked predictions of a shimmering sky show.
Now it seems aurora fans may be in for another treat: A solar flare spotted Saturday by NASA's Solar and Heliospheric Observatory was even more powerful than the previous eruption. Although this time the bulk of the plasma burst isn't aimed right at Earth, scientists say it could still trigger another round of colorful auroras.
Photograph courtesy Ian Diamond
An aurora created by last week's coronal mass ejection glimmers over Laurentian Beach, on the shores of Lake Manitoba in Canada (map), on August 6.
Auroras happen when energized particles from the sun wash over Earth and flow down the planet's magnetic field lines toward the Poles. Along the way, the charged partices bang into nitrogen, oxygen, and other atoms in our atmosphere.
The charged solar particles give Earth's atmospheric atoms an energy boost, which then gets released as light, producing the shimmering curtains of greens, reds, blues, and other colors. (Related: "Aurora 'Power Surges' Triggered by Magnetic Explosions.")
Seen from the shores of Lake Superior, an August 4 aurora glows green topped with blueish-purple. Auroras glow in different colors based on the types of atoms in the atmosphere and how high they are in the sky.
Humans mostly see auroras in shades of yellowish-green. That's because the eye sees a light show created by oxygen atoms at lower altitudes, about 62 to 186 miles (100 to 300 kilometers) above the surface.
Blues and purples are created by lighter gases such as hydrogen and helium, while low-level nitrogen can add red fringes to the bottoms of green auroral curtains.
Green light curls over Oslo, Norway, August 4 in an auroral display caused by last Sunday's coronal mass ejection.
In the Northern Hemisphere, auroras are more commonly seen at high latitudes near the Arctic Circle, such as northern Alaska, Canada, and Scandinavia.
But scientists predicted that last Sunday's strong burst would bring the light show to slightly lower parts of the globe. In fact, sky-watchers were snapping pictures of auroras as far south as Oslo, Lake Manitoba in Canada, and Lake Superior (map) in the United States.
Photograph courtesy Otto L. Motzke
The view from a Michigan shore of Lake Superior included multicolored auroras on August 3, as seen in a panoramic picture.
A solar storm headed for Earth isn't a guarantee of auroras. Without more sun-watching satellites, scientists are hard-pressed to know the exact effects a coronal mass ejection will have on Earth's atmosphere.
Last week Leon Golub of the Harvard-Smithsonian Center for Astrophysics had put the odds of the August 1 solar flare producing auroras at about 50-50.
Clouds over Manitoba, Canada, are bathed in green light from an August 4 aurora.
The solar ejections that cause auroras can also create geomagnetic storms that can affect spacewalking astronauts, Earth-orbiting satellites, and even communications and power systems on the ground.
But like the previous solar storm, which was strong but relatively slow, the oncoming burst of charged particles shouldn't create significant problems for people, according to the website SpaceWeather.com. Instead, the site says, "high-latitude sky-watchers should be alert for auroras when the cloud arrives, probably on August 10."
Photograph courtesy Miguel Yetman
Green Glow, Orange Clouds
Green aurora borealis curtains light up the skies over a beach in Grimstad, Norway (map), just after midnight on August 4.
The cloud of solar particles that sparked last week's northern lights weighed about a billion tons, according to NASA. The space agency's Solar Dynamics Observatory and other sun-studying spacecraft are about more than just advance notice of sky shows.
Because solar wind can disrupt communications systems and power grids, "one of the main reasons to have these instruments in space is so that you can issue alerts or warnings, pretty much like you would with a hurricane," Golub said.
That way "people can know ahead of time when there is a possibility of an event that will have an impact on Earth."
A bright swirl (left of center) on the sun marks the spot where a tumult of plasma sent out the August 1 coronal mass ejection that sparked last week's auroras, as seen in a mosaic picture taken by the Solar Dynamics Observatory. Different colors show regions of temperature variation on the sun.
Solar activity rises and falls on a regular cycle of about 11 years. The last period of peak activity ended in 2001, and it led into a long-lasting quiet spell. (See "Sun Oddly Quiet—Hints at Next 'Little Ice Age'?")
Along with a recent flurry of sunspots, the August eruptions seem to be signs of the star's reawakening—good news for aurora fans, but potential trouble for satellites, astronauts, and some Earthly technologies.