Solar flare? Yes, but tonight's northern lights have a more spectacular cause.

Skywatchers in the northern US tonight may become the beneficiaries of a major burp from the sun that took place June 7.

While the solar storm did include a solar flare, the giant pulse of plasma, electricity, and matter that fountained across nearly half the sun's surface was a coronal mass ejection – and that's what we can thank for the auroral display filling the skies Wednesday night.

If the oncoming hordes of charged particles from that event reach Earth at the right time, aurora could be visible on the northern horizon as far south as Washington, D.C., according to an alert today from the University of Alaska's Geophysical Institute.

Given the intensity of the outburst, "it is reasonable to expect [conditions] that would put the aurora over Milwaukee, and visible on the northern horizon along a line from Portland, Ore., southern Nebraska, southern Indiana, to Washington, D.C.," say aurora forecasters. Weather permitting, of course.

If you live in the northern half of the country, look north around midnight local time, say experts. Keep checking their website, too, for ongoing updates on viewing conditions.

Residents in the southern hemisphere would see the mirror opposite of any aurora in the northern hemisphere.

Tuesday's solar storm

The solar outburst, which peaked at 1:41 a.m. Eastern Daylight Time on Tuesday, was captured in spectacular fashion by NASA's Solar Dynamics Observatory. The spacecraft was launched in February 2010 on a five-year mission to observe the sun and its effects on space weather around Earth.

Tuesday's activity on the sun included a solar flare and a minor radiation storm, according to researchers at the Goddard Space Flight Center in Greenbelt, Md.

But the eye-popping event was a coronal-mass ejection – an enormous release of energy that sends several billion tons of matter, largely in the form of protons and electrons, hurtling from the star at speeds measured in millions of miles an hour.

If the region on the sun where the outburst originates is aimed toward Earth, the ejected particles slam into Earth's magnetic field when they arrive. The collision in effect compresses the magnetic field on the daylight side of the planet. It also adds energy to the field, stretching it on the night side into a comet-like tail – far longer than the modest tail Earth hosts during quiet solar moments.

Effects on the ground

The movement of these ejected, charged particles also represents an electric current – which means it carries its own magnetic fields.

When these fields and currents interact with Earth's magnetic field at ground level, it can induce unusually large electric currents in pipelines and long-distance utility power lines. This can increase corrosion in the pipelines and potentially overload portions of the grid.

In effect, these important pieces of infrastructure "become antennas sensing the changes in Earth's magnetic field," says Antii Pulkkinen, a physicist at Goddard who is involved in modeling the interaction between the sun and Earth's upper atmosphere.

Effects in the atmosphere

Meanwhile, above the planet's surface, the compression of Earth's magnetic field from the solar smackdown has other effects.

Spacecraft in geostationary orbit, ordinarily protected by the magnetic field, will suddenly be exposed to higher, potentially damaging levels of cosmic radiation. They will also be vulnerable to the build-up of electrical charges on their surfaces, which can suddenly discharge and zap sensitive electronics.

And over on Earth's night side, the magnetic field can only stretch so far. When it can take no more energy from the passing mass of solar particles, the stretched field will snap back in what researchers call "magnetic reconnection."

This motion accelerates charged particles already in the magnetic field, plus others the CME has donated, back toward Earth. These particles travel along magnetic-field lines, plowing into the upper atmosphere at Earth's geomagnetic poles.

The electrons in this subatomic race home collide with atoms of hydrogen, oxygen, and nitrogen in the upper atmosphere, causing them to fluoresce in the familiar reds, blues, and greens of the northern and southern lights.

[Editor's note: The original subhead and original photo caption were altered to distinguish more clearly between a solar flare and a coronal mass ejection.]