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The solar storm that seemed to be more fizzle than fury got much stronger early Friday before fading again.
At its peak, it was the most potent solar storm since 2004, US space weather forecasters said.
No power outages or other technological disturbances were reported from the solar storm that started to peter out late Friday morning.
Solar storms, which cannot hurt people, can disturb electric grids systems and satellites. They can also spread colorful Northern Lights farther south than usual, as the latest storm did early Friday.
More storms are coming, according to the US government's Space Weather Prediction Center, which says the same area of the sun erupted again Thursday night, with a milder storm expected to reach Earth early Sunday.
The latest storm started with a flare on Tuesday, and had been forecast to be strong and direct, with one scientist predicting it would blast Earth directly like a punch in the nose.
But it arrived Thursday night New Zealand time at mild levels at the bottom of the US government's 1-5 scale of severity. It strengthened to a level three for several hours early Friday as the storm neared its end. Scientists say that is because the magnetic part of the storm flipped direction.
"We were watching the boxer, expecting the punch. It didn't come,'' said physicist Terry Onsager at the National Oceanic and Atmospheric Administration's space weather center. "It hit us with the back of the hand as it was retreating.''
Forecasters can predict a solar storm's speed and strength, but not the direction of its magnetic field. If it is northward, like Earth's, the jolt of energy flows harmlessly around the planet, Onsager said. A southerly direction can cause power outages and other problems.
Thursday's storm came in northerly, but Friday switched to the fierce southerly direction. The magnetic part of the storm spent several hours at that strong level, so combined with strong radiation and radio levels, it turned out to be the strongest solar storm since November 2004, said NOAA lead forecaster Bob Rutledge.
By late Friday morning the storm was essentially over, forecasters said. But they had a new flare from the same sunspot region to watch. Preliminary forecasts show it to be slightly weaker than the one that just hit, arriving somewhere around 1 a.m. EST Sunday.
The storms are part of the sun's normal 11-year cycle, which is supposed to reach a peak next year.
"This is what we're expecting as we approach solar maximum,'''' Onsager said. "We should be seeing this for the next few years now.''