The largest solar flare in five years is racing toward Earth, threatening to disrupt power grids, GPS and flights, while also putting on a brilliant aurora display.

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The sun erupted on Tuesday, and the effects should start smacking Earth later tonight, according to forecasters at the US Space Weather Prediction Center. They say the flare is growing as it speeds outward from the sun.

"It's hitting us right in the nose," said Joe Kunches, a scientist for the National Oceanic and Atmospheric Administration.

The solar storm was likely to last through Friday, but the region that erupted could still send more blasts our way, Kunches said. He was predicting brilliant auroras.

Astronomer John Field, from the Carter Observatory in Wellington, said he was "crossing his fingers", hoping auroras would be spotted in the sky above New Zealand.

He said it was likely the rays of colours would be seen in the far south, near Invercargill, when the solar flares hit the earth around 6.30pm.

"Plus or minus seven hours depending on how fast the flares are travelling."

Field said the auroras could be seen as north as Wellington, depending on the weather, but the "further south you are the better".

Kunches said another set of active sunspots was ready to aim at Earth.

But for now, scientists were waiting to see what happened when the charged particles hit Earth at 6.4m kmh.

NASA solar physicist Alex Young added: ''It could give us a bit of a jolt. But he said this is far from a super solar storm.''

"The storm is coming after an earlier and weaker solar eruption happened Sunday," Kunches said.

There was the potential for widespread problems. Solar storms have three ways they can disrupt technology on Earth: with magnetic, radio and radiation emissions. This was an unusual situation when all three types of solar storm disruptions were likely to be strong, Kunches said.

"That means a whole host of things could follow," he said.

The magnetic part of the storm has the potential to trip electrical power grids. Kunches said power companies around the Earth had been alerted for possible outages. The timing and speed of the storm determined whether it would knock off power grids, he said.

In 1989, a strong solar storm knocked out the grid in Quebec, leaving 6 million people without power.

Solar storms could also make global positioning systems less accurate, which was mostly a problem for precision drilling and other technologies, Kunches said. There also could be GPS outages.

The storm could cause communication problems and added radiation around the north and south poles, which would probably force airlines to reroute flights. Kunches said some had already done so.

Satellites could be affected by the storm, too.
NASA spokesman Rob Navias said the space agency wasn't taking any extra precautions to protect astronauts on the International Space Station from added radiation from the solar storm.

- AP