OCTOBER 29, 2003 ( REUTERS ) - A shockwave from the Sun hit the Earth today, the final burst from a solar hurricane that has hampered some space satellite transmissions and led electric grid operators to curb power transmissions as a precaution. Scientists said the cloud of charged particles, known as a coronal mass ejection (CME), unleashed by a hyperactive Sun was traveling at more than 5 million mph, reaching the Earth in just 19 hours.

Power plants from Sweden to New Jersey cut production to limit how much electricity was flowing over transmission grids, preparing to absorb any sudden surge in energy that might result in coming days from lingering effects of the storm.

"It arrived at six this morning (1 a.m. Eastern time) and was going much faster than people thought," said Mike Hapgood, a space expert at the Appleton Laboratory in England. "There were some problems starting yesterday because of the effects that precede the arrival of this shock wave from the sun."

"We expect this storm to continue through the day and tomorrow," said Larry Combs, a space weather forecaster at the National Oceanographic and Atmospheric Administration's (NOAA) Space Environment Center in Boulder, Colo.

The center, which acts as the official U.S. space weather agency, advises power utilities, airlines and communications network operators of potential threats from space. It first warned of the storm a week ago (see story).

The gaseous cloud dumped energy into the magnetic field that surrounds the Earth, creating a geomagnetic storm; it was the final wave in a three-stage solar storm that began peppering the Earth with X-rays yesterday. These X-rays, traveling at the speed of light, forced air traffic controllers to scramble to find alternative communications channels and affected satellite transmissions of images back to Earth, weather experts said.

In the second wave, a pulse of solar radiation hit the Earth. Image transmissions from the Solar and Heliospheric Observatory (SOHO) satellite, which first detected the solar blast, degenerated into salt-and-pepper images for a time yesterday, forcing its operators to put the spacecraft into rest mode, NOAA said.

Hapgood and other scientists suspect the CME produced an amazing aurora, or light show, over Alaska and the Far East, as well as radio communication problems. "The higher up you are ... the bigger the effect you see," said Lucie Green, a solar physicist at the Mullard Space Science Laboratory in England.

CMEs come around every few years but the one that arrived today may rank as one of the strongest.

The X-ray and solar radiation storms rank as the second largest such events recorded in the latest 11-year cycle, according to NOAA data. Records of solar cycles date from 1755. This is the tail end of the 23rd cycle, Combs said.

The geomagnetic particle storm that hit earlier today measured G5, or extreme. How long the storm remains in Earth's atmosphere will determine whether it ranks as one of the biggest storms ever.

A U.S. utility said it had reduced power at two nuclear units in southern New Jersey to avoid possible disruptions. The electric grid operator for New England warned that it could reduce power imports from Quebec and exports to New York.