Recent satellite observations have revealed the largest breach yet seen in the magnetic field that protects Earth from most of the sun's violent blasts.

Researchers reported the discovery on Tuesday, which they made last summer using Themis, a fleet of five small NASA satellites.

Scientists have long known that the Earth's magnetic field, which guards against severe space weather, is similar to a drafty old house that sometimes lets in violent eruptions of charged particles from the sun. Such a breach can cause brilliant auroras or disrupt satellite and ground communications.

Observations from Themis show the Earth's magnetic field occasionally develops two cracks, allowing solar wind — a stream of charged particles spewing from the sun at 1.6 million km/h — to penetrate the upper atmosphere.

Last summer, Themis calculated a layer of solar particles to be at least 6,500 kilometres thick in the outermost part of the Earth's magnetosphere, the largest tear of the protective shield found so far.

"It was growing rather fast," Themis scientist Marit Oieroset of the University of California, Berkeley told an American Geophysical Union meeting in San Francisco. Such breaches are temporary, and the one observed last year lasted about an hour, Oieroset said.

Solar flares are a potential danger to astronauts in orbit, but generally are not a risk to people on the surface of the Earth.

NASA and the U.S.'s National Science Foundation funded the research.

Scientists initially believed the greatest solar breach occurred when the Earth's and sun's magnetic fields are pointed in opposite directions. But data from Themis found the opposite to be true. Twenty times more solar wind passed into the Earth's protective shield when the magnetic fields were aligned, Oieroset said.

The Themis results could have bearing on how scientists predict the severity of solar storms and their effects on power grids, airline and military communications and satellite signals.

The Themis satellites were launched to find the source of brief powerful geomagnetic disturbances in the Earth's atmosphere.

Earlier this year researchers using Themis data discovered that an explosion of magnetic energy a third of the way to the moon powers solar substorms, the sudden brightenings and rapid movements of the aurora borealis, or northern lights.