Earth's natural defenses are routinely compromised by huge cracks that open up for hours, allowing space storms to pour through like a hurricane through an open window, scientists announced today.

The finding is expected to help space weather forecasters make better predictions of the potentially damaging effects of solar storms that buffet the planet. The tempests threaten satellites at the fringes of the atmosphere and power grids on the surface.

"We discovered that our magnetic shield is drafty, like a house with a window stuck open during a storm," said Harald Frey of the University of California, Berkeley. "The house deflects most of the storm, but the couch is ruined."

Frey led the work, which is detailed in the Dec. 4 issue of the journal Nature.

Earth's magnetic field emanates from the poles and extends beyond the atmosphere and past the highest Earth-orbiting satellites.

The magnetic field absorbs the brunt of a solar storm, which is a huge cloud of charged particles, ions and electrons. The Sun constantly spits out a "wind" of these particles. During intense activity, it can shoot a coronal mass ejection (CME) our way. A CME -- the most damaging sort of solar storm -- is to the solar wind what a hurricane is to a summer breeze.

Each CME has a magnetic orientation. If it is oriented south -- opposite the northern orientation of Earth's magnetic field -- the cloud has greater potential to get through. That much scientists knew.

As early as 1961, theorists suspected that this opposing scenario might generate cracks in Earth's magnetosphere, as the magnetic field lines of Earth and the storm connect. A rupture was first detected in 1979, but scientists have since wondered if they were fleeting or of longer duration.

Discovering that the gaping breaches indeed last hours involved some fancy formation flying by a fleet of NASA satellites.

In one set of observations, the Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite looked upward and monitored ions slamming into an area of the upper atmosphere above Earth's North polar region. It was determined to be a crack almost as big as California.

During the event, a team of four other satellites, called Cluster, flew above the IMAGE craft and directly through the crack. Cluster detected the solar wind ions streaming through the rip in Earth's defense system.

The crack, which widened at higher altitudes, appeared to remain open continuously for nine hours.