Breach of the Shield: Magnetic links between sun and Earth last hours

Each time the sun hurls a planet-size cloud of charged particles toward Earth, there’s a potential for power outages and satellite damage. But it’s when the magnetic field carried by these billion-ton clouds points opposite to Earth’s magnetic field that geomagnetic storms are most severe. In that configuration, our planet’s field, which usually shields Earth from the sun’s outbursts, connects directly to the field accompanying the cloud. That magnetic handshake opens up a hole in Earth’s shield, permitting energetic ions and electrons from the sun to gush through and induce large electrical currents in and around the planet.

STORMY WEATHER. Ultraviolet image (above) shows a proton aurora (ring). Brightest spot (arrow) is about the size of California. NASA
Simulation of a breach (white spot) in Earth’s magnetic shield. NASA

A report in the Dec. 4 Nature reveals that once such breaches are created, they can persist for hours, rather than closing up soon after they’ve formed. The finding ends a 2-decades-long debate about the duration of holes in Earth’s magnetic shield, says Ron Zwickl of the National Oceanic and Atmospheric Administration’s Space Environment Center in Boulder, Colo. The results, he adds, should be incorporated into models of geomagnetic storms and could be crucial for placing regional hot spots–places where solar ions most easily punch through–on global space-weather maps.

In the study, a NASA satellite called IMAGE (Imager for Magnetopause to Aurora Global Exploration) observed two powerful proton auroras in the arctic portion of Earth’s upper atmosphere, or ionosphere. Proton auroras are generated by ions in the solar wind, the stream of charged particles blown out by the sun. Unlike the colorful northern and southern lights, proton auroras shine only in ultraviolet light.

During the proton aurora on Feb. 18, 2002, a constellation of four spacecraft known as Cluster was perfectly situated for recording solar-wind ions streaming through a breach in Earth’s magnetosphere.

The breach occurred along magnetic field lines that traced down to a bright spot in the proton aurora, which IMAGE observed for 4 hours.

Although Cluster’s orbit enabled the spacecraft to observe the breach for only 5 minutes, IMAGE’s observations of the aurora indicate that the breach lasted for at least 4 hours, assert study coauthor Harald U. Frey of the University of California, Berkeley and his colleagues.

At the boundary of Earth’s magnetic shield, about 60,000 kilometers above the planet’s surface, the breach’s diameter was greater than that of Earth.

Other scientists recently got a taste of just how powerful geomagnetic storms can be when the magnetic field carried by a cloud of solar material opposes that of Earth. Soon after a recent eruption on the sun, dubbed the Halloween solar storm, a high-altitude belt of energetic electrons that cradles Earth was pushed inward and the radiation it emitted greatly increased.

From Nov. 1 to Nov. 10, the center of the belt, normally 19,000 to 25,000 km above Earth’s equator, descended to an altitude of about 9,600 km. “We have never seen such a powerful enhancement and distortion” of the belt during 11 years of observations, notes Daniel Baker of the University of Colorado at Boulder. He reported the findings this week at the fall meeting of the American Geophysical Union in San Francisco.


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