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A Coronal Hole Targets Earth


23rd Feb 2000

High speed solar wind particles from a large coronal hole are buffeting Earth's magnetosphere.

The end of February could mark another good weekend for aurora watchers, thanks to a large coronal hole currently straddling the visible disk of the sun. High speed solar wind particles streaming out of the hole reached the Earth today and began to buffet our planet's magnetosphere. Space weather forecasters expect moderate to high levels of aurora borealis with minor impacts on satellite operations and high latitude power grids for the next few days. Right: This x-ray image of the sun, captured on Feb 21, 2000, by the Japanese Yohkoh X-ray Observatory shows the coronal hole that has rotated into a favorable position to send high-speed solar wind particles toward Earth. The resulting gusts of solar wind is expected to strike Earth's magnetic field and trigger moderate geomagnetic disturbances over the next few days. Coronal holes are easy to spot by looking at the Sun through an x-ray telescope. They appear as very dark areas that contrast with bright spots overlying sunspot groups. Hot gas around sunspots is captured by magnetic fields (see 3D Fields) that rise up out of one sunspot and bend back to reconnect at another spot nearby. These glowing 'magnetic bottles' shine brightly at x-ray wavelengths. The magnetic fields around coronal holes are different. Instead of looping back to reconnect on the sun's surface, these magnetic fields are essentially open. They extend far out into the solar system and no one knows exactly where they reconnect. Rather than trapping the hot gas, the open field lines of a coronal hole allow high-speed solar wind particles to escape.

Above: These data from NASA's Advanced Composition Explorer (ACE) spacecraft show the speed of the solar wind measured 1.5 million km from Earth at the L1 Sun-Earth libration point. The wind speed began to increase around 0900 UT on February 23, 2000, as the solar particles from a coronal hole reached ACE. Solar wind gusts measured by ACE reach Earth's magnetosphere about an hour later. The solar wind flows away of the Sun in all directions, not just from coronal holes, but the wind speed is high (up to 800 km/s) over coronal holes and much lower (300 to 400 km/s) elsewhere. The higher pressure streams from coronal holes squeeze the Earth's magnetic field and can produce geomagnetic activity. The coronal hole visible on the sun now is at least 7 months old. It has been seen by the Yohkoh soft x-ray telescope during each of the past seven solar rotations. (The sun rotates on its axis once every 27 days.) After this apparition, there is every reason to expect its return 27 days hence for another bout of solar wind gusts and geomagnetic activity. It's not yet clear how much activity will result from this week's coronal hole. The Geophysical Institute at the University of Alaska predicts: "Auroral activity will be high today (February 23). Weather permitting, highly active auroral displays will be visible overhead from Barrow to Anchorage, and visible low on the horizon from Bethel, Soldotna and southeast Alaska." Observers in Alaska, Canada, and the upper tier of U.S. states should be on the lookout for aurora in the coming days. The best time to look is just before local midnight, before the waning gibbous moon rises. If auroral activity increases, with displays visible at middle-latitudes, notices will be posted at

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