A Week of Solar Blasts:

The Space Weather Event of May 1998

The Active Week:

The shock caused by this speedy cloud was detected passing SOHO only 53 hours later -- record travel time. For all its speed, though, the cloud caused only an average geomagnetic storm at Earth. On May 2, the Dst (disturbance storm time) index, which is an indicator of the strength of geomagnetic disturbances, fell to -85, signaling that a minor magnetic storm was in progress. The storm pushed the northern lights as far south as Chicago and Detroit that night.

While the Earth was feeling the impact of the April 29 cloud, however, a much stronger storm was brewing. On May 1 and 2, two more halo CMEs were spotted by SOHO, along with a huge X-class solar flare. High-energy particles from the flare flew the 92 million miles to SOHO in just 30 minutes. When the May 1-2 clouds reached Earth on May 4, the largest magnetic storm of that solar cycle broke.

The Dst index fell to -205; electric currents in the ionosphere jumped above 2000 nanotesla, three times the typical solar-minimum strength. Two thousand gigawatts of power (three times the power generated by all U.S. utilities) surged through the upper atmosphere, lighting up auroras over Boston, London, and Chicago and forcing New England power companies to reconfigure their grids. Nearly two-million-mile-an-hour winds, the fastest solar winds detected by the Wind spacecraft since its launch, smashed into the sun-side of the Earth's magnetic field, ramming this border from 45,000 miles above Earth in to only 15,300 miles, well within the 22,300-mile-high orbits of geosynchronous satellites. Spacecraft normally protected by the magnetosphere found themselves out in the solar wind.

The storm raged for about a day, both delighting aurora-watchers and frustrating those whose communications were down (high-frequency radio failed in Antarctica from May 2-6 because of the storm's interference with the ionosphere). Things had just calmed down again when, on May 6, the Sun threw a last punch in the form of three CMEs, none of which hit Earth, and another X-class flare whose protons upset instruments on the Polar spacecraft and forced a temporary shutdown. By the middle of that day, the Sun's week of fits had ended, leaving the Earth to face the aftermath of the storm.