A Week of Solar Blasts:
The Space Weather Event of May 1998


The Active Week:
On April 29, 1998, a week of wild activity began in space. Around noon Eastern Daylight Time, the Extreme-ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory (SOHO) satellite saw the Sun begin to give off a large cloud of plasma called a coronal mass ejection (CME). SOHO's coronagraph saw the cloud expand into the Sun's faint corona by 16:58 and form a lopsided halo around the Sun, indicating that it was headed straight for the satellite and the Earth beyond. Initial measurements showed that the cloud was moving at 900 km/sec, more than twice the average speed of the solar wind.

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.

The Aftermath:
Earth-dwellers probably thought the space storm had completely blown over. Indeed, the clouds of plasma had passed, and their immediate effects had died down; the longer-term effects, however, had just begun. Two rings of plasma called the Van Allen radiation belts normally lie trapped around the Earth between the orbits of high-flying geosynchronous satellites and the low-orbiting Space Shuttle. When the shock wave associated with the May 4 storm smashed into the radiation belts, the electrons trapped there were energized to new heights and were spun around the Earth to new locations. Satellite data showed that a new radiation belt had formed between the normal two by the end of May 4 and did not subside until the middle of June.

That the high-energy electrons of the radiation belts are known as "killer electrons" does not bode well for satellites flying near the belts. During the period of disturbed conditions that May, the Equator-S satellite failed; the Polar satellite suffered blackouts; four of Motorola's Iridium satellites were lost. The most famous failure, however, was that of the Galaxy IV communications satellite, handler of some 90 percent of U.S. pager transmissions and several television and radio feeds. At 6:00 PM EDT on May 19, Galaxy IV's onboard control system and backup switch failed, proving to at least 40 million pager customers across the United States the fragility of satellite communications. Over the next few days, PanAmSat, Galaxy IV's owner, scrambled to transfer signals to other satellites and fix the communications blackout.

Although PanAmSat dismissed any solar causes for the satellite outage, blaming instead circuit-damaging "tin whiskers", some scientists disagree. An October 1998 article in the newspaper Eos stated boldly that "killer electrons" flooded the satellite and damaged its electronics. Since high-energy electron levels were far above normal for at least three weeks, the article stated, Galaxy IV slowly fried to death. The true cause can never be determined, but the concept suggested is striking -- extended periods of space weather such as the week of activity in early May 1998 can create highly abnormal conditions for satellites and inconvenience millions of people at one blow.

Quotes and Anecdotes:

Chronology (all times Universal Time):
[April 29, 1998]
 16:00 -- Halo CME first visible in SOHO/EIT images
 16:58 -- CME appears in SOHO/LASCO-C2
[May 1, 1998]
 21:15 -- Shock detected by SOHO/PM
 23:56 -- Partial halo CME in progress, seen from LASCO
[May 2, 1998]
 13:40 -- X-class flare from region 8210
 14:06 -- Bright partial halo CME begins in NW quadrant of Sun
 14:20 -- High-energy protons arrive near Earth from flare
[May 4, 1998]
 02:30 -- Solar wind speed jumps from 500 to 700 km/s (at Wind)
 03:30 -- Wind speed jumps again to 800 km/s
[May 6, 1998]
 00:02 -- Bright partial halo CME appears in LASCO-C2
 02:28 -- Small, slow CME appears in LASCO-C2
 05:29 -- Wide, fast CME associated with X-class flare appears
[May 19, 1998]
 22:00 -- Galaxy IV satellite's onboard control system fails
 22:30 -- ABC and CBS video feeds freeze
[May 21, 1998]
 10:30 -- 85% of PageNet customers back online
 16:00 -- Most of Galaxy IV's transmissions switched to another satellite
Useful Sources:
ISTP Sun-Earth Connections Event, May 1998: http://www-spof.gsfc.nasa.gov/istp/events/1998may1/
A listing of the scientific data available for the event, with some summary information about the satellite observations.
Aurora Monitor Project Status: http://angwin.ece.uiuc.edu/~haunma/aurora/status/07May98/
Some background on the storm, with a focus on auroral observations from early May 1998.
Russell, C.T. et al. "The Extreme Compression of the Magnetosphere on May 4, 1998, as Observed by the Polar Spacecraft." http://www-ssc.igpp.ucla.edu/personnel/russell/papers/Extreme_comp/
Scientific article discussing the compression of the Earth's magnetic field during the May event.
"'Whiskers' Caused Satellite Failure," Wireless Week, 5/17/99: http://www.wirelessweek.com/News/may99/ffour517.htm
Article on the cause of the Galaxy IV failure, dismissing solar activity as a possible contributor.
"The Satellite Fix Is In," ABC News, 5/22/98: http://www.abcnews.go.com/sections/tech/DailyNews/satellite980519.html
From ABC News, this article covers the Galaxy IV outage, effects, and attempts to recover from the outage.

Images from the Event:
Flare on EIT
SOHO's EIT captures the May 2 solar flare

May 2 X-Class Flare
X-ray flux plot shows an X-class flare on May 2

May 2 Aurora
May 2 aurora seen by UVI on Polar

May 3 Aurora
VIS on Polar sees the aurora on May 3

Aurora of May 4
Plot shows auroral oval from DMSP, with white dots indicating ground observations

May 4 Aurora
The aurora, as observed May 4 by Polar's PIXIE

Aurora from Laramie
Brian Rachford's image of the May 4 aurora from Laramie, WY

SOHO/LASCO images a CME on May 6

May 6 X-Class Flare
X-ray flux plot shows an X-class flare on May 6