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Scientists report dramatic drop in solar wind over three-day period

For three days last May, scientists at the Department of Energy's Los Alamos National Laboratory and MIT watched as the solar wind all but disappeared.

The wind, a stream of plasma from the sun that normally buffets the Earth at speeds close to one million miles per hour, suddenly decreased in velocity to roughly 626,000 miles per hour as the particle density of the wind decreased from its typical 5 to 10 protons per cubic centimeter of space to 0.2 protons per cubic centimeter. The event, which Los Alamos scientists believe was caused by a coronal mass ejection from the sun, has quickly become one of the mysteries of space weather studies.

In an analysis of the event given December 13 in San Francisco at the 1999 fall meeting of the American Geophysical Union, scientists presented data gathered on May 10-12, 1999. Data came from the Solar Wind Electron Proton Alpha Monitor (SWEPAM) flying aboard NASA's Advanced Composition Explorer (ACE) satellite and the Solar Wind Experiment (SWE) aboard the WIND satellite. SWEPAM was designed and built at Los Alamos. The Faraday Cup portion of SWE was designed and built by MIT.

"This event is rather baffling in a number of ways," said John Steinberg, a Los Alamos scientist in the Los Alamos-MIT collaboration. "First of all, it is extremely rare for the density of the solar wind to drop so low for so long. The mystery is compounded by the low speed of the wind during the event. Not only that -- when the wind finally kicked back in, riding in the wind were some of the largest solar wind waves that space scientists had ever seen, causing the speed to change in a matter of a few hours."

The solar wind is the result of the supersonic expansion of the sun's corona. As the coronal plasma flows away from the sun in all directions, it creates a constant stream of charged particles. Occasional coronal mass ejections add to the stream as the sun "burps" a bubble of gas moving at several million miles per hour and containing billions of tons of matter.

"The presence of counterstreaming solar electrons throughout most of the period suggests the event is associated with a solar coronal mass ejection," Dr. Steinberg said. "But if that's the case, this is not the garden-variety coronal mass ejection we are used to seeing in the solar wind. Since the particle density of the solar wind typically drops below three protons per cubic centimeter only 5 percent of the time, for it to drop to 0.2 is really quite unusual at any time, including within a coronal mass ejection."

Because the solar wind is an ionized gas (that is, all the particles carry an electric charge), the wind sees Earth's magnetic field as an object to flow around. When the solar wind runs into Earth's magnetic field, a bow shock forms -- a shock wave similar to a jet's sonic boom on Earth. The region of space inside the bow shock, where Earth's magnetic field dominates, is referred to as Earth's magnetosphere. The bow shock normally forms about 50,000 miles sunward from Earth.

"The size of the bow shock during this event was incredible," said Alan J. Lazarus, an MIT senior research scientist in physics. "This was one of the few times I know of that the Earth's bow shock has gone all the way out to the moon." The event provided geophysicists with a rare opportunity to examine solar-terrestrial interactions at a time when the magnetosphere was extremely inflated.

In the May event, Earth's magnetosphere ballooned to many times its normal size as the bow shock was measured by the IMP 8, Interball 1, Geotail and WIND satellites at increasing distances out to 200,000 miles, or roughly the same as the average distance of the Earth to the moon. The absence of the wind also gave the magnetosphere less of a comet-like shape shortening the tail that normally extends out from the Earth's night side.

Launched in 1997, ACE orbits at a distance of roughly a million miles from the Earth to provide scientists with information about the solar wind. ACE was designed to provide elemental and isotopic composition measurements of the solar wind and cosmic rays as well as to provide warnings of potential geomagnetic storms caused by coronal mass ejections that can destroy satellites and disrupt electronic communications and electrical power grids.

NASA launched WIND in 1994 as part of the Global Geospace Science initiative and the International Solar-Terrestrial Physics Project.WIND's task is to provide plasma, energetic particle and magnetic field data for magnetospheric and ionospheric studies of the Earth, as well as to investigate plasma processes occurring in the near-Earth solar wind.

Several groups are studying the May event. At MIT are Drs. Lazarus and Matthias Aellig, a postdoctoral associate at the Center for Space Research. At Los Alamos, Dave McComas, Jack Gosling and Ruth Skoug join Dr. Steinberg in the solar wind studies. Also at Los Alamos, Reiner Friedel, Michelle Thomsen, Joe Borovsky and Tom Cayton are analyzing the response of the magnetosphere to the event. Both institutions work in collaboration with researchers from NASA Goddard Space Flight Center and the Bartol Research Center at the University of Delaware.

A version of this article appeared in MIT Tech Talk on December 15, 1999.

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