LANL CONTACT: Todd A. Hanson, 505-665-2085, tahanson@lanl.gov
MIT CONTACT: Deborah Halber, 617-258-9276, dhalber@MIT.EDU
THE DAY THE SOLAR WIND DISAPPEARED
LOS ALAMOS, N.M., Dec. 13, 1999 — For three days last May, scientists at the Department of Energy’s Los Alamos
National Laboratory and the Massachusetts Institute of Technology 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 a million miles per
hour, suddenly decreased in velocity to roughly 626,000 miles per hour. At the same time, 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 today 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 counter-streaming solar electrons throughout most of the period suggests the event is associated
with a solar coronal mass ejection,” Steinberg said. “But if that is 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 3 protons per cubic centimeter only 5 percent of the time, for it to drop to 0.2 is really quite unusual at
anytime, 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 creating 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 Lazarus, an MIT space scientist. ” 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 May event provided geophysicists with a rare opportunity to examine solar-terrestrial interactions at a time
when the magnetosphere was extremely inflated. In the 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 cometlike shape by shortening the tail that normally extends out from
the Earth’s night side and making the Earth’s magnetic field more analogous to the field of a dipole bar magnet.
These unusual boundary conditions for the magnetosphere also had a rather dramatic effect on the particle population
inside the Earth’s magnetosphere. While low energy plasma particles decreased to some of the lowest density levels
ever observed near geosynchronous orbit, the energetic particles trapped by the near-Earth’s magnetic field
expanded much further out than normally possible.
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 Alan J. Lazarus and Matthias Aellig. At Los Alamos, Dave
McComas, Jack Gosling and Ruth Skoug join 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.
Los Alamos National Laboratory is operated by the University of California for the Department of Energy.
