Don Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

William Steigerwald

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-5017)

RELEASE: 99-145


From May 10-12, 1999, the solar wind that blows constantly
from the Sun virtually disappeared — the most drastic and
longest-lasting decrease ever observed.

Dropping to a fraction of its normal density and to half its
normal speed, the solar wind died down enough to allow physicists
to observe particles flowing directly from the Sun’s corona to
Earth. This severe change in the solar wind also changed the
shape of Earth’s magnetic field and produced an unusual auroral
display at the North Pole.

Starting late on May 10 and continuing through the early
hours of May 12, NASA’s ACE and Wind spacecraft each observed that
the density of the solar wind dropped by more than 98%. Because
of the decrease, energetic electrons from the Sun were able to
flow to Earth in narrow beams, known as the strahl. Under normal
conditions, electrons from the Sun are diluted, mixed, and
redirected in interplanetary space and by Earth’s magnetic field
(the magnetosphere). But in May 1999, several satellites detected
electrons arriving at Earth with properties similar to those of
electrons in the Sun’s corona, suggesting that they were a direct
sample of particles from the Sun.

“This event provides a window to see the Sun’s corona
directly,” said Dr. Keith Ogilvie, project scientist for NASA’s
Wind spacecraft and a space physicist at NASA’s Goddard Space
Flight Center, Greenbelt, MD. “The beams from the corona do not
get broken up or scattered as they do under normal circumstances,
and the temperature of the electrons is very similar to their
original state on the Sun.”

“Normally, our view of the corona from Earth is like seeing
the Sun on an overcast, cloudy day,” said Dr. Jack Scudder, space
physicist from the University of Iowa and principal investigator
for the Hot Plasma Analyzer on NASA’s Polar spacecraft. “On May
11, the clouds broke and we could see clearly.”

Fourteen years ago, Scudder and Dr. Don Fairfield of Goddard
predicted the details of an event such as occurred on May 11,
saying that it would produce an intense “polar rain” of electrons
over one of the polar caps of Earth. The polar caps typically do
not receive enough energetic electrons to produce visible aurora.
But in an intense polar rain event, Scudder and Fairfield
theorized, the “strahl” electrons would flow unimpeded along the
Sun’s magnetic field lines to Earth and precipitate directly into
the polar caps, inside the normal auroral oval. Such a polar rain
event was observed for the first time in May when Polar detected a
steady glow over the North Pole in X-ray images.

In parallel with the polar rain event, Earth’s magnetosphere
swelled to five to six times its normal size. NASA’s Wind, IMP-8,
and Lunar Prospector spacecraft, the Russian INTERBALL satellite
and the Japanese Geotail satellite observed the most distant bow
shock ever recorded by satellites. Earth’s bow shock is the shock
front where the solar wind slams into the sunward edge of the

According to observations from the ACE spacecraft, the
density of helium in the solar wind dropped to less than 0.1% of
its normal value, and heavier ions, held back by the Sun’s
gravity, apparently could not escape from the Sun at all. Data
from NASA’s SAMPEX spacecraft reveal that in the wake of this
event, Earth’s outer electron radiation belts dissipated and were
severely depleted for several months afterward.

“The May event provides unique conditions to test ideas about
solar-terrestrial interactions,” Ogilvie noted. “It also
strengthens our belief that we’re beginning to understand how the
Sun-Earth connection works.”

– end –

EDITOR’S NOTE: Images, movies, captions, and more detailed
background information associated with this release are available
on the Internet at:

A NASA Video File relating to this story will air on December
13 at Noon EDT. NASA Television is available on GE-2, transponder
9C at 85 degrees West longitude, with vertical polarization.
Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz.
Video File Advisories can be found at