News from ISTP on the Wind Satellite
December 1999: On the Day the Solar Wind Disappeared,
- December 1999: On the Day the Solar Wind Disappeared,
Scientists Sample Particles Directly from the Sun
- May 1999: Polar, Wind, Interball Verify Dungey
Theories of Reconnection
- March 1999: Wind WAVES Observes New Solar Radio
- October 1998: Wind Detects Magnetar,
Atmosphere on Moon
- April 1998: Wind Spacecraft to Begin Petal Orbits
- January 1998: Scientists Tracking Ejection from Sun that
Reached Earth January 6
- October 1997: Wind, Ulysses Triangulate, Pinpoint
- August 1997: Wind WAVES Adds Diplomacy, Radio
Astronomy to its Resume
- July 1997: Wind Detects Collapsing Plasma Waves
Scientists Sample Particles Directly from the Sun
>From May 10-12, 1999, the solar wind that blows constantly from the Sun virtually disappeared in 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 drastically changed the shape of Earth's
magnetic field and produced a rare auroral display at the North Pole.
Starting late on May 10 and continuing through the early hours of May 12, the density of the solar wind
dropped by more than 98%. Because of the drop-off of the wind, energetic electrons from the Sun arrived at
the 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 Goddard Space Flight Center. "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
(HYDRA) on NASA's Polar spacecraft. "On May 11, the clouds broke and we could see clearly."
Scudder, Ogilvie, and other scientists affiliated with the International Solar-Terrestrial Physics program (ISTP)
presented their findings at the Fall Meeting of the American Geophysical Union in San Francisco's Moscone
Center. Researchers working with more than a dozen spacecraft observed various facets of this event.
Fourteen years ago, Scudder and Dr. Don Fairfield of NASA 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
because those electrons are slowed and depleted by too many collisions in interplanetary space. 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 should precipitate directly into the polar caps, inside the normal
Such a polar rain event was observed as a steady glow in X ray images and confirmed for the first time in May
1999. Aurora were observed at the North Pole, which can only happen if these energetic electrons are coming
directly from the solar wind.
"While we saw weak aurora in the south, in the north we saw the effects of intense, energetic electrons on the
upper atmosphere in the form of X rays," said Dr. Dave Chenette, a space physicist at Lockheed Martin and
principal investigator of the Polar Ionospheric X-Ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft.
"These X-ray emissions are the most intense that we have ever seen at the north magnetic pole since Polar
was launched in 1996."
According to Chenette and Scudder, the fact that the aurora appeared only at one pole in May 1999 suggests
that the North Pole is connected to the end of the magnetic field from the Sun, while the South Pole is
connected to the end of the Sun's magnetic field that extends to the outer reaches of the solar system.
"The May event provides unique conditions to test ideas about solar-terrestrial interactions," Ogilvie noted. "It
also strengthens our belief that we understand how the Sun-Earth connection works."
Under typical conditions, the Sun emits a tenuous gas of protons, helium, and electrons - the solar wind -- in
all directions across the solar system. Carrying energy and magnetic fields from the Sun, the solar wind
varies but usually stays within 5 to 10 particles per cubic centimeter (cc) and between 400-600 kilometers
per second. The pressure from this solar wind buffets and confines Earth's magnetic field, ramming it up
against the planet on the day side and stretching a long magnetic tail on the night side.
But on May 11, the drop in the density of the solar wind (to less than 0.2 particles per cc) allowed Earth's
magnetosphere to swell unimpeded to five to six times its normal size. 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 gravity, apparently could not escape from the Sun at all. NASA's Wind, IMP-8, and Lunar
Prospector spacecraft and the Japanese Geotail satellite observed Earth's bow shock - the region where the
solar wind slams into the sunward edge of the magnetosphere - moving out to 238,000 miles from Earth
(380,000 kilometers). The event produced the most distant bow shock ever recorded by satellites; the norm is
41,500 miles (67,000 km) from Earth toward the Sun.
In addition, the Earth's magnetic field took on a more dipolar shape - similar to the shape of iron filings spread
around a magnet - as Earth's field would appear if there was no solar wind. And data from NASA's SAMPEX
spacecraft reveal that in the wake of this event, Earth's radiation belts dissipated and nearly disappeared for
several days afterward.
Nearly a dozen spacecraft observed this unusual event, including NASA's Polar, Wind, ACE, IMP-8, SAMPEX,
FAST, and Lunar Prospector satellites. Contributions also were made by Interball (Russian Space Agency),
Geotail (Japan's Institute for Space and Astronautical Science), and by satellites operated by the National
Oceanic and Atmospheric Administration and the U.S. Department of Defense.
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
May 1999: Polar, Wind, Interball
Verify Dungey Theories of Reconnection
J. W. Dungey predictions, before satellites were: IMF south,
reconnection on dayside, low latitude (previously verified by ISEE);
IMF north (reconnection on nightside of cusp, had not been
verified); tests of strong north IMF (quantified in simulation by
Fedder and Lyon, 1995). Reconnection now verified, observed by
Reconnection is one of the most important plasma processes
in the universe, a key method of energy exchange.
- Measures strong northward IMF -- Dungey's condition -- on May
- Solar wind pressure 4 times normal squeezes magnetosphere so
much that Polar approached magnetopause
- Magnetic field shows major variations from models
- New MHD simulation including nightside reconnection explains
- Plasma measurements shows trapped magnetosheath plasma on
closed field lines on day side -- which simulation predicts
March 1999: Wind WAVES Observes New
Solar Radio Features
Using the WAVES instrument on Wind, Reiner and Kaiser have
observed a potentially new type of solar event, and a complex,
striated topology (magnetic and plasma) to the solar corona around
2.5 solar radii. WAVES measures 1-14 MHz, in the gap between ground
and spacecraft observations. Observing a special form of Type
III radio bursts -- historically known as shock-accelerated (SA)
events -- they found that shocks do not necessarily cause these
events, as previously predicted (Cane et al, 1981). Electron
acceleration begins in the corona ~1.04 solar radii (seen as Type
III by ground-based telescopes). At 2-2.5 solar radii -- where solar
wind forms -- the electron beam producing Type III burst is disrupted.
The result of unusual, chaotic magnetic fields and plasma struture?
SA emission reappears at 4 solar radii. Normal Type III shocks
proceed to Earth unimpeded; newly observed signals wash out, then
Wind Satellite Detects Magnetar,
Atmosphere on Moon
The Wind spacecraft was the first to detect the 27 August 1998
gamma ray burst that led to confirmation of the existence of
magnetars. The wave of gamma rays hit the night side of Earth and
ionized the atoms in the upper atmosphere to a level usually seen
only during daytime.
Investigators using KONUS detected the burst from the magnetar.
KONUS is the first Russian instrument to fly on a U.S. satellite
since civil space cooperation resumed in 1987.
In other research, scientists have used the Suprathermal Ion
Spectrometer on Wind to identify oxygen, silicon, and aluminum ions
in the atmosphere of the Moon. Investigations of the lunar
atmosphere will intensify in November when Wind returns.
A movie of the gamma ray burst.
WIND Petal Orbit
WIND Petal Orbit
April 1998: WIND Spacecraft to Begin Petal
After returning from several months at the L1 Lagrangian
point--the point where the gravitational and centrifugal pull of
the Sun and Earth cancel each other--ISTP's Wind spacecraft will
soon make two passes by the Moon. Having spent the past few months
cross-calibrating its instruments with those from the ACE
spacecraft, Wind will now begin a six-month series of "petal"
orbits that will take it out of the ecliptic plane.
Starting in October 1998, Wind will fly in an orbit that brings it
as close as 10 Earth radii (about 63,000 km) and as far as 80 Earth
radii from our planet. More importantly, the orbit will take Wind
at an angle of 60 degrees from the ecliptic plane--the plane of
Earth and most of the planets. Wind's trips above and below the
ecliptic will allow the spacecraft to sample regions of
interplanetary space and of the magnetosphere that have never
before been studied.
January 1998: Scientists Tracking Ejection from Sun that
Reached Earth January 6
Researchers from the International Solar-Terrestrial Physics (ISTP)
program are currently
tracking a coronal mass ejection (CME) that left the Sun late on
January 2 and began
arriving at Earth around 10 a.m. Eastern Time on January 6. CMEs
charged gas from the Sun that can trigger magnetic storms around
eruptions--which are becoming more frequent as the Sun builds up
toward the maximum
of its 11-year cycle--occasionally disturb spacecraft, navigation
systems, and electric power grids.
The Wind, Polar, and Geotail spacecraft, as well as a network of
smaller satellites and
ground-based observatories are now monitoring the interplanetary
storm as it crosses
paths with Earth. Scientists are observing changes in the strength
of Earth's magnetic
field and radiation belts, while gathering images of Earth's
Forecasters at the Space Environment Center of the National Oceanic
Administration predicted the CME would begin arriving during the
latter half of January
6 and would continue through January 7. The disturbance to Earth's
magnetic field and
space environment is not expected to be particularly strong;
however, observers at high
latitudes (Canada, Scandinavia, etc.) are likely to see aurora
tonight and tomorrow.
On January 2, scientists operating the Solar and Heliospheric
spacecraft detected a "halo" type coronal mass ejection erupting
from the Sun at
approximately 500 km/s (more than 1 million miles per hour). The
SOHO team alerted
the rest of ISTP to the possibility of an Earthbound storm. In
research presented at the
December meeting of the American Geophysical Union, ISTP
researchers announced that
"halo" CMEs almost always result in magnetic activity at Earth.
Halo CMEs are so
named because they appear as expanding halos around the Sun when
seen from Earth.
ISTP is a joint, comprehensive effort to observe and understand our
star, the Sun, and its
effects on Earth's environment in space. The primary participating
NASA, the European Space Agency (ESA), the Japanese Institute of
Astronautical Sciences (ISAS), the Russian Space Research Institute
To view the same data and images as ISTP scientists, visit the
Event web page here.
information about ISTP and the physics of the Sun and Earth, go
For the official U.S. space weather forecast, visit
An image to accompany this story is available here.
Current images of Earth's aurora as seen from space are available
October 1997: Wind, Ulysses Triangulate, Pinpoint Radio Burst
Flares or other explosive events on the Sun can produce "Type III"
radio bursts. These radio emissions are caused when streams of
electrons from the Sun ram into the interplanetary medium, creating
radio emissions along the trajectory (see figure). Traditionally, individual
spacecraft have been able to determine the direction of such radio
bursts, but not the true distance from the Sun. Scientists were
forced to rely on gross estimates for that. Now, through a rare
alignment of the Wind and Ulysses spacecraft--the only craft
equipped to track such radio bursts--scientists have been able to
triangulate and determine the precise location of a Type III radio
burst in three-dimensional space. If equipped to do this on a
regular basis--with a long-term, stereo alignment of
spacecraft--researchers would be able to map the interplanetary
magnetic field; track the electron streams from Sun to Earth;
measure the size, density, and intensity of radio bursts and
perhaps the events on the Sun that produce them; and track some
elements of space weather.
August 1997: Wind WAVES Adds Diplomacy,
Radio Astronomy to its Resume
Receivers on Wind's Radio and Plasma Waves experiment (WAVES) are
contributing to radio astronomy and diplomacy as well as space physics.
Researchers from Russia's NIRFI laboratories are experimenting with
techniques of modifying Earth's ionosphere. By heating the ionosphere above a
radar site, researchers can create an atmospheric lens through which they can
detect low frequency radio emissions. Wind-WAVES helped prove the concept
by monitoring changes in the intensity of the radio signals from Russian
ground stations as the ionosphere was modified (see figure). Similarly, WAVES
has helped American researchers using Air Force over-the-horizon radar to
prove that OTH signals can escape the atmosphere. Such signals could be used
to study the surface of the Sun.
July 1997: Wind Detects Collapsing
For reporters and editors seeking more information
about any ISTP news item, contact Mike Carlowicz, science writer
for ISTP, at
Theory holds that when Langmuir waves (high frequency electron waves)
grow to large amplitudes, they focus and intensify (top figure). In a
runaway process, the waves eventually collapse into small packets of
electrical energy, then change into radiation or heat. The Time Domain
Sampler on Wind has detected tightly focused wavepackets in the
foreshock area between the Earth and the solar wind (bottom figure). The
collapse of electron plasma waves has been predicted by equations but
never before observed in space. This may help explain the bright radio
emissions found at Earth's bow shock.