News from ISTP on the Polar 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
- October 1999: Polar VIS Captures the Ultraviolet
- May 1999: Polar, Wind, Interball Verify Dungey
Theories of Reconnection
- March 1999: ISTP, ACE Assist Sounding Rocket Launch
- December 1998: Earth's Own Magnetosphere, Not Solar Wind,
Accelerates the Particles
of the Radiation Belts
- December 1998: Solar Wind Squeezes Some of Earth's
Atmosphere into Space
- June 1998: ISTP Observes Effects of New
- May 1998: Succession of Flares, CMEs Upsets Spacecraft,
Presages Solar Maximum
- March 1998: Polar's Visible Imaging System
Traces Shadow of Eclipse
- January 1998: Scientists Tracking Ejection from Sun
Reached Earth January 6
- August 1997: Polar Spies Plasma Leaving Atmosphere
- July 1997: Polar Confirms New Comet Tail
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
October 1999: Polar VIS Captures the
The Visible Imaging System (VIS) on the Polar spacecraft observed
the Moon's shadow as it moved across the face of the Earth during
the total solar eclipse of August 11, 1999.
VIS used its Earth Camera, which is sensitive to the ultraviolet
atmospheric emissions at 130.4 nm. Besides using the opportunity
for public outreach, the VIS science team got some science out of
this: by studying the scattering of ultraviolet light in and around
the shadow, they hope to learn something about the chemistry of
atomic oxygen in the upper atmosphere.
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: ISTP, ACE Assist Sounding
The Polar and ACE missions, and ISTP ground stations in Canada,
were instrumental in achieving maximum return from the recent launch
of the Enstrophy sounding rocket.
Launched Feb. 11 from Poker Flat,
the rocket studied the fine structure of auroral electric currents.
The launch sent four magnetometers through the poleward edge of the
ACE solar wind data told researchers that conditions
were right for loading Earth's magnetic tail. UVI and VIS images
from Polar showed the auroral structure through which the rocket was
launched. CANOPUS ground magnetometer data indicated approaching
auroral activity, so the rocket could be launched as the auroral arc
reached the site.
December 1998: Earth's Own Magnetosphere, Not Solar Wind,
Accelerates the Particles
of the Radiation Belts
EMABARGOED FOR RELEASE ON DECEMBER 7 AT 8:30 A.M. PST
Forty years after James Van Allen discovered the radiation belts, scientists have found that Earth's space
environment is a massive particle accelerator, boosting electrons to near light speed in a matter of
minutes. By using the coordinated measurements from two dozen spacecraft together with sophisticated
computer models, scientists should soon be able to make "weather maps" of this acceleration, allowing
predictions of the intensity of the radiation belts and the location of the most active regions. The
acceleration of particles inside the radiation belts can affect the operation of satellites.
The Van Allen radiation belts are a pair of doughnut shaped rings of ionized gas (or plasma) trapped in
orbit around Earth. The outer belt stretches from 19,000 km (11,500 miles) in altitude to 41,000 km
(25,000 miles); the inner belt lies between 13,000 km (7600 miles) and 7,600 km (4,500 miles) in
For decades, space physicists theorized that the Sun and its solar wind provided most of the high-energy
particles found in Earth's radiation belts. But new observations from the International Solar-Terrestrial
Physics (ISTP) program and other missions suggest that Earth's own magnetic shell in space, or
magnetosphere, is a more effective and efficient accelerator of particles.
According to Dr. Geoffrey Reeves of Los Alamos National Laboratory and an investigator for ISTP, the
solar wind and Sun are insufficient sources for the radiation belts. "There are just not enough
high-energy electrons in the solar wind to explain how many we observe near Earth," said Reeves, who
discussed the findings on December 7 in San Francisco during the Fall Meeting of the American
Data from NASA's Polar and SAMPEX spacecraft, as well National Oceanic and Atmospheric
Administration (NOAA) and the Department of Defense satellites, show that the radiation belts change in
response to a variety of solar events. High-speed streams of solar wind, coronal mass ejections, and
shock waves from the Sun all can compress and excite the magnetosphere. But it is the pressure and
energy of these events, not the particles buried in them, that energizes the particles trapped inside the
"It is amazing that the system can take the chaotic energy of the solar wind and utilize it so quickly and
coherently," said Dr. Daniel Baker of the University of Colorado, an investigator for ISTP and SAMPEX.
"We had thought the radiation belts were a slow, lumbering feature of Earth, but in fact they can change
on a knife's edge."
Discovered in 1958, the radiation belts have long been treated as a relatively stable and predictable
phenomenon. But in studying recent space weather events, space physicists have found that the
intensity of the belts can vary by 10, 100, or even 1000 times in a matter of seconds to minutes. "The
radiation belts are almost never in equilibrium," said Reeves. "We don't really understand the process,
but we do know that things are changing constantly."
For instance, in early May 1998, a series of solar events provoked the most powerful storm in the
radiation belts of the current solar cycle. Following a succession of coronal mass ejections and flares on
the Sun, several major magnetic storms brought auroras to Boston and Chicago, and ISTP ground
observatories in Canada and Antarctica measured electric currents in the ionosphere about 3-4 times
the norm. The leading edge of the magnetosphere, which usually sits at 76,000 km (45,000 miles) from
Earth toward the Sun, was pushed in to 25,000 km (15,300 miles).
In the wake of this disturbance, the natural gap (or "slot" region) between the two radiation belts was
filled by a new radiation belt, as energized particles were trapped where they wouldn't naturally settle.
The new belt lasted for nearly six weeks.
"The May 1998 event was a harbinger of what may come during the approaching solar maximum," said
Baker. At the height or maximum of the 11-year solar cycle -- predicted for 2000-2001 -- coronal mass
ejections and other solar events that disturb the radiation belts are likely to be much more common.
Observations from the May event are prompting researchers and space weather forecasters to reconsider
the radiation belt models relied upon by the engineers who design and operate satellites. "We now have
a fleet of satellites that gives us a more complete picture of what's going on in the radiation belts," said
Reeves. "We are using this data to construct pictures, essentially 'weather maps' of what's going on in
the radiation belts."
"Within the research community, there has been continuous progress in modeling the space
environment, but very little of that research has made it into the space weather operations community,"
said Dr. Terrance Onsager of NOAA's Space Environment Center. "Most of the models in use today do a
reasonable job of predicting average conditions, but few of them take into account the dynamics and
how quickly the system can change."
"Some of the new models that we are developing will allow us to visualize the radiation environment
over vast regions of space and then specify and predict the conditions at any location," Onsager added.
"We are beginning to synthesize mature models with the new stream of real-time measurements from
space in order to give industry and the government the information it needs to work in space."
December 1998: Solar Wind Squeezes
Some of Earth's Atmosphere Into Space
EMBARGOED FOR RELEASE ON DECEMBER 8, 1998 AT 10 A.M. PST
Using NASA's Polar spacecraft, researchers have found the first direct evidence that bursts of energy
from the Sun can cause oxygen and other gases to gush from Earth's upper atmosphere. Space physicists
have observed that the flow of "polar wind" increased substantially when a storm from the Sun smacked
into Earth on September 24- 25, 1998. In effect, pressure from the solar ejection squeezed gas out of the
Scientists have known since the early 1980s that Earth's upper atmosphere leaks oxygen, helium, and
hydrogen ions (atoms that have gained or lost an electron) into space from regions near the poles. But it
was not until the Polar spacecraft flew through this fountain of ionized gas in September 1998 that
scientists confirmed that the flow of ions is caused by solar activity.
"We now have direct, quantifiable evidence that disturbances in the solar wind produce changes in the
flow of ions out of the ionosphere," said Dr. Thomas E. Moore of NASA's Goddard Space Flight Center,
principal investigator for Polar's Thermal Ion Dynamics Experiment (TIDE). "This solar wind energy
essentially cooks the atmosphere off of the Earth." Moore's observations were presented on December 8
in San Francisco during the Fall Meeting of the American Geophysical Union.
On September 22, 1998, the Sun ejected a mass of hot, ionized gas (plasma) toward Earth. This magnetic
cloud of plasma (known as a coronal mass ejection) increased the density and pressure of the solar wind
and produced a shock wave similar to a sonic boom. When that cloud arrived at Earth late on September
24, it rammed into and compressed Earth's magnetic shell in space, or magnetosphere. The shock and
pressure excited the plasma trapped in Earth's ionosphere to a point where some ions gained enough
energy to escape gravity and flow downwind of Earth.
The amount of oxygen and other gases lost from the ionosphere amounted to a few hundred tons,
roughly equivalent to the mass of oxygen inside the Louisiana Superdome. "This is the supply of plasma
that makes things interesting in space," said Moore. "Much of the gas ejected from the ionosphere is
caught in Earth's wake. It then flows back toward the Earth while being heated and accelerated by the
same processes that create auroral particles and the radiation belts."
The ionosphere is a series of invisible layers of ions and electrons that are suspended in Earth's
atmosphere at about 50 to 400 kilometers (25 to 250 miles) in altitude. These particles are produced
when the Sun's ultraviolet light ionizes the atoms and molecules in the upper atmosphere. The
ionosphere makes long distance radio communication possible by reflecting radio waves back to Earth.
It also is home to the aurora and the electrical currents that heat the atmosphere during magnetic
"Our research shows that Earth"s own ionosphere is a major contributor to the growth of space storms,"
said Dr. Barbara Giles, a co-investigator on the TIDE team and researcher at NASA-Goddard. "These new
observations will help us understand the conditions that enable space storms to form, thereby moving
one step closer to the forecasting of the most damaging storms."
Prior to the launch of Polar, such observations of ions flowing out of the ionosphere were nearly
impossible. But the TIDE instrument was specifically designed to neutralize the electrical charge that
naturally builds up on the surface of spacecraft due to sunlight (about 40 to 50 volts). By squirting a
small plume of Xenon ions and electrons, TIDE offsets the charge on the spacecraft and allows detectors
to observe cold plasmas like the oxygen ions seen during the September event.
June 1998: ISTP Observes Effects of New
Observations made by ISTP scientists suggest that a new radiation
belt was formed in May, an unusual phenomenon not observed
Following 7 coronal mass ejections and 2 X-class flares in early
May, the population of relativistic electrons in Earth's radiation
belts achieved "killer" energy levels, according to investigators
Dan Baker and Geoff Reeves. The boost in radiation lasted longer
and achieved higher energy than any event since the last solar
During one magnetic storm in May, the disturbance storm-time
(or Dst) index reached -218 on the scale of 0 to -220---the largest
storm of the current solar cycle. An ISTP ground station
(CANOPUS) measured electric currents in the ionosphere well
above 4000 nanotesla, about 6-8 times the norm for solar
As a result of the consecutive doses of radiation, the Polar
spacecraft was upset to the point of being shut down for several
hours. ISTP investigators also have found compelling evidence
that several other satellite failures may have been related to
radiation belt activity.
May 1998: Succession of Flares, CMEs Upsets Spacecraft,
Presages Solar Maximum
>From April 27 through May 6, ISTP spacecraft and observatories got
of what is to come during the maximum of the solar cycle. In the
10 days, the Sun produced seven coronal mass ejections, two X-class
(the most energetic type), and at least two energetic particle
Earth, several major magnetic storms upset several spacecraft,
auroras to lower-than-normal latitudes, and forced power companies
reconfigure the grid in New England.
On April 29, a "halo" CME left the Sun and its shock arrived at the
spacecraft within 53 hours. The shock arrived faster than any
detected so far by SOHO. A magnetic storm followed on May 2.
rumors and anecdotal reports, the failure of the Equator-S
not necessarily a result of CME or magnetic storm.
ISTP observers have noted that sunspots are becoming more
moving with a clockwise rotation--telltale properties of proton
On May 1-2, ISTP observed two halo CMEs, as well as an X-class
High-energy protons from the flare arrived at SOHO within 30
a major magnetic storm developed on May 4.
During the storm, the disturbance storm-time index (Dst) reached
the scale of 0 to -220. It is the largest storm of the current
An ISTP ground station (CANOPUS) measured electric currents in the
ionosphere well above 2000 nanotesla, about 3-4 times the norm for
minimum. The January 1997 event that knocked out Telstar 401 had
of 1800 nT.
In response to the magnetic storm, power companies in New England
their power sharing capacity with Canadian utilities. Auroras were
reported as far south as Boston and Chicago.
May 6 also was an extremely active day: ISTP observatories detected
CME, a moderate-speed halo CME, and a fast CME, though none led to
storms. However, an X-class flare burst from the Sun, raining
protons toward Earth. The protons upset the Polar spacecraft,
controllers to shut it down for several hours. All of the
electronics have since been restored.
March 1998: Polar's Visible Imaging System
Traces Shadow of Eclipse
On February 26, while most eyes and cameras were trained on the Sun
as it hid behind the Moon, ISTP's Polar spacecraft turned to look
at Earth. From 1715 UT to 1909 UT, Polar's Visible Imaging
System--designed and operated by Lou Frank and John Sigwarth of the
University of Iowa--observed the shadow of the eclipse as seen from
50,000 km above the northern hemisphere.
The path of the eclipse's shadow began near the equator in the southern
Pacific Ocean, progressed across the northernmost tip of South
America, crossed through the southern Caribbean and ended over the
The VIS Earth Camera obtained global views of the Earth throughout
the eclipse while the Low Resolution Camera acquired magnified
views of the Moon's shadow on Earth. Real time images were
available on the World Wide Web within minutes of their exposure,
generating several thousand web hits per second.
January 1998: Scientists Tracking Ejection from Sun
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
August 1997: Polar Spies Plasma Leaving Atmosphere
Using the Thermal Ion Dynamics Experiment (TIDE) on Polar, NASA scientist
Thomas Moore and colleagues have taken the first accurate, high-altitude
measurements of the polar wind, the plasma that escapes Earth's ionosphere
into the tail of the magnetosphere. TIDE found that hydrogen plasmas flow out
of Earth's polar holes faster than predicted, and they are hotter than expected
from polar wind theories. Researchers also observed a substantial amount of
oxygen flowing along Earth's magnetic field lines and out of the atmosphere.
The flow of oxygen is in excess of the amount predicted by most models. The
hydrogen and oxygen ions in the polar wind are produced by the breakdown of
water molecules in the upper atmosphere. Essentially, Earth is losing water to
space through the poles. Moore concludes that more energy is going into the
plasmas of the polar wind than theory predicts. This finding will improve
understanding of space weather and of how planetary atmospheres evolve.
July 1997: Polar Confirms New Comet Tail
While viewing comet Hale-Bopp, the Visible Imaging System on Polar
independently confirmed the existence of a new type of comet tail --
made up of electrically neutral sodium atoms. The yellow-tinted "neutral
sodium tail" was initially discovered by European astronomers at a
ground-based observatory. The sodium tail is narrower and straighter
than the dust tail, and it lies between the dust and ion
tails. The images were made possible by special filters that allow
Polar's cameras to view objects too close to the Sun to be observed with
conventional and orbiting telescopes. For more information, go to
Observations on Hale Bopp.
For reporters and editors seeking more information
about any ISTP news item, contact Mike Carlowicz, science writer
for ISTP, at