>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 auroral oval.
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 ftp://ftp.hq.nasa.gov/pub/pao/tv-advisory/nasa-tv.txt
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.
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 Wind, Polar.
Reconnection is one of the most important plasma processes in the universe, a key method of energy exchange.
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 auroral zone.
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.
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 altitude.
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 Geophysical Union.
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 radiation belts.
"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."
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 ionosphere.
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 storms.
"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.
Observations made by ISTP scientists suggest that a new radiation belt was formed in May, an unusual phenomenon not observed since 1991.
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 max.
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 minimum.
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.
|>From April 27 through May 6, ISTP spacecraft and observatories got a sample of what is to come during the maximum of the solar cycle. In the span of 10 days, the Sun produced seven coronal mass ejections, two X-class flares (the most energetic type), and at least two energetic particle events. At Earth, several major magnetic storms upset several spacecraft, brought auroras to lower-than-normal latitudes, and forced power companies to 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 complex and moving with a clockwise rotation--telltale properties of proton flare sunspots.
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 -218 on the scale of 0 to -220. It is the largest storm of the current solar cycle.
An ISTP ground station (CANOPUS) measured electric currents in the ionosphere well above 2000 nanotesla, about 3-4 times the norm for solar minimum. The January 1997 event that knocked out Telstar 401 had currents of 1800 nT.
In response to the magnetic storm, power companies in New England reduced 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 a slow CME, a moderate-speed halo CME, and a fast CME, though none led to magnetic storms. However, an X-class flare burst from the Sun, raining high-energy protons toward Earth. The protons upset the Polar spacecraft, forcing controllers to shut it down for several hours. All of the instruments and electronics have since been restored.|
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 Atlantic Ocean.
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.
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 are eruptions of electrically charged gas from the Sun that can trigger magnetic storms around Earth. Such eruptions--which are becoming more frequent as the Sun builds up toward the maximum of its 11-year cycle--occasionally disturb spacecraft, navigation and communications 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 auroras.
Forecasters at the Space Environment Center of the National Oceanic and Atmospheric 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 Observatory (SOHO) 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 institutions include NASA, the European Space Agency (ESA), the Japanese Institute of Space and Astronautical Sciences (ISAS), the Russian Space Research Institute (IKI).
To view the same data and images as ISTP scientists, visit the
Event web page at http://pwg.gsfc.nasa.gov/istp/events/1998jan3.
For more information about ISTP and the physics of the Sun and Earth, go to http://pwg.gsfc.nasa.gov/istp/outreach.
For the official U.S. space weather forecast, visit http://www.sec.noaa.gov/today.html.
An image to accompany this story is available at http://pwg.gsfc.nasa.gov/istp/events/1998jan3/images.jpg.
Current images of Earth's aurora as seen from space are available
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.
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 email@example.com