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Coordinated Studies

News from ISTP on Coordinated Studies



December 1999: On the Day the Solar Wind Disappeared,
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 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




May 1999: Polar, Wind, Interball Verify
Dungey Theories of Reconnection

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 Wind, Polar.

Reconnection is one of the most important plasma processes in the universe, a key method of energy exchange.

Wind Observations:

Polar Observations:


April 1999: Example Successes -- ISTP, ACE, SOHO

Live From the Sun Live From the Sun

Backdrop Images and Animations


March 1999: ISTP, ACE Assist Sounding Rocket Launch

Earth 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.



December 1998: Earth's Own Magnetosphere,
Not Solar Wind, Accelerates the Particles of the Radiation Belts

Radiation Belt 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."




September 1998: ISTP on the Road: Workshops Foster Collaboration

Logo Image In September, ISTP conducted its first workshop in Europe (Rutherford Appleton Labs) to foster collaboration with Equator-S, Cluster, Interball.

Over 150 scientists from a dozen countries participated. Russian, Japanese, and Czech labs were well-represented. Highlights included: first review of May 1998 solar-terrestrial events; discussion of first simultaneous observations of reconnection in the magnetopause. By coincidence, a CME lifted off the Sun, the fourth time a CME coincided with an ISTP workshop.

Also in September, ISTP project scientists participated in a NATO workshop in Kosice, Slovakia. Much of the meeting focused on greater participation of Russian spacecraft in ISTP.



June 1998: ISTP Observes Effects of New Radiation Belt

Relative Fluences of Highly Relativistic Electrons 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.



May 1998: Succession of Flares, CMEs Upsets Spacecraft,
Presages Solar Maximum


Figure 1 >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.
Figure 2 On April 29, a "halo" CME left the Sun and its shock arrived at the SOHO spacecraft within 53 hours. The shock arrived faster than any other detected so far by SOHO. A magnetic storm followed on May 2. Contrary to rumors and anecdotal reports, the failure of the Equator-S satellite was 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.
Figure 3

Figure 4

Figure 5 		On May 1-2, ISTP observed two halo CMEs, as well as an X-class flare. High-energy protons from the flare arrived at SOHO within 30 minutes, and 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.
Figure 6
Figure 7
 Figure 8

Figure 9

 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.



April 1998: ISTP Conducts Successful Workshop

Latest ISTP workshop -- held April 7-9 at GSFC -- attracted 187 participants from at least 15 countries.

Included interactive panels on the highlights of ISTP and the future of the mission. Science team suggested that time has come for an ISTP monograph.

Workshop included a preview of some of the 200+ ISTP-related papers to be presented next month at AGU.

Highlights included evidence for reconnection in Earth's magnetic tail and plans for Wind's exploration of high-latitude magnetosphere.

Outreach session showcased a dozen efforts being made to share space physics with students and the public.



April 1998: New Poster Conveys Excitement,
Relevance of Sun-Earth Connection

CME Poster ISTP has created a new 22" by 34" poster for high school students and adult science buffs. The poster highlights the dynamic, electric connection between Sun and Earth by using more than two dozen images (front & back), a magazine-length article on space weather, and a CME tracking exercise. 28,000 copies of "Storms from the Sun" are now being produced for distribution at the April meeting of the National Science Teachers Association and other meetings. It will also be available through NASA Educator Resource Centers. A Spanish-language version of the poster will be produced later this year.



January 1998: Scientists Tracking Ejection
from Sun that Reached Earth January 6

Image of sun 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 Sun-Earth Connections Event web page here.
For more information about ISTP and the physics of the Sun and Earth, go here.
For the official U.S. space weather forecast, visit http://www.sec.noaa.gov/today.html.

NOTE: An image to accompany this story is available here. Current images of Earth's aurora as seen from space are available at http://eiger.physics.uiowa.edu/~vis/images/.



December 1997: Space Physicists Find the Energy
that Powers Explosive Coronal Mass Ejections
and Discover Signatures of Their Origins and Impact

Using spacecraft and supercomputers, scientists from the International Solar Terrestrial Physics (ISTP) program have developed a new theory for the explosive, high velocity coronal mass ejections (CMEs) that will erupt from the Sun with increasing frequency during the maximum of the new solar cycle. CMEs are eruptions of electrically charged gas from the Sun that can trigger magnetic storms around Earth. Such storms occasionally disturb spacecraft, navigation and communications systems, and electric power grids.

Recent experimental and theoretical observations from ISTP indicate that the interaction of magnetic fields high above the Suns surface, not low in the corona, as was previously thought, allows tremendous energy to build up and to release CMEs at speeds approaching 2000 kilometers per second. Scientists also have determined that halo CMEs almost always lead to geomagnetic storms, an observation of great consequence for the prediction of space weather. Finally, ISTP researchers have been able to create realistic visual simulations of the effects that CMEs can have on Earth's magnetic field.

Coronal mass ejections are the largest structures that erupt from the Sun and they one of the principal ways that the Sun ejects material and magnetic energy into the solar system. Most CMEs travel at 400 km per second, but fast CMEs can reach speeds double or triple that. The first fast CME of the new solar cycle was observed on November 6, 1997. Slow or fast, the eruptions produce magnetic clouds that can cause geomagnetic storms and increase the intensity of the aurora (Northern and Southern lights). Fast CMEs also can accelerate protons in interplanetary space to the point where they can harm spacecraft or astronauts in space.

Inspired by images of CMEs collected by the Large Angle Spectrometric Coronograph on the SOHO spacecraft, Dr. Spiro Antiochos of the U.S. Naval Research Laboratory in Washington, DC, has proposed a new answer to the long-standing question of how the Sun can build up the energy to produce the violent explosions of fast CMEs. Using supercomputers from NASA and the Department of Defense, Antiochos has created a model that simulates the complex, interwoven magnetic structures of the Sun. After observing how magnetic fields abut and interact, Antiochos theorizes that the Sun's magnetic fields tend to restrain each other and force the buildup of tremendous energy.

When a stressed magnetic field low in the Sun's atmosphere pushes higher into the corona, it gets held back by the surrounding and overlying fields. As the stressed field builds up more potential energy, it pushes harder against these magnetic ropes and moves higher into the corona. Eventually, through a process known as "magnetic reconnection"-- where opposing magnetic lines of force merge and cancel--the field is released from its bonds and escapes the Sun as high speed.

Antiochos compares the process to that of filling a helium balloon. "If you fill it without anchoring it, the balloon will slowly drift upward," he said. "But if you hold the balloon down as you fill it, you can generate a lot of upward force, which makes the balloon take off at higher speed once you release it. Fast CMEs are released in the same way." Antiochos added that such high-speed bursts will be more frequent as the Sun approaches its maximum period of activity in 2000-2001.

In other research, ISTP teams headed by David Webb of Boston College and the Research Laboratory at Hanscom Air Force Base and by Dr. Guenter Brueckner of the Naval Research Laboratory used ISTP's SOHO and Wind spacecraft to study nine "halo" type CMEs that occurred from December 1996 to May 1997. They found that such events 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 the perspective of Earth.

According to Dr. Nancy Crooker, a space physicist at Boston University, detection of halo events "will vastly improve" methods of predicting geomagnetic storms. "If you see a halo CME, you are pretty sure that you are going to see a storm at Earth," Crooker said. "In the past, scientists could only look at solar flares to know if a storm was headed toward Earth. And while some flares have accompanying CMEs, some don't. But now we have instruments sensitive enough to see the CMEs themselves."

ISTP does not predict space weather. Forecasting is the domain of the Space Environment Center of the National Oceanic and Atmospheric Administration.

Crooker and Webb also have identified the signature of halo CMEs on the Sun's surface. These remnants of CMEs show up in X rays and ultraviolet light as the sudden brightening of magnetic arches, with dimming areas on either side. These dimming areas seem to mark the roots of CMEs already launched into space.

Downwind from those eruptions, the invisible magnetic shell (or magnetosphere) that shields Earth from the Sun's particles and radiation is regularly shaped and shorn by CMEs. To better understand this interaction, Dr. Charles Goodrich, of the University of Maryland, has developed a series of animations that depict how the magnetosphere responds to the shock of a CME.

Using a Cray C-90 and other powerful computers, Goodrich and colleagues have for the first time depicted the evolution of the magnetosphere as it is bombarded by a real CME. The simulation reconstructs 42 hours of time centered around the arrival of a CME on January 10, 1997. The simulation reveals the shape and orientation of Earth's magnetic field, as well as the density of plasma in Earth's space environment.

In fact, the real January CME produced magnetic storms and spectacular auroral displays, and poured as much as 1400 Gigawatts of electricity into the atmosphere (almost double the power generating capacity of the United States). The magnetic cloud from the CME smacked the magnetosphere with a burst of plasma 30 times denser than the normal solar wind. The shock compressed the leading edge of the magnetosphere inside geosynchronous orbit, where many satellites are positioned.

"We have created a global picture of what is going on in the magnetosphere," said Goodrich. "Since we only have a few spacecraft, and they can only make point measurements, this is the only way to look at the whole system." Such simulations will provide researchers with insights about the conditions that lead to--and perhaps trigger-- geomagnetic storms, Goodrich added.

In the past year, the primary spacecraft of ISTP--SOHO, Wind, Polar, and Geotail--and the program's network of cooperating satellites, ground sensors, and theory centers have monitored approaching interplanetary storms for the first time, from their genesis to their impact on Earth. ISTP was designed as a joint, comprehensive effort to observe and understand our star, the Sun, and its effects on Earth's environment in space. Participating institutions include NASA, the European Space Agency (ESA), the Japanese Institute of Space and Astronautical Sciences (ISAS), the Russian Space Research Institute (IKI), as well as the Max Planck Institute, the U.S. National Oceanic and Atmospheric Administration, the Los Alamos National Laboratory, the U.S. Air Force, the Canadian Space Agency, the British Antarctic Survey, the U.S. National Science Foundation, and the Johns Hopkins Applied Physics Laboratory.




illustration of the radio emissions along the trajectory border=0> </td> <td> <a name=1097><font size=4><b> October 1997: Wind, Ulysses Triangulate, Pinpoint Radio Burst </b></font> <br><br></a> Flares or other explosive events on the Sun can produce (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.


illustration of a coronal mass ejection
October 1997: September Space Weather Events Defy Predictions

 On September 24, a coronal mass ejection lifted off the Sun with the classic signature of an event that should cause geomagnetic storms. Scientists detected a Moreton wave (see figure), several flares, and Type II radio bursts associated with the shock fronts of a CME. NOAA issued a press release and alerted the physics community about a likely strong impact on Earth. Three days later, the CME missed SOHO, Wind, Polar, and Earth, reminding scientists that spectacular solar events don't necessarily mean effects at Earth.

illustration of a coronal mass ejection On September 28, SOHO spied another CME (see figure) leaving the Sun. Like the effective January 1997 event, this "halo" CME did not have the markings of a storm-inducing event--no Moreton waves, no flares, no Type II radio bursts, no X-ray signatures. NOAA predicted near-normal space weather, but ISTP alerted scientists to a possible impact. On October 1, Polar and ISTP ground stations detected bright aurora, a geomagnetic storm, and substorms heralding the arrival of the CME at Earth.



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