>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
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.
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
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 1
WIND Petal Orbit 2
WIND Extended Mission
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.
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
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.
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.
||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.|