>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 monograph New Perspectives on the Earth's Magnetotail has been selected by the Association of American Publishers as the best professional and scholarly book in physics and astronomy for 1998.
New Perspectives was edited by Atsuhiro Nishida, director-general of ISAS and project scientist for the Geotail mission; Daniel N. Baker of University of Colorado and an ISTP investigator; and Stanley W.H. Cowley of the University of Leicester (U.K.).
More than half of the book's 19 chapters were penned by ISTP scientists. Most of the results reported in the book were derived from or confirmed by Geotail observations.
Geotail discovered intense bursts of energized particles flowing in Earth's magnetosphere at nearly twice the speed of typical plasma flows. Though short-lived--about a minute or less--these bursts transport hot plasma as fast as 2000 km/s. Such high-speed flows occur on a local, small scale, which helps explain why they've never before been detected.
In the case studied by Don Fairfield of NASA-GSFC, a high-speed flow of plasma was detected about 13 Earth radii downwind in the magnetosphere. The flow coincided with the onset of a magnetic substorm at Earth. Substorms cause brilliant auroral displays, intense electric currents in the ionosphere, and energization of particles in the radiation belts.
One of the leading theories of the origin of substorms says they
are caused by "magnetic reconnection"--conversion of magnetic energy
into particle energy--about 20-25 Re downwind from Earth. The
location and intensity of Fairfield's plasma bursts lends new
evidence to that disputed theory.
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
Tsugunobu Nagai of the Tokyo Institute of Technology has found that the
near-Earth neutral line of the magnetotail typically lies about 25 Earth radii
(Re) downwind of our planet. (Previous data had put it anywhere from 15 to 30
Re.) The near-Earth neutral line is a point at which plasmas are accelerated
either toward or away from Earth
(see figure). Knowing the location and dynamics of that region is key to
understanding the origin of substorms--the most common form of magnetic
disturbance in our atmosphere. Physicists theorize that magnetic
reconnection--when field lines of opposite polarities connect to form a new
magnetic loop--occurs at the neutral line. Reconnection can lead to
compression of other field lines and to the conversion of magnetic energy into
plasma energy. According to Nagai, acceleration of plasma at the neutral line
tends to precede the onset of substorms, implying that reconnection initiates
storms. These observations of tail reconnection also could improve our
understanding of the reverse process taking place on Earth's day side, where
energy from the solar wind plasma is turned into magnetic field energy.
For reporters and editors seeking more information about any ISTP news item, contact Mike Carlowicz, science writer for ISTP, at email@example.com