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Sun-Earth Connection at NASA
NASA Missions Wind, Polar, Geotail, Soho, Cluster
Current Satellites in Orbit for Space Weather
SOHO stands for Solar Helioseismology....
SOHO stands for Solar and Heliospheric, where the heliosphere is the sphere around the sun (Helios).
Space scientists around the world are today celebrating the first anniversary of the European Space Agency's revolutionary Cluster mission to explore near-Earth space and study the interaction between the Sun and Earth.
This groundbreaking mission began exactly one year ago, on 16 July 2000, when two of the four Cluster spacecraft were launched from Baikonur Cosmodrome in Kazakhstan. Within a month, a second pair of identical satellites joined them in similar orbits that pass over the Earth's poles.
After the most complex period of spacecraft commissioning ever undertaken for any space mission - including the verification and testing of 44 separate instruments and 64 boom deployment sequences - full scientific operations started on 1 February 2001.
Since then, the Cluster quartet has been carrying out the most comprehensive exploration of the Earth's environment ever undertaken. For the first time, scientists have been able to explore the magnetosphere - the magnetic bubble that surrounds the Earth - with a flotilla of four identical spacecraft.
"Although Cluster has only been fully operational for five months, we have already gathered a huge amount of new information about the Sun - Earth connection," said Professor David Southwood, ESA Director of Science. "More than 200 scientists around the world are currently analysing this remarkable treasure trove of data."
"It has been a very challenging, but satisfying year," said Philippe Escoubet, Cluster project scientist. "Cluster is a completely new type of scientific mission, so it took us a while to find out how to get the best out of the satellites and their suite of instruments. Now we are receiving exciting new information about the magnetosphere and making new discoveries all the time."
By flying in a close, tetrahedral (lop-sided pyramid) formation, the four spacecraft have provided scientists with their first small-scale, three-dimensional views of near-Earth space.
"Cluster's new three-dimensional 'picture' of the magnetosphere is rather like looking at photos of an old familiar scene, but instead of the dull black-and-white pictures, we now have the same view in brilliant colours," said Professor Andre Balogh of Imperial College, London, principal investigator for the FGM experiment on Cluster.
Monitoring the magnetic shield around Spaceship Earth
Like a spacecraft orbiting another world, our Earth is trapped on an everlasting journey around the Sun. During its eternal voyage, Spaceship Earth is continuously exposed to the solar wind, a perpetual blast of plasma (electrically charged particles) sweeping outwards from our nearest star.
Fortunately for us, the Earth is protected by a powerful magnetic field which forces the supersonic solar wind to sweep around the planet. In the process, the magnetic field is shaped into a gigantic teardrop that typically extends approximately 65 000 km towards the Sun and more than two million km - five times the distance to the Moon - in the opposite direction.
However, a continuous struggle for supremacy rages as gusts in the solar wind cause the magnetosphere to balloon in and out. The fluctuating fortunes of the magnetic field are monitored by the Cluster flotilla as it flies through different regions of this unpredictable teardrop - the bow shock, the magnetopause, the cusps and the tail.
On the sunward side of the Earth lies the bow shock, where particles of the solar wind slam into the magnetosphere at a speed of about 400 km/s (around 1.5 million km per hour). This creates an enormous shock wave similar to a sonic boom ahead of a supersonic aircraft.
The new three-dimensional view from Cluster reveals a fast-moving, complex surface, in contrast to the motionless snapshots, frozen in time, provided by previous spacecraft measurements. During their encounters with the bow shock, the Cluster satellites have found that this turbulent boundary moves through space at 5 to 6 km/s (about the same speed that the International Space Station travels around the Earth).
Cluster has also provided the first confirmation of waves along the magnetopause - the outer limit of Earth's magnetic field. Until now, these plasma waves have only existed in computer simulations, but the Cluster spacecraft have surfed these waves and confirmed their existence. The speed of the waves has been estimated at around 70 km/s - equivalent to travelling from London to Paris in 4.5 seconds.
One of the real surprises concerned the polar cusps - 'windows' above the northern and southern polar regions where the particles from the solar wind can penetrate the magnetic shield. These cusps rapidly shift position due to gusts in the solar wind. The Cluster quartet has shown that they pivot through space at between 10 and 30 km/s - the first time this motion has been directly measured by spacecraft.
Closer to Earth, the mini-armada has flown through the plasmasphere - a doughnut-shaped region of dense plasma, mostly electrons and protons, that lies between the Earth's two magnetic poles. By flying in formation through the narrow part of the doughnut, Cluster has provided the best data yet on its complex ingredients of particles, electric and magnetic fields.
Radio signals from lightning, auroras (the curtains of red and green light that illuminate the polar skies) and particles that are trapped in the Earth's radiation belts have also been detected by Cluster. The new data are enabling scientists to find out where these signals originate and how they travel through near-Earth space.
Most dramatic of all have been the Cluster observations of solar storms. With the Sun now at maximum activity in its 11 year cycle, numerous powerful sunstorms are expected to occur. When one of the biggest solar storms on record began on 8 November 2000, instruments on Cluster were used to monitor the dramatic changes around Spaceship Earth.
About 8 minutes after a huge cloud of hot gas, known as a Coronal Mass Ejection (CME), was blasted from the Sun, the WHISPER instrument on Cluster detected an intense radio emission. Several days later, when the CME arrived at the Earth, it punched into the magnetosphere, pushing the magnetic shield toward the planet and leaving the Cluster spacecraft exposed to the solar wind, where they stayed for many hours.
The best is yet to come...
The latest chapter in Cluster's exciting exploration began in June, when ESA's intrepid flotilla began to explore the elongated magnetotail which stretches far beyond the Moon. During the next few months, Cluster will cast new light on this region where storms of high energy particles are generated. When these particles arrive at the Earth, they can cause intense auroras on the nightside of the Earth. A less attractive consequence is their ability to cause power cuts, damage satellites and disrupt communications.
"Cluster will provide us with a mass of new information about what takes place inside this magnetic 'power station' and help us to find out what generates such surges of energetic particles," said Dr. Escoubet.
"As we pass Cluster's first launch anniversary, we are all looking forward to even more exciting results in the months ahead," he added. "The best is yet to come."
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NASA-funded Earth Science researchers have discovered that during periods of increased solar activity much of the United States becomes cloudier, possibly because the jet stream in the troposphere moves northward causing changes to regional climate patterns.
The new study supports earlier findings by suggesting there is a relationship between increased cloud cover over the United States and the solar maximum, the most intense stage of activity on the Sun.
Previous studies have shown that during the solar maximum, the jet stream in the Northern Hemisphere moves northward. The jet stream guides storms and plays an important role in cloudiness, precipitation and storm formation in the United States.
Dr. Petra Udelhofen, a NASA-funded researcher at the Institute for Terrestrial and Planetary Atmospheres at the State University of New York at Stony Brook, is the lead author of a paper that discusses this topic, appearing in the July 1 issue of Geophysical Research Letters.
"Based on these results and because the location of the jet stream influences cloudiness," said Udelhofen, "we suggest that the jet stream plays an important role in linking solar variability and cloud cover."
The jet stream is a ribbon of fast-moving air in the upper troposphere that blows from west to east. Storms beneath the jet stream follow its path. A shift in the jet stream can alter the location of clouds and precipitation across the U.S.
The troposphere is the region of the atmosphere that extends from the Earth's surface out to about 50,000 feet and is the focus of local, regional and global weather research. The stratosphere extends above the troposphere to about 150,000 feet and is the region where the ozone layer is formed.
The Sun's energy output varies over an 11-year cycle, sending more ultraviolet radiation towards the Earth during times of increased activity. While the Sun's total energy output only varies by about one-tenth of one percent between periods of low and high solar activity, the ultraviolet radiation that affects ozone production in the stratosphere can change by more than 10 percent.
Ultraviolet radiation is absorbed in the Earth's stratosphere and creates the protective ozone layer. When the ozone absorbs ultraviolet radiation, it warms the stratosphere, which may affect movement of air in the troposphere where clouds form.
Solar cycle effects of ultraviolet radiation absorption by ozone in the stratosphere, its impact on atmospheric circulation and the location of storm tracks have been the subject of recent Earth Science research.
"Our results show that cloudiness varies on average by about two percent between years of solar maximum and minimum. In most parts of the U.S., cloud cover is slightly greater in years of solar maximum," noted Udelhofen.
Though more investigation is needed to better understand just how changes in the Sun's ultraviolet energy output is linked to atmospheric winds, the study helps people identify potential large-scale mechanisms that affect local and regional climates.
Scientists continue to investigate mechanisms that may link solar variability with weather. These new results support the idea of a link between stratospheric chemistry and meteorology, and support other recent theoretical studies associated with the impact of stratospheric chemistry on climate change and weather.
"It is important for future studies to identify and explain in detail the link between solar variability, ozone, the atmospheric circulation and cloud cover," Udelhofen said.
This research is part of the NASA Earth Science Enterprise program, which is dedicated to understanding how Earth is changing and what consequences these changes have for life on Earth.
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Space physicists have made the first direct observations of the process that causes auroras and magnetic disturbances -- or space weather -- around the Earth. Settling a fifty-year-old debate, scientists have directly measured the transfer of energy from the solar wind into the magnetic space around Earth, or magnetosphere, and down to the atmosphere. Such events can affect radio communications, spacecraft operations, and the control of electric power systems on Earth.
Relying on observations collected by NASA's Polar spacecraft and Japan's Geotail spacecraft, scientists associated with the International Solar-Terrestrial Physics (ISTP) program have gathered the first direct evidence that a process known as magnetic reconnection occurs naturally in the Sun-Earth system. Until now, reconnection had only been observed under contrived conditions in a few physics laboratories.
During reconnection, magnetic fields that are heading in opposite directions -having opposite north or south polarities -- break and connect to each other. In space, reconnection between the magnetic fields of the Earth and Sun allows the solar wind to break through the planet's magnetic shell and flow into the space around Earth. Along the way, magnetic energy gets converted to bursts of particle energy that create auroras - "northern or southern lights" -- and space weather storms.
Indirect evidence of reconnection has provoked debate for more than half a century, as space physicists could only detect signs of reconnection after it had happened. But recently, the Polar spacecraft flew through a region on the sunlit side of Earth where reconnection was in progress, gathering the first eyewitness account of the process. Using data collected from Geotail's dozens of passes through Earth's magnetic tail, scientists also have pinpointed the area on the night side where reconnection occurs, and have shown for the first time a clear association between reconnection and auroras.
"Reconnection is the fundamental process for transferring and exchanging energy in the Sun-Earth system," said Dr. Atsuhiro Nishida, a researcher with the Japan Society for the Promotion of Science and the recently retired Director-General of Japan's Institute of Space and Astronautical Science (ISAS). "Reconnection on the day side of Earth is critical for allowing solar wind energy to come into the magnetosphere. Night-side reconnection is critical for the transfer of that energy down to the atmosphere."
Nishida and colleagues presented their results today at the spring meeting of the American Geophysical Union, held in Washington, D.C.
While crucial for understanding space weather, the direct observation of reconnection around Earth has implications for many fields of physics. Reconnection on the Sun likely plays a role in the development of solar flares and of coronal mass ejections. Similar magnetic activity outside our solar system may explain some of the galactic X-rays that astronomers have detected. And observations of reconnection in nature may aid the study of nuclear fusion and other plasma processes in the laboratory. The magnetosphere is the only place where reconnection has been observed first-hand as it occurs naturally.
A popular misconception holds that auroras and space weather are caused when electrically charged particles from the Sun plunge directly into Earth's atmosphere near the magnetic poles. But in fact, the Sun provides the energy -- but not necessarily the particles -- to drive space weather activity around Earth. And rather than a direct trip from the solar atmosphere to Earth's poles, solar wind and storms from the Sun must pass through these small and elusive reconnection regions before they can stir up space weather.
"The magnetosphere acts like a great magnetic cocoon around the Earth," said Dr. Jack Scudder, professor of physics at the University of Iowa and principal investigator for the Hot Plasma Analyzer (HYDRA) on NASA's Polar spacecraft. "There are often times when the solar wind creates tears in this cocoon, allowing charged particles and energy from the Sun to enter the space around Earth. This tearing - reconnection - is what we directly observed with Polar."
Once these "tears" open up - scientists call them "reconnection regions" - the magnetic field of the solar wind becomes directly linked to the magnetosphere. Solar energy floods into the system, overloading and destabilizing it. The energy excites the particles already trapped around the Earth and stretches the magnetic tail like taut rubber bands, forcing reconnection to happen again -- this time inside Earth's space. As magnetic field lines on the night side snap and reconnect, they shoot energy stored in the tail down toward the auroral zones near the poles and into the radiation belts.
When the solar wind and magnetospheric fields reconnect, it opens a valve or faucet that lets the solar wind energy cross the magnetopause and pour into the magnetosphere," said Dr. Jeffrey Hughes, chairman of the department of astronomy at Boston University. "Without reconnection, the magnetosphere would be a very benign place."
Over the past eight years, ISAS's Geotail spacecraft has systematically studied and surveyed the magnetic tail of Earth in search of this process. As a result, scientists have been able to pinpoint the area where reconnection happens in the tail, about 85,000 to 96,000 miles (140,000 to 160,000 kilometers) downwind of the Earth. They have also been able to show that reconnection frequently occurs in the tail shortly before auroras and magnetic disturbances begin in Earth's atmosphere. Nishida and colleagues interpret those results to mean that reconnection is the source of energy behind the auroras and storms.
The International Solar-Terrestrial Physics program is a joint scientific study between NASA, ISAS, and the European Space Agency (ESA), with contributions from Russia's Institute for Space Research and many other international science institutions. The primary spacecraft of ISTP include ISAS's Geotail, NASA's Polar and Wind spacecraft, and the joint ESA/NASA Solar and Heliospheric Observatory (SOHO).
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Significant discoveries about the Sun made during and outside of solar eclipses.
From the book Totality: Eclipses of the Sun, 2nd ed., by Mark Littmann, Ken Willcox, and Fred Espenak (Oxford University Press, 1999) http://sunearth.gsfc.nasa.gov/eclipse/TOTALITY/TOTALITYchron.html