This newsletter is a joint project of SIRS Publishing, Inc. and the University Corporation for Atmospheric Research

Fall 2001 — VOLUME 8, NO. 2

Copyright | Note to Teachers

Space Weather: Living with a Turbulent Sun

Rising each morning, setting each evening— what could be more constant than the Sun? While the total energy emitted by the Sun changes only about 0.1% over a typical 11- year sunspot cycle, research indicates that the Sun is actually a dynamic and turbulent place. Ninety- five percent of the Sun’s energy reaches Earth as sunshine, the light and heat that make life possible. The remaining five percent, including energetic x- rays to long- wave radio waves can vary significantly over the solar cycle. Space weather is caused by variations in both solar electromagnetic radiation (Figure 1) and solar wind, the highvelocity, ionized gases or plasma with energetic particles that accelerate away from the Sun and fill planetary space.

The Solar Dynamo

Figure 1: The Sun emits energy from the complete electro magnetic spectrum from short wave X- rays through visible light in intermediate wavelengths and long- wave radio waves.

The Sun is an immense dynamo whose layered structure generates complex magnetic fields and energy. The core of the Sun is thought to rotate nearly rigidly, like a solid planet, but the outer part of the Sun rotates more rapidly at the equator than at the poles. Progressing outward from the core is a radiation zone (pale orange in Figure 2); then a convection zone. In between these two zones, rotation within the sun varies enough to create a shear layer, generating the Sun’s magnetic fields. As magnetic fields shift, materials of different temperatures and densities in the Sun’s interior and its atmosphere circulate in amazing ways, almost like ocean currents do on Earth.

In the 1940s, Walter Orr Roberts, founder of the High Altitude Observatory (HAO, now a division of the National Center for Atmospheric Research– NCAR), was a pioneer in viewing the Sun’s complex atmosphere, or corona. He used a coronagraph, an instrument that allows scientists to create an artificial solar eclipse. The coronagraph contains a metal disk used to block, or occult, the bright light from the solar surface. This allows the much dimmer corona to be seen. As in a natural solar eclipse, the solar corona appears as a faint, hot, magnetized sheath surrounding the Sun (Figure 3A). Coronagraphs allow scientists to observe and study dynamical events in the corona. At the Sun’s surface, temperatures are about 6,000º Celsius (C), while the outer atmosphere of the corona, where extremely hot gases are expanding, reaches a million degrees C. Many space weather events occur in association with eruptions from the corona.

Figure 2: The Sun's internal structure.

All manifestations of solar activity are due to the Sun’s dynamo magnetic fields. In bright, high- density regions of the corona, gases following magnetic fields arc upward and are then pulled back into the atmosphere (Figure 3B). Darker, lower- density areas of the corona occur where the magnetic field and plasma stream outward from the Sun, escaping into space as solar winds reaching beyond the orbit of Pluto (Figure 3C). There is a constant tug- of- war underway between the outward and inward magnetic field forces controlling the movements of solar materials. Even so, all the matter that escapes into space from the Sun each day in solar winds is comparable to the mass of Utah’s Great Salt Lake. And this happens every day, day after day, year after year.

Sunspots are dark, circular features on the Sun’s surface that form where denser bundles of magnetic field lines rise from the interior. Sunspots reach a maximum number at the height of the 11- year solar cycle. They can produce huge solar flares that heat particles rapidly, flinging them away from the Sun at speeds up to a million miles per hour. Flares can grow rapidly, rotating like hurricanes and lasting from tens of seconds to many minutes.

Violently released bubbles or tongues of gas along with their magnetic fields are known as coronal mass ejections (CMEs). A large CME can create solar wind containing more than a million tons of matter that travels into space at speeds up to 450,000 m/ sec or a million miles an hour. Associated with CMEs, prominences may be seen as loops of gas projecting above the edge of the sun from lower, cooler, and more dense regions of the corona. When seen with the disk of the Sun in the background, they look like and are called filaments.

Figure 3: Coronagraph. (A) Above the solar surface extends the solar atmosphere known as the corona. (B) Brighter regions represent high- density, closed magnetic fields where gases and plasma are trapped and recaptured by the Sun. (C) Darker regions are composed of lower- density plasma and gases following open magnetic field lines that expand outward to fill interplanetary space.

Wind in Space

Figure 4: With the discovery of solar wind, physicists realized that the magnetic field of Earth is pushed away from the Sun.

Constantly flowing, the solar wind fills the entire solar system. It consists mostly of hydrogen, with small amounts of helium, and traces of heavier elements, such as iron and silicon that become ionized by being stripped of their electrons. This stream of ionized particles compresses Earth’s magnetic field on the side facing the Sun. The magnetic field then stretches into a very long tail on the side away from the Sun. This complex magnetic envelope surrounding Earth is called the magnetosphere (Figure 4).

The magnetosphere protects us from the solar wind. In addition, the Earth’s upper atmosphere absorbs most of the Sun’s damaging radiation. Some of the magnetized particles, however, enter the lower atmosphere and can even reach the planet’s surface near Earth’s poles where magnetic field lines converge.

The solar wind and compression of Earth’s magnetosphere combine to form a vast electrical generator that converts the kinetic energy of solar wind particles into electrical energy. Although the very complex interactions between plasma and the currents in the magnetosphere are not fully understood, scientists have discovered that the compression of the magnetosphere close to the surface of Earth on the daytime side of the planet causes the geomagnetic field to fluctuate wildly. Such an event is called a geomagnetic storm.

Impacts of Space Weather

At ground level, a geomagnetic storm induces powerful surges of electrical currents in conductors. The first recorded impacts from space weather were disruptions of telegraph communication in 1847. As a result of our growing dependency on electricity, space weather’s impacts have continued to grow. Power grids and radio communications can be knocked out of service during geomagnetic storms, and oil pipelines and power stations can be severely damaged.

In Earth’s outer atmosphere, solar winds affect satellites by perturbing their orbits, striking their surfaces, and disrupting communications. Since 1994, 13 satellite losses and damage to the gyroscope on the Hubbell telescope have been blamed on space weather events. Currently many hundreds of satellites are in orbit, and with the development of space stations, there is increasing potential for radiation injury to astronauts and damage to spacecraft resulting from high- energy particles emitted by the Sun.

At night, we can sometimes see a geomagnetic storm in action as solar materials collide with, ionize, and excite molecules in Earth’s ionosphere near the poles. The result is the famed aurora borealis (the aurora australis in the Southern Hemisphere). Auroras are created as these e n e rgized molecules emit a wide spectrum of light from infrared to ultraviolet wavelengths. The most common aurora features a whitish- green light produced by atomic oxygen; pink emissions come from excited molecules of nitrogen.

Only in the last few decades have we realized space weather can profoundly affect people and their activities. The list of consequences grows in proportion to our increasing dependence on technological systems. As our use of space increases and electrical grids and communication systems become increasingly complex, we need to learn more about space weather in order to better predict it and protect ourselves from its consequences.

Meet a Solar Scientist

Joan Burkepile, NCAR Scientist

Joan Burkepile is an associate scientist in HAO at NCAR who studies the Sun’s coronal mass ejections throughout the solar cycle. She analyzes signatures of solar activity responsible for major geomagnetic storms.

“We have always looked to the stars as a source of inspiration and wonder,” says Burkepile. “The proximity of our own star provides us with a glimpse of the complexity and dynamic behavior that stars can possess.” She appreciates that technological advances have given us the tools to observe our star as never before and led us to the realization that the entire solar system is bathed in the solar wind. She believes we need to study and understand the effects of space weather to better protect our tools, ourselves, and our society.

“The school children of today are tomorrow’s first space colonizers,” adds Burkepile. “It is imperative that we gain a firm understanding of our dynamical Sun, so we can prepare for life on space stations, the moon, Mars, and beyond. I can think of no more exciting work than to study our changing Sun and our backyard interplanetary space.”

Science Now is jointly published by the Walter Orr Roberts Institute at the University Corporation for Atmospheric Research and SIRS Publishing, Inc. (Social Issues Resources Series.) Science Now is published three times during the school year and is distributed to SIRS subscribers. Comments and questions should be directed to Joyce Gellhorn via Internet at jgellhorn@sprynet.com. You can also contact your SIRS representative or write to:

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Editors:
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Writer:
Joyce Gelhorn

Scientific Advisor:
Joan Burkepile

UCAR is a consortium of over 60 universities in the U.S. and Canada with doctoral programs in atmospheric and related sciences. UCAR manages and operates the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Anyone who undertakes any of the activities described herein shall do so at their own risk; UCAR and SIRS Publishing, Inc. assume no liability, whatsoever, for any injury or harm, which may result therefrom.

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