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The Sun
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Although the Sun nurtures life on Earth, it can create costly problems for society. Geomagnetic storms, spawned by disturbances in the vast solar corona, may scramble radio waves, disrupt navigational systems, burn up electrical transformers and exert a drag on low-orbiting satellites. Even small shifts in the radiative output of the Sun may drive global changes in climate.

Scientists in NCAR’s High Altitude Observatory, using computer models and observing tools, are sharpening the view on how these vast forces shaping solar magnetism. This research may point the way to better predictions of solar storms and other events with impacts on the atmosphere, such as the Sun’s 11-year cycle. In addition, other NCAR scientists are raising important questions about the effect of the Sun on Earth’s climate, and its role in past and future climate change.

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A Prodigious Celestial Furnace

If not for the Sun, we wouldn’t have any weather. Sunlight provides the energy that drives atmospheric circulation and creates such weather events as precipitation and wind. The Sun also has profound impacts on our upper atmosphere, shaping the magnetic field that surrounds our planet. Brief ejections of mass and energy from the Sun sometimes set off electrified disturbances in this magnetic field that affect sensitive communications and energy systems worldwide. Solar researchers at NCAR and elsewhere use an array of instruments to determine the complex processes in the Sun's interior and atmosphere, as well as its impacts on Earth’s atmosphere.

The Sun is often described as an average star, but it is actually a prodigious and very stable producer of energy. Earth, at a distance of 93 million miles (150 million kilometers), receives less than a billionth of the Sun’s energy output. But that’s enough to warm the planet, enable photosynthesis in plants, and sustain our web of life.

Is the Sun’s output always the same? Scientists talk about the solar constant, which amounts to 1,368 watts of energy per square meter (9 square feet) at the outer edge of the Earth’s atmosphere. (An average-sized person positioned there, facing the Sun, would intercept the energy equivalent of more than 13 100-watt light bulbs.)

In fact, however, precise satellite measurements show the Sun’s total energy output rises and falls by as much as 0.1 percent over the 11-year solar cycle. This total output is dominated by visible, near-ultraviolet, and near-infrared radiation, but radiation in the extreme ultraviolet can vary a hundred times more over the solar cycle. It is this extreme ultraviolet radiation that controls Earth’s upper atmosphere, and NCAR scientists use observations of its variability, along with theoretical atomic physics and computer modeling, to study its effects on the upper atmosphere.

As powerful and stable as the Sun is, it won’t be around forever in its current form. In another several billion years, it will transform into a red giant star. Its surface will likely expand and encompass the inner planets, including Earth. Eventually, it may contract into a relatively small, cool star known as a white dwarf. But, thankfully, that’s all far in the future.

If not for the Sun, we wouldn’t have any weather. Sunlight provides the energy that drives atmospheric circulation and creates such weather events as precipitation and wind.

The Solar Interior

The Sun’s energy is created deep within its core. This is a place where the temperature is so high (15 million degrees Celsius) and the pressure so great (340 billion times the atmospheric pressure at sea level) that nuclear reactions take place. The nuclei of hydrogen atoms collide at incredibly high speeds, fusing together in groups of four to form a helium nucleus, which scientists call an alpha particle. This particle has slightly less mass than the four protons. The difference in mass is released as energy, which gradually works its way to the Sun’s outer surface—the luminous area known as the photosphere, or “sphere of light.”

The photosphere looks extremely bright because it is about 6,000ºC, which is the temperature at which maximum amounts of radiation in the visible region of the spectrum are emitted. But it also contains dark blemishes, known as sunspots, that are cooler (about 4,000ºC). These regions of concentrated magnetic fields tend to develop in groups, with some individual spots covering areas 20 times larger than a circle the diameter of Earth . They may last for weeks or months and develop in cycles, with the maximum number of sunspots occurring about every 11 years. The most recent solar maximum occurred in 2001.

Scientists have long wondered why the solar cycle averages 11 years and what the reasons for sunspot patterns might be. In 2004, NCAR researchers unveiled a computer model indicating the source of concentrated magnetic fields that can create sunspots. These fields are transported by a circulating current of gas, which flows on the Sun's surface, from its equator to its poles, and then sinks, returning to the equator some 200 million meters below the surface—about a third of the way to the center of the Sun—at the base of the solar convection zone. This motion, sometimes compared to a conveyor belt, would explain why sunspots, originating from the base of the convection zone, occur at certain times and in certain areas of the Sun. Scientists are beginning to use this theory to make predictions about solar activity, which could eventually help society better prepare for solar storms.

Much remains unknown about the solar interior because it is impossible to observe directly. NCAR researchers and colleagues elsewhere who study solar magnetism and activity rely in part on helioseismic observations, which are measurements of movements of the Sun’s surface. These movements, analogous to movements produced by earthquakes on our planet, are caused by pressure fluctuations from deep within the Sun’s surface. Using these observations, scientists are gaining insights into temperature, density, and movement deep within the Sun.

The solar equator spins around in 28 days (pink/red) while the poles take 35 days (blue/black). The model simulates this differential rotation. (Image by Mark Miesch, NCAR, ©UCAR.)

The Solar Atmosphere

The Sun's turbulent atmosphere, at more than a million degrees C, is a place of constant churning and frequent explosions. Loops of magnetic fields arc above the surface, filled with clouds of electrified gas. Known as plasma, the electrified gas forms when temperatures become so hot that atoms break apart into charged particles. The charged plasma particles are blown away from the Sun in every direction, moving millions of miles per hour—which is enough speed to escape the gravitational pull of the Sun. This vast flow is known as the solar wind, and it extends beyond the far reaches of our solar system.

The Sun’s outer atmosphere, or corona is visible from the ground only during a total solar eclipse (or an artificial eclipse created by a specialized telescope called a coronagraph), when it appears as a pale cloud encircling the Sun. But whether or not an eclipse is in progress, observers should never look directly at the Sun. Even after traveling 93 million miles, the energy we call sunlight can damage the eye. NCAR's Advanced Coronal Observing System observes the corona in several different wavelengths daily.

NCAR researchers are working on an instrument, known as a multichannel polarimeter, designed to examine the Sun’s magnetic fields by focusing on wavelengths emitted by a type of iron that is a common and easily visible solar element. The polarimeter will give researchers insights into the forces that twist and ultimately tear apart magnetic loops.

When the solar wind leaves the corona, it flows around obstacles such as planets. Those planets—each with its own magnetic field—respond in particular ways. The shape of Earth's magnetic field resembles the pattern formed when iron filings align around a bar magnet. Under the influence of the solar wind, Earth's magnetic field lines are compressed in the direction of the Sun and stretched out downwind. This creates the magnetosphere, a complex, teardrop-shaped cavity around Earth.

The solar wind also has profound impacts on Earth’s upper atmosphere, a region known as the mesosphere and lower thermosphere/ionosphere. This zone, 40–110 miles (60–180 km) above Earth’s surface, is difficult to probe. Ground-based instruments can detect only a small portion of it, and sounding rockets provide just a brief picture of the region before falling back into the lower atmosphere. Scientists want to know more about the upper atmosphere, partly to bolster communications networks and help keep satellites on course, and partly to learn how it influences temperature and energy in the lower atmosphere.

Researchers capture data on the mesosphere and lower thermosphere/ionosphere in a variety of ways. One of the most important research tools is a NASA satellite named TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics). The four instruments on board measure winds, temperatures pressure, energy from auroras, information on the chemistry of important gases, and other features. NCAR scientists developed and run TIDI (TIMED's Doppler Interferometer), which measures the speed and direction of high-atmosphere winds across the globe. Researchers expect measurements from TIDI to shed light on other related phenomena at the edge of Earth's atmosphere.

NCAR scientists are incorporating TIMED data into computer models being developed to better represent the complex physical and chemical processes in the upper atmosphere.

Earth is shown as a relatively small dot within the teardrop-shaped cavity of the magnetosphere in this illustration. The flow of particles from the Sun known as the solar wind gives the magnetosphere its shape.

Solar Storms

Though the Sun looks calm in the sky, it is actually the site of violent activity driven by magnetic mayhem. The most dramatic solar storms are known as coronal mass ejections. These are associated with the corona’s strong magnetic fields. When the fields are closed, often in loops above groups of sunspots, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas, ejecting billions of tons of matter. This may create a shock wave in the solar wind that accelerates particles to dangerously high energies. Behind the shock wave, the coronal mass ejection expands into a huge cloud that engulfs any planet in its path with plasma.

Coronal mass ejections can strike Earth’s magnetosphere, the magnetic field surrounding our planet. This collision leads to changes in the outer atmosphere that disrupt satellite orbits and ground-based communications and power systems. During these solar storms it's also unsafe for astronauts to work outside their spacecraft.

NCAR’s Mauna Loa Solar Observatory in Hawaii trains advanced observation tools on the corona to examine the forces that lead to coronal mass ejections. NCAR scientists are also working with their counterparts across the country on a comprehensive program, known as the Center for Integrated Space Weather Modeling, to simulate solar storms and their impacts on Earth.

Huge clouds of cool gas, known as prominences, are often carried outward with coronal mass ejections, forming spectacular twisted arches as they move away from the Sun. To better understand the behavior of prominences, NCAR scientists are developing techniques to measure the magnetic fields that hold prominences above the solar surface.

Researchers are interested in related types of solar disturbances as well. Associated with many coronal mass ejections are brief but enormous explosions, known as flares, which spew high-energy extreme-ultraviolet radiation and x-rays into space. Flares, which may be caused by the tearing and reconnecting of magnetic fields, can last less than an hour but heat material to many millions of degrees and release as much energy as a billion megatons of TNT.

Coronal mass ejections can strike Earth’s magnetosphere, the magnetic field surrounding our planet. This collision leads to changes in the outer atmosphere that disrupt satellite orbits and ground-based communications and power systems. During these solar storms it's also unsafe for astronauts to work outside their spacecraft.

Space Weather

Solar storms can have dramatic effects on Earth’s atmosphere. When many tons of magnetized gas strike the Earth's magnetic field (the magnetosphere), the result can be geomagnetic storms that spark million-ampere electric currents and distort the magnetosphere. This can scramble radio waves, disrupt navigational systems, and pump extra electricity into power lines.

In March 1989, a magnetic storm burned up a transformer in New Jersey, collapsing the entire power grid in Quebec and leaving six million people without power. Such storms also heat the upper atmosphere, exerting a drag on low-orbiting satellites. Astronauts in space can be subjected to potentially lethal doses of radiation.

How best to prepare society for these inevitable storms? As part of their work for the Center for Integrated Space Weather Modeling, NCAR scientists are creating new computer models of the Sun and the upper regions of Earth’s atmosphere. This work may lead to numerical forecasts of solar activity, much the way weather forecasters use computer models to generate forecasts of rain, snow, hail, and wind storms. In time, forecasts of coronal mass ejections and other major solar disturbances may become as commonplace as forecasts of thunderstorms.

The same research will lead to forecasts of the beautiful atmospheric lights known as the aurora borealis (in the Northern Hemisphere) and aurora australis (Southern Hemisphere). The aurora is produced when the solar wind interacts with Earth’s magnetic field to produce high-energy particles. These particles are guided by the magnetic field toward the upper atmosphere above Earth's high latitudes, where they collide with oxygen, nitrogen, and other molecules. Each molecule is electrically excited and emits light in a characteristic hue. For example, oxygen emits red and yellow-green, and nitrogen emits blue.

Although the aurora are often called northern or southern lights because they are most visible in polar regions, viewers in the Northern Hemisphere can sometimes see them as far south as Los Angeles, Rome, and Tokyo. Another benefit of accurate forecasts will be alerting the public to the best times and places to view these spectacular sky shows.

NCAR scientist Stanley Solomon captured this aurora unfolding above NCAR's Mesa Laboratory on the evening of 20 November 2003.

Solar Variation and Earth's Climate

Everyone agrees the Sun has profound influence on our atmosphere. But what, exactly, are its impacts on climate? Researchers believe that changes in sunspot activity or other solar events may affect Earth in ways that are indirect but that can have a significant impact.

For example, the virtual disappearance of sunspots between 1645 and 1715 coincided with a period of intensely cold winters in Europe, part of the period dubbed the Little Ice Age. The lack of sunspots may have reduced solar radiation by a small amount, perhaps a quarter of a percent—enough to contribute to famines in Europe and allow glaciers to expand.

Although sunspots send comparatively little solar radiation into space, they are surrounded by bright areas with a high energy output. As a result, periods of sunspot activity see more overall solar radiation reaching Earth. Satellite measurements have detected a 0.1 swing in the Sun's total output during the course of an 11-year sunspot cycle.

That change appears to be too small to significantly affect global average temperatures in the lower atmosphere. But the ebb and flow of solar radiation can heat and cool the stratosphere enough to change its circulation patterns, which may have significant impacts on regional climate. In the case of the Little Ice Age, for example, Europe and North America felt the temperature drop most strongly.

The Sun may have other, more subtle climate impacts. Some researchers speculate that energy from the Sun may influence global temperatures indirectly by affecting the formation of clouds. Others speculate that plant growth, which appears to vary during solar cycles, may respond to variations in solar energy.

NCAR researchers develop powerful computer models to simulate the impact of the Sun on our climate. One such effort, the Whole Atmosphere Community Climate Model (WACCM), helps researchers home in on interactions among different levels of the atmosphere, ranging from the surface of Earth to the upper atmosphere and the edge of space. The modeling work is combined with analyses of data from observing instruments aboard satellites to track the impacts of solar energy throughout the atmosphere.

Researchers believe that changes in sunspot activity or other solar events may affect Earth in ways that are indirect but that can have a significant impact.