The Sun-Earth Connection

  Understanding the Turbulent Star Next Door

This illustration shows how magnetic fields are recycled to produce sunspots within the solar convection zone (the top 30% of the solar interior, shown in white, surrounding the radiative core, in orange). Because the sun rotates faster at the equator than the poles, the north-south (poloidal) magnetic field (a) gets twisted into an east-west (toroidal) field (b). Pockets of enhanced toroidal field rise to the surface, twisting in the process, and emerge to create sunspots (c, upper right). Magnetic flux emerges and spreads outward as the spots decay. Panels (d) and (e) show the solar dynamo, the conveyor belt of plasma flow (yellow) carrying the surface magnetic flux toward the poles—reversing the polar field—and eventually downward and back toward the equator. New sunspots eventually form in the poloidal field (f), which is now reversed from that in (a). Click here or on the image to enlarge. (Illustration by Mausumi Dikpati, NCAR.) View a streaming animation of the solar dynamo, or sunspot conveyor belt (Animation by Mausumi Dikpati, NCAR.)

A 42-year quest pays off     Peter Gilman (Photo by Carlye Calvin, UCAR.) The American Astronomical Society honored Peter Gilman with the society's prestigious Hale Prize in 2006. Gilman was recognized "for his unique insights and substantial scientific achievements" in understanding the Sun's behavior during a career spanning four decades. Underlying his recent collaboration with Mausumi Dikpati on the mechanisms behind sunspots and the causes of the 11-year solar cycle is Gilman's long interest in the fluid dynamics and magnetic behavior of the Sun, stars, and planets. "For me this quest began in 1964, when I was a graduate student in meteorology at MIT," Gilman recalls. His adviser suggested examining the Sun to see whether the way its rotation varied with latitude could be explained using meteorological principles. The inspiration for this quest to explain the Sun's differential rotation came from diagrams that plot the movement of sunspots from the Sun's poles toward its equator during the 11-year solar cycle. The tilted pattern on either side of the equator created by differential rotation gave these images the nickname "butterflydiagrams." (View the enlargement of a recent diagram from NASA here.) Gilman has been closing in ever since on the long-sought explanation of the processes that drive sunspots into this intriguing pattern. One breakthrough came in the 1990s, with advances in helioseismology. This technique analyzes the structure, composition, and movement deep within the Sun's interior by detecting pulsations on its surface created by that activity. This new data revealed variations in internal rotation with depth as well as latitude. Gilman's work combines the principles of convection, shallow-water turbulence, and other concepts from Earth-systems science to examinations of the Sun's physical composition and magnetic behavior. He and colleagues are now testing explanations of the behavior of active longitudes, which are preferred places on the Sun where magnetic fields are strong and sunspots recur over time. In the future, Gilman predicts the team led by Dikpati will stretch the capabilities of the sunspot model to produce a unified theory of the solar cycle and active longitudes. And, he notes, since the model works so well for the Sun, it ought to work for understanding the behavior of many other stars.

We're now in the lull between the end of one 11-year sunspot cycle and the start of the next, but it's never quiet on the solar front at NCAR, whose High Altitude Observatory has been studying the Sun for over six decades.

Because of our increasing reliance on satellite-driven technology and far-flung power grids, the Sun and its magnetism can wreak havoc on society in a matter of hours.

The vast solar corona, the outermost part of the Sun's atmosphere, holds key clues to when, where, and how powerfully the next solar storm will assail satellites, mobile phones, or electrical grids. It's the launching pad for energized particles that can trigger geomagnetic storms in Earth's atmosphere.

New instruments and a new way of portraying the flow of magnetism around the Sun are advancing our understanding of the long-term solar cycle as well as the brief but enormous eruptions, known as coronal mass ejections, that fuel solar storms.

What's new under the Sun? A breakthrough forecast

The next sunspot cycle will be 30-50% stronger than the last one and begin as much as a year later than average, according to a breakthrough forecast using a computer model of solar dynamics developed by NCAR scientists.

The scientists have confidence in the forecast because, in a series of test runs, the newly developed model simulated the strength of the past eight solar cycles with more than 98% accuracy. The forecasts are generated, in part, by tracking the subsurface movements of the sunspot remnants of the previous two solar cycles.

"Our model has demonstrated the necessary skill to be used as a forecasting tool," says NCAR scientist Mausumi Dikpati, the leader of the forecast team at NCAR's High Altitude Observatory that also includes Peter Gilman and Giuliana de Toma.

Scientists for years have known about the current of plasma, known as the Sun's meridional flow, which moves at a pace of around 72 kilometers per hour (45 miles per hour) near the surface. But they had not previously connected it to sunspot activity. The meridional flow appears to act as a sort of conveyor belt by slowly transporting remnant magnetic signatures of the sunspots of previous cycles from the Sun's surface to the interior. Inside the Sun, the remnants give rise to a new generation of magnetic fields that produce new sunspots at the surface.

"In our model, we can show how physical processes relate the surface signatures of solar magnetic fields from old cycles to that of the new cycle," explains Dikpati. The model results are consistent with observed solar features, she adds.

Ten years from now, Dikpati hopes the model will be able to make some estimates of sunspot count for the next cycle. The model might also help society brace for an extended period of unusual solar activity, such as that during the Little Ice Age, when the number of sunspots dropped dramatically and temperatures cooled in some regions of the globe.

A new eye on the Sun: The Solar Optical Telescope

NCAR researchers helped design the Solar Optical Telescope and plan its research mission. The telescope is one of three aboard the Hinode satellite, which flew into orbit on September 22, 2006. (Photo courtesy NAOJ.)

A new era in solar observing began with the launch of the Solar-B satellite on September 22, 2006. On launch by the National Astronomical Observatory of Japan, the satellite was given the name Hinode, meaning sunrise in Japanese.

Hinode promises to improve understanding of the eruptions on the Sun that create space weather, which puts modern societies at risk by disrupting the technologies we depend on, from navigational satellites to electrical power lines.

The satellite is in Sun-synchronous polar orbit, which positions it in continual twilight. This will allow researchers to view the Sun continually for 9 or 10 months of each year during Hinode's three-year life expectancy.

• Solar Optical Telescope (SOT) – photosphere (surface)
• X-Ray Telescope (XRT) – corona (outer atmosphere)
• Extreme-ultraviolet Imaging Spectrometer (EIS) – between the chromosphere (lower atmosphere) and the corona

Researchers in NCAR's High Altitude Observatory (HAO) have long understood the potential for a type of measurement called spectro-polarimetry to advance understanding of magnetic fields at the solar surface. The technique allows scientists to infer both the strength and the direction of the magnetic field from these precise measurements of light that has been polarized by the magnetic activity.

Hinode spreads its solar-panel wings in this artist's conceptual drawing. (Illustration courtesy NAOJ).

An HAO collaboration with the National Solar Observatory has been gathering such measurements from the ground since 1995. The opportunity to gather data from space, free from interference by Earth's atmosphere, led NCAR scientist Bruce Lites and colleagues to team with scientists and engineers at Lockheed Martin's Solar and Astrophysics Laboratory to create a key part of the Solar Optical Telescope. Together, they developed focal plane instrumentation for the SOT that will provide spectro-polarimetric data about the magnetic fields at the solar surface, or photosphere.

The telecope's main aperture, measuring 50 centimeters (19.5 inches), breaks previous size records for satellite-borne solar telescopes. The engineering makes it the most advanced solar telescope yet to be flown in space.

NCAR researchers have a longstanding interest in the Sun's magnetic fields, as well as the behavior of its corona, or outer atmosphere. Hinode's XRT will provide new measurements of the faint, hard-to-detect energy in the corona, supplementing ground-based observations from NCAR's Advanced Coronal Observing System on Mauna Loa, Hawaii. The third telescope, the EIS, has a sensitivity to exreme-ultraviolet rays that reveal motions and energy release in the lower corona.  Together, the satellite's instrument package is expected to reveal the heating mechanism and physical motions of the solar corona.

Besides NCAR, the team of SOT designers and developers includes National Astronomical Observatory of Japan (NAOJ), Lockheed Martin Advanced Technology Center, Mitsubishi Electric Corporation, NASA, and the Japan Aerospace Exploration Agency. Hinode replaces Solar-A, better known as Yohkoh (sunbeam), which streamed remarkable images of the Sun's corona and solar flares to earthbound researchers for a decade until sustaining irreparable damage in 2001.

"First light" images from Hinode's three telescopes are posted at the NAOJ Web site.

Another Sunrise is poised to peer over the horizon in 2008. NCAR scientists and engineers, working with colleagues in Germany and Spain, are building a unique telescope that will capture images of the Sun from a perch high in the atmosphere.

At the heart of the Sunrise project is a lightweight, 1-meter (39-inch) telescope. Carried aloft by a balloon, it will circle Antarctica for about two weeks at an altitude of approximately 130,000 feet. Its advanced instrumentation will offer another view of the Sun’s photosphere, providing further clues at high resolution of the small-scale magnetic fields that drive solar variability and profoundly affect Earth’s atmosphere. The international team expects to launch the telescope in late 2008.

Predicting mighty magnetic eruptions

Sunspots get attention in part because they sometimes cause a far larger type of solar disturbance, one that can propagate all the way to Earth's atmosphere. About once a week when the Sun is relatively quiet and about two or three times a day at the peak of the 11-year solar cycle, a great bundle of plasma escapes from the Sun's surface. This coronal mass ejection, or CME, accelerates through the corona in only a few hours. If it's pointed at Earth, it can irradiate astronauts, disable the circuitry in satellites, knock out surface power grids, degrade the accuracy of the Global Positioning System, and paint the high-latitude skies with shimmering auroras.

The Transition Region and Coronal Explorer (TRACE) satellite captured this image of a magnetically active region at the edge of the Sun. The cylindrical loops connect one magnetic polarity to the other. (Image courtesy NASA.)

Forecasts issued by NOAA shortly after a CME emerges from the Sun provide warnings from hours to several days in advance of a potential geomagnetic storm.

But to predict one before it erupts, scientists will have to learn the precursors of CMEs. They'll need to illuminate the plasma contortions below the solar surface that give birth to a CME and the coronal magnetic fields that shape its evolution.

Since 1998, NASA's Transition Region and Coronal Explorer satellite (TRACE) has measured how the magnetic field shapes coronal plasma from the photosphere up through the corona at an exceptionally fine horizontal resolution.

But the arching structures uncovered by TRACE denote only a few of the corona's intricately nested magnetic field lines—arches within arches, as it were. To help see these multilayered structures, many coronal specialists have turned to animation.

This computer graphic shows areas likely to be heated during coronal eruptions (group of purple field lines), overlaid on other sample field lines from an emerging magnetic flux rope. Color contours at lower boundary represent the normal magnetic field at the Sun's surface. (Illustration by Sarah Gibson, based on modeling from Yuhong Fan, NCAR.)

At NCAR, Sarah Gibson is using visualization routines based on modeling by colleague Yuhong Fan to see how an idealized twisted tube of magnetic flux—a CME in the making—might appear in observations. She concentrates on a sigmoidal (S-shaped) portion of the twisted field. This zone is the interface between field lines that are firmly tethered to the dense solar surface and those that have a portion suspended in the atmosphere and thus move more freely. "This is the region where heating is likely to happen during an eruption," says Gibson.

Recent x-ray data show that hot coronal gas can take on a sigmoidal structure a few days to weeks before a CME emerges in the same area. While the relationship isn't guaranteed, and it currently has limited use as a forecasting tool, "the link is definitely intriguing from a scientific point of view," says Gibson. "If we can figure out the science behind the eruptions, we'll be in a much better position for making future forecasts."

A Sun-wide glimpse of coronal magnetism

Attempts to observe the solar corona have long been thwarted by the Sun's far-brighter surface, as if someone were trying to decipher a whisper amid a thunderstorm. Eclipses help muffle the visual noise of the solar disk, and filters can artificially block it, but each approach has its limitations.

In 2004, a handful of NCAR researchers fulfilled a long-sought measurement dream. At the National Solar Observatory in New Mexico, they collected the first-ever data on magnetic fields across the entire solar limb (the slice of the Sun's corona perpendicular to Earth). Animations from their instrument, the Coronal Multichannel Polarimeter (CoMP), reveal turbulent, high-velocity magnetic features spewing outward from the Sun's surface.

"People have measured coronal magnetism before," says HAO's Steven Tomczyk, "but we believe this is the first time it's being done in a time sequence like this, where you can see an evolving structure. I think we're making important steps and demonstrating that this technology works."

These images show the brightness, magnetic field strength, and Doppler velocity of an erupting solar prominence taken with the Coronal Multi-Channel Polarimeter on March 9, 2004. The images were taken in a wavelength region in the near-Infrared spectrum corresponding to emission from Helium atoms. Positive and negative polarities of magnetic fields are indicated by the yellow and white colors of the middle image, while velocities directed towards and away from the observer are indicated by the blue and red colors of the rightmost figure. Click here or on the image to view an animation (running time approximately six seconds, representing one hour of data at one minute per frame).

Connecting Earth and space weather NCAR researchers study the Sun and its influence on our planet, including space weather. But what about Earth's influence on space? This false-color image shows ultraviolet light from two plasma bands in the ionosphere that encircle the Earth over the equator. Bright, blue-white areas are where the plasma is densest. Solid white lines outline the continents; Africa is on the left; North and South America are on the right. Dotted white lines mark regions where rising tides of hot air indirectly create the bright, dense zones in the bands. The picture is a composite built up from 30 days of observations with NASA's IMAGE satellite (March 20 to April 20, 2002). Click here or on the image to enlarge. (Image courtesy NASA/University of California, Berkeley.) Researchers recently discovered the first evidence that air motions triggered by thunderstorms in the tropics are changing the structure of the ionosphere—the region of electrically charged gas in Earth's outer atmosphere, at the edge of space. The team analyzed two known bands of plasma, or electrically charged gas, that hover high above the tropics. They discovered wave-like variations in the plasma bands by analyzing data from NASA's IMAGE satellite (see illustration). To envision what might be causing the variations, they used simulations by an NCAR computer model. NCAR scientist Maura Hagan was on the team that made the thunderstorm connection. Hagan is the lead developer of the Global Scale Wave Model, which simulates the large air motions called atmospheric tides. The connection between surface and space is indirect, by way of the atmospheric tides and an intermediate layer of air that picks up electrical charge during daylight hours. The NCAR model suggests that atmospheric tides excited by thunderstorms modify this E-layer, which is part of the ionosphere. The research team was led by the University of California, Berkeley, and included scientists from Japan's National Institute of Information and Communications Technology, Utah State University, Johns Hopkins University, and NCAR Learn More First Global Connection Between Earth and Space Weather Found (NASA News Release) The Global Scale Wave Model is one ingredient nested within a unique larger model built at NCAR that links the thermosphere, ionosphere, and mesophere and makes studies of Earth's upper atmosphere possible. That larger model has recently been merged with a global climate model, forming a hybrid that stretches from ground level to more than 140 km high. The Whole Atmosphere Community Climate Model (WACCM) will help scientists analyze how climate change at lower altitudes will influence the highest reaches of the atmosphere. Experts are already dreaming of connecting WACCM and solar models so that one day the entire region from Earth to Sun can be depicted.

(Fall 2006)

Near the Sun's surface—especially in the photosphere, the lowest part of the Sun's atmosphere—magnetism has been traced for over a decade by ground- and space-based instruments, such as NCAR's Advanced Stokes Polarimeter. These devices infer the magnetic field by measuring several components of visible radiation. The brightness of the polarized light is proportional to the strength of the magnetic field along the line of sight.

But until recently, there was scant hope for using this technique to analyze magnetism in the Sun's corona. Although its temperatures are scorching—as high as a million degrees Celsius (1.8 million Fahrenheit)—the corona is far too thin to yield a strong signal. CoMP capitalizes on a new generation of super-sensitive, low-noise infrared sensors that has made the impossible possible for coronal research. "The technology has only recently come on line," explains NCAR software engineer J. Anthony Darnell, who worked closely with Tomczyk and engineer Gregory Card in designing the instrument.

The NCAR team also devised a way to measure two wavelength components simultaneously. Earth's atmosphere scatters a continuously varying amount of background light from the brighter disk into the coronal line of sight. The simultaneous measurements in CoMP allow the varying background signal to be accurately removed while preserving the faint coronal signal.