Solar physicists from Lockheed Martin, the National Center for Atmospheric Research, The Institute of Theoretical Astrophysics of the University of Oslo, and the Institute for Solar Physics of the Royal Swedish Academy of Sciences have analyzed the highest resolution images ever taken near the solar limb (or visible edge of the sun), and found a surprising variety of structure. Their results, which are being reported today at the American Astronomical Society's Solar Physics Division meeting in Laurel, Maryland, address long-standing theories on how the brightness of the Sun varies over the course of its magnetic cycle. Such changes may influence the Earth's climate on long timescales.

Image of a solar active region taken on July 24, 2002 near the eastern limb of the Sun at heliographic coordinates S15 E53 degrees. The limb is towards the top of the image. The tick marks are 1000 km apart. The smallest resolvable features in the image are about 70 km in size. The image was taken by Prof. Goran Scharmer and processed by Dr. Mats G. Löfdahl, both of the Institute for Solar Physics of the Royal Swedish Academy of Sciences.

"Until recently we thought of the solar photosphere as the relatively flat and featureless 'surface' of the Sun, punctuated only by an occasional sunspot," said Dr. Tom Berger, principal investigator on the study, and solar physicist at the Lockheed Martin Solar and Astrophysics Lab (LMSAL) at the company's Advanced Technology Center in Palo Alto, Calif. "Now, using the newly commissioned Swedish one-meter Solar Telescope (SST) on the island of La Palma, Spain, we have, for the first time, imaged the three-dimensional structure of the convective 'granules' that cover the photosphere."

The solar surface consists mostly of an irregular cellular pattern caused by temperature variations. The cells, called granules, are evidence of convection that transports heat to the surface in the same manner as boiling water on a stovetop or thermal plumes rising over hot fields to form thunderstorms. Each granule on the sun is about the size of Texas. At the 75 km resolution of the SST, sunspots and smaller dark "pores" are seen to be sunken into the surrounding granulation. This so-called "Wilson depression" has been inferred from lower resolution observations of large sunspots but never directly resolved until now.

Most importantly from a terrestrial climate perspective, the images show clearly that the granulation in regions of smaller magnetic fields outside of sunspots is both raised up and has brighter walls than the granulation in non-magnetic regions. Bright structures near the limb of the Sun have been seen for centuries in lower resolution images and are called "faculae" (Latin for "little torches"). Faculae are significant because scientists believe that their brightness is responsible for the increased solar irradiance (on the order of 0.1 to 0.15%) that occurs during periods of maximum solar magnetic activity.

At solar maximum, the Sun is covered by the greatest amount of dark sunspots in its 11-year cycle. It would be expected that the solar irradiance reaching Earth during that time might decrease. But beginning in the 1980s, satellite radiometer instruments, such as the Active Cavity Radiometer Irradiance Monitor instrument (ACRIM I) on the Solar Maximum Mission (SMM) spacecraft, revealed that while sunspots cause a decrease in the solar irradiance on time scales of days to weeks, the long-term solar irradiance actually increases as sunspot (magnetic) activity increases.

The source of this "extra" irradiance has been traced to the bright faculae near the limb of the Sun. Based on earlier low resolution images of faculae, scientists have created models that attribute most of the brightness of faculae to small magnetic "flux tubes" or "micropores". These models suggest that micropores act like tiny holes in the surface of the photosphere. When looking at disk center, we see only the relatively cool "floors" of the flux tubes. When seen at an angle near the limb, the models predict that the "hot walls" of the magnetic holes shine brightly compared to the relatively cooler surrounding granules.

The SST images may help resolve discrepancies between the "hot-wall" flux tube model and observations of facular brightness near the solar limb. Most of the bright structures seen are between 150 and 400 km tall and are typically elongated towards the limb. Simultaneous measurements of the magnetic field establish that the bright faculae are exactly aligned with the magnetic fields. However the faculae in these images appear more like bright walls of granulation that have somehow been "piled up" by the presence of magnetic fields than like micropores seen at an angle.

Theoretical models of solar convection developed by Dr. Neal Hurlburt of LMSAL support this "raised wall" picture. "The model that has been used to explain the brightness of faculae," Dr. Hurlburt reflects, "usually assumed that the rest of the solar atmosphere was an innocent bystander. However it is known that magnetic fields are swept aside as hot gas rises and spreads across the solar surface and confines the field to regions of down-flows. Many groups have modeled the dynamics of such magnetoconvection, but we have never gotten around to detailed comparison with sources of irradiance variations. We frequently find that the gas in our models is denser or hotter at the edges of the magnetic fields -- which might result in brightenings very much like what these images show."

As the ultimate source of all energy input to the Earth, understanding solar irradiance and its variation with magnetic activity on the Sun is an important factor in understanding climate variation on Earth. "Raising the hot material above the photosphere enhances facular emission at low angles to the solar surface" according to Prof. John Lawrence of California State University Northridge. "Low angles cover the greater part of the solar 'sky' as seen from the perspective of a facula, so this discovery impacts our estimate of the contribution of faculae to solar brightness changes. With this new discovery, we can hope to incorporate the effects of magnetoconvection into solar irradiance models to better predict variations in solar output."

Preliminary analyses of the some of the images are in a paper by Dr. Bruce Lites of NCAR, Prof. Goran Scharmer of the Royal Swedish Academy of Sciences, and Drs. Alan Title and Tom Berger of Lockheed Martin Solar and Astrophysics Lab that has been submitted for peer-review to the journal Solar Physics.

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Low- and high-resolution JPEG image files of the discovery are available at the following URL: http://www.lmsal.com/Press/SPD2003.html.