Solar physicists from Lockheed
Martin [NYSE: LMT], 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.
“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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NOTE TO EDITORS: Low- and high-resolution JPEG image files of the
discovery are available at the following URL:
