Measuring the Corona's Magnetic Field
by Jeffrey Kuhn
A contour map of the coronal magnetic field strength measured by
Haosheng Lin and Jeffrey Kuhn plotted over an image of the corona taken
by the Extreme Ultraviolet Imaging Telescope.
Since the late 1930s, scientists have known that the outer part of the
Sun, its corona, is much hotter than its surface. The corona is important
because it extends far into space and eventually influences Earth's environment.
Measuring the corona's magnetic field is a major unsolved problem because
it cannot be observed directly. Although magnetic fields have been measured
on the visible surface of the Sun for several decades, until now scientists
have been unable to make useful observations of them into the corona. Instead,
we have depended on hypothetical models based on invisible magnetic
fields to explain the corona. This is our "dark energy" problem,
and solving it is important because the corona's magnetic fields are responsible
for virtually all of the Sun's explosive and dynamic phenomena, and they
play a major role in driving variations in the Sun's energy output.
My colleague Haosheng Lin and I recently produced a "coronal magnetogram"—a
map of the magnetic field in the corona. To see these fields, we used the
world's largest coronagraphic telescope, the Solar Observatory for Limb
Active Regions and Coronae (SOLARC), on Haleakala and a new type of spectrograph built
by Lin, an imaging infrared spectropolarimeter. After several hours of
continuous observing, we achieved the sensitivity needed to detect the
coronal magnetic field. The magnetic fields were measured about 62,000
miles (100,000 km) above the surface of the Sun, which is comparable in
strength to what a compass needle feels on Earth's surface.
Obtaining magnetic maps of the Sun with such sensitivity is a critical
first step to understanding the variations in the amount and kinds of solar
radiation that affects Earth's magnetosphere, ionosphere, and ultimately
SOLARC and its spectrograph are new instrument concepts that will soon
be expanded to a much larger scale. Recently, the National Solar Observatory
announced plans to build the
Advanced Technology Solar Telescope (ATST)
on Haleakala. ATST's mirror diameter will be nearly 10 times
larger than that of SOLARC, but its optical design follows the design concepts
used in SOLARC. Its first-light instrumentation is also patterned after
our Haleakala imaging spectropolarimeter.