SOHO Scopes the Sun

(An expanded version of an article from the March 2003 Eyepiece )


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The Sun through SOHO's EIT (Extreme Ultraviolet-Imaging
Telescope) instrument, in resonance lines of eight
and nine times ionized iron (Fe IX/X) at 171 Angstroms.
[SOHO is a project of international cooperation
between ESA and NASA.]


The Solar and Heliospheric Observatory (SOHO) has been an overachiever, revolutionizing our knowledge of the structure and dynamics of the solar interior, corona and solar wind, advancing the fledgling science of space weather (how the Earth is affected by solar storms: flares and coronal mass ejections) and helping to determine how Earth's climate is affected by (mostly cyclical) variations in the Sun's luminosity.

That was a central point in a February 8 address to the New York chapter of the National Space Society by Pal Brekke of the European Space Administration, a deputy SOHO project scientist based at NASA's Goddard Space Flight Center. He spoke on "The Sun Through the Eyes of SOHO: New Insight Into Our Star and Its Impact on the Earth's Environment. "

SOHO, a joint NASA-European Space Agency project, was launched in 1995 to help us better understand the structure and dynamics of the solar interior, the heating and dynamics of the solar corona, and the acceleration and composition of solar wind. SOHO has made great strides in these areas, thanks to its suite of 11 instruments and multidisciplinary cooperation by an international community of solar scientists.

Helioseismology, the study of the propagation of sound waves within the Sun, is an area where SOHO has made strides, Brekke noted. When the waves hit the solar surface from within, they cause it to oscillate or "ring." At any time, there may be hundreds of these waves traveling through the Sun at different speeds. They cannot be observed directly but can be deduced by measuring surface oscillations. Two instruments, GOLF and MDI, employ the familiar Doppler technique for measuring these motions. Other instruments, such as the aptly named GONG, also measure these oscillations.

Helioseismology has profoundly increased our knowledge of the dynamics of the Sun's interior and its large-scale convection flows as well as visible features such as sunspot regions and flares. At its core, the Sun is 50 million degrees Fahrenheit, and its surface is about 6,000 degrees, and the process by which heat migrates to the surface is just now beginning to be understood.

The SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph has been used by SOHO to create a spectral atlas of the Sun in ultraviolet. EIT, the Extreme Ultraviolet Telescope, provides a range of ultraviolet images of the Sun filtered to show the spectral lines of helium II and several isotopes of iron. These reveal details in active regions of the photosphere: sunspot zones, flares, prominences. Brekke showed monthlong EIT animations of the Sun during minimum and maximum which illustrated the huge difference in solar activity through the 11-year cycle.

Brekke also discussed coronal mass ejections (CMEs), enormous eruptions of plasma (clouds of gas containing both positive and negative ions) that burst from the Sun between one and five times a day (depending on the Sun's level of activity), and are visible on the LASCO C2 and C3 coronagraph images. When the eruptions are pointed in Earth's direction, the interaction of the charged particles with the Earth's magnetic field can cause geomagnetic storms replete with aurorae, and power and communications disruptions. These storms have contributed to the failure of several satellites.

CMEs were unknown until first revealed by coronagraphs in the 1970s. Although SOHO can detect them, it can't triangulate their direction of motion and determine how likely they are to hit Earth; that will have to wait for a new generation of solar telescopes, the twin spacecraft of the Stereo project, scheduled for launch in 2005, which will generate 3-D images of these eruptions.

SOHO is also providing valuable data on global climatic change. By accurately measuring solar irradiance at different wavelengths over time, scientists can determine how much of Earth's temperature variations are due to changes in the Sun's luminosity; at present, only about half can be explained that way.



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