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Sun-Earth Day Presents: Eclipse, In a Different Light

Seeing a magnetic field is an easy trick with iron filings and a magnet, but harder to do with the whole sun!


Mt. Wilson Magnetogram

This is a solar magnetogram obtained at Mt. Wilson in 1961. The brightness of each spot indicates the magnetic intensity. The slant indicates the polarity: right slant = north; left slant = south. (Courtesy: Babcock (Mt. Wilson)

Soon after George Ellery Hale discovered a way to detect magnetism on the sun in 1908, large telescopes were created to make more detailed observations using the Zeeman Splitting Effect.

The light from excited atoms will normally produce spectral lines at specific wavelengths. However, in the presence of magnetic fields, the atomic electrons produce light at two distinct wavelengths for each normal line. The stronger the magnetic field, the greater the difference in wavelength between the atomic lines. This is known as the Zeeman Effect. Because the electrons ‘spin’ in opposite directions in the two lines, they produce oppositely-polarized light. The effect is very minute, so special telescopes have to be designed to make the splitting easily detectable within the light from the sun.

Solar Surface/Magnetic Field Comparison

A side-by-side comparison of the solar surface, and the magnetic image showing (red color) intense regions of magnetism near sunspots. (Courtesy; UCAR - National Solar Observatory)

One of the largest of these telescopes, the Mt. Wilson 150-foot tower telescope, was built in 1912. Astronomers immediately used it to create detailed studies of solar magnetism. In 1914, for example, Hale discovered that polarities of sunspots in the north and south hemispheres of the sun reversed from cycle to cycle. At first, only fields stronger than 1,000 Gauss could be detected. These were related to sunspots. As the technology got better, fields as weak as 0.1 Gauss were detectable. By comparison, Earth’s magnetic field is about 0.7 Gauss at the surface. At these strengths, the entire ‘weak’ solar field could at long last be detected and mapped. This mapping process would be very tedious if done by hand, especially considering that the sun is, itself, rotating!

Horace W. Babcock developed the principle of the modern magnetograph and built the first magnetograph in 1955. An improved copy of this instrument was built and installed at the 150-foot Mt. Wilson solar telescope in 1957, and took about one hour to make a complete image. This new technique allowed astronomers to actually take a photograph of a TV screen, onto which the magnetic data was displayed. Since the light from the entire sun was involved in the spectroscopic image, the photograph displayed the intensity of the solar magnetic field in terms of ‘magnetic brightness’.

Solar Surface/Magnetic Field Comparison

This magnetogram was obtained by the SOHO satellite on January 26, 2006. It shows the intense magnetism of a sunspot on the west (right) limb, and the more diffuse magnetism from a larger magnetic region near the center. (Courtesy SOHO-MDI)

The NASA-Marshall Space Flight Center Vector Magnetograph Facility was assembled in 1973 to support the Skylab mission. Major improvements to the vector magnetograph occurred in 1982, when the SEC Vidicon camera was upgraded to a CCD. In 1985, other changes were made to increase the field-of-view from 5 x 5 arc min (2.4 arc sec per pixel) to 6 x 6 arc min with a resolution of 2.81 arc sec. At this facility, daily magnetograms can be accessed online since 2000.

Meanwhile in space, the NASA/ESA SOHO satellite creates magnetograms of the full sun at resolutions of 1.4 arc seconds, and can measure changes in the solar magnetic field of as little as 0.5 gauss.

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