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New Instrument for Investigating Solar Magnetic Fields

by Sarah Jaeggli, IfA Graduate Student

solar magnetic field

This image, taken during the testing of the FIRS spectropolarimeter, shows the magnetic field strength of a sunspot. The weakest areas are in purple and the strongest, in red, and the black lines show the direction of the magnetic field. The sunspot's diameter was 38,000 km (over 23,000 miles). Image by Haosheng Lin and Sarah Jaeggli.

Dunn Solar Telescope
The Dunn Solar Telescope at Sacremento Peak, New Mexico. Photo by Sarah Jaeggli.

In 2007, IfA solar astronomer Haosheng Lin began construction of the Facility Infrared Spectropolarimeter (FIRS) for the Dunn Solar Telescope of the National Solar Observatory (NSO) to study the Sun's magnetic activity. The instrument was completed in July 2009 through a collaboration between the IfA and the NSO, and the instrument has been released for general use by the solar community. FIRS is the precursor to a spectrograph that the IfA will build for the Advanced Technology Solar Telescope (ATST), proposed for Haleakala, and proof of concept for the techniques that will be used in the next generation of solar instruments.

People have been aware of solar activity for thousands of years. Large sunspots are observable with the naked eye when the Sun is mostly obscured by thin clouds. Following the invention of the telescope, scientists were able to view sunspots in more detail. They found that they were highly structured and similar in appearance to iron filings arrayed around a magnet, and they began to suspect that sunspots must be magnetic. George E. Hale confirmed the presence of strong magnetic fields in sunspots in 1908 during his observations at Mt. Wilson. He found that within sunspots, absorption lines of hydrogen became split, and recognized this as the Zeeman effect, the splitting of a spectral line due to a magnetic field, for which Pieter Zeeman had won the Nobel Prize for physics in 1902.

Why is the Sun magnetic? The Sun spins, and the circulation of hot, electrically charged gases gives rise to magnetic fields. This is called the solar dynamo. Solar magnetic fields play a role in virtually every kind of activity we see on the Sun. They reach out from the Sun's hidden inner depths, through the surface layer called the photosphere, into the thin gases of the chromosphere and corona, and beyond into the solar system. The emergence of strong, localized magnetic fields on the Sun creates cool, dark sunspots in the photosphere, while filaments and prominences form above in the chromosphere and corona where flares, explosive events that send material flying out into the solar system, are likely to occur. Because of its magnetic fields, our Sun is a dynamic place that changes every second.

Atoms in the cooler outer layers of the Sun absorb light exiting from the hotter interior. This sunlight energizes the electrons surrounding the atomic nucleus to a particular energy state that depends on the wavelength of the absorbed photon. Astronomers use "spectroscopy" to analyze the absorption (or emission) from atoms. Light from an object is isolated into a line by a narrow slit and then dispersed into its constituent colors by a prism or grating, resulting in a spectrum. Absorption and emission of energy by atoms are known as "spectral lines."

As Pieter Zeeman discovered, magnetic fields cause spectral lines to split. The amount of the splitting is proportional to the magnetic field strength. Atoms also orient themselves according to the direction of the magnetic field, and as a result, only light with a given orientation, or polarization, can be absorbed. Absorption lines made by iron, calcium, and helium in the Sun exhibit particularly strong responses to its magnetic fields. These lines form at different heights in the solar atmosphere. Iron absorption lines at infrared wavelengths come from very deep in the photosphere, and iron absorption lines at visible wavelengths form near its top. The calcium absorption line comes from the lower chromosphere, and the helium absorption lines come from an area close to the inner coronal boundary. Observations of the energy splitting and the polarization enable scientists to construct 3-D maps of the Sun's magnetic fields. But despite all our studies of the Sun, we still don't understand how its magnetic features form and what they mean.

FIRS has an unusual design that drastically improves instrument performance through the use of new technologies. The spectrograph slit is scanned across an area to create a map of the magnetic field, with spectra taken at every position. To increase the speed and efficiency of these mapping observations, and take advantage of new, large-format cameras, it has four adjacent slits instead of one, and it builds maps of the magnetic field four spectra at a time. The spectrograph grating in FIRS provides very high spectral resolution (about 10 times higher than that of a high-resolution nighttime spectrograph) to be able to study the detailed physics of the absorption lines.

FIRS has not one, but two, cameras for taking spectra. These dual infrared and visible cameras make it possible to simultaneously measure the magnetic field at two different heights in the solar atmosphere. Specially tuned narrowband filters (called dense wavelength division multiplexers, or DWDMs, and adapted from optical telecommunications) prevent overlap of the spectra produced by the four slits at the focal plane. The polarization of the spectra is obtained using liquid crystal variable retarders (LCVRs) placed before each of the cameras. The LCVRs alternate between four different polarization states to determine the full polarization signal.

FIRS has been placed on the DST, located at Sacramento Peak, New Mexico because DST's adaptive optics and large collecting area allow for the highest achievable spatial and spectral resolution. It should provide a great deal of interesting data as solar activity increases during the new solar cycle.