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NOAO Newsletter - NOAO Highlights! - March 1999 - Number 57


Mapping Sunspot Magnetic Fields at 12 Ám

The McMath-Pierce telescope and Celeste cryogenic spectrometer have produced the first mid-infrared (12 Ám) magnetograms in a sunspot. The field is measured by recording spectra of a magnesium (MgI) line at 12.3 Ám wavelength. This 12 Ám line exhibits the largest Zeeman splitting of any line which is currently observable, and its resolved splitting gives a direct measure of the magnetic field strength. Celeste is a high-resolution liquid-helium-cooled grating spectrometer built by NASA Goddard Space Flight Center. Individual measurements record the MgI spectrum at each point along a 2.4' slit. An infrared polarimeter placed at the spectrometer input creates a Stokes-V profile of the line at each position. Data cubes (two-dimensions spatial, one-dimension spectral) are created by stepping the slit across the region being imaged. The image dimensions are approximately 0.9' by 2.4'.

The magnetic field image illustrated is a slice of the data cube at a wavelength where only spectral components corresponding to fields of 1700 Gauss contribute. The maximum field in the sunspot umbra was 2200 gauss; however, the umbra appears dark in the image due to a much weaker line intensity. The morphology of the field can be studied by examining the variation with field strength using a series of these slices of the data cube. In some regions, more than one field strength is present, and these separate fields can be resolved in the spectrum due to the great Zeeman sensitivity of the line.

This research is part of an ongoing NASA-sponsored project to measure the magnetic structure above the photosphere using the infrared MgI line. The unique infrared capability and large aperture of the McMath-Pierce telescope makes this work possible. This investigation is a collaboration between Goddard Space Flight Center (D. Jennings, D. Deming, G. McCabe, and T. Moran) and the Universidad de Monterrey, Mexico (P. Sada).

     


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