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The Advanced Stokes Polarimeter is a new instrument for measuring the polarization of lines in the solar spectrum, thus allowing scientists to infer both the strength and direction of magnetic fields in the solar atmosphere. This new instrument, a collaborative project between the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado, and the National Solar Observatory (NSO) of the National Optical Astronomy Observatories, has become operational at the NSO Vacuum Tower Telescope in Sunspot, New Mexico. The instrument is now providing information about the structure of magnetic fields in the solar atmosphere with unprecedented clarity and precision. Scientists will use this instrument to explore the the physical nature of solar activity, from its origin within the Sun to the upper reaches of the solar atmosphere. The outer layers respond to the presence of magnetic fields by heating, emission of radiation, and disturbance of the interplanetary magnetic field. This knowledge will be used to better understand the variability of the Sun and the influence of this variability on the Earth.
This new instrument represents a major advance in scientific capability for understanding solar magnetic fields and related phenomena. Its angular resolution is roughly a factor of 10 better than its predecessors, and, with its precise polarization calibration and simultaneous wavelength coverage of lines of differing sensitivity to the Zeeman effect, it is now possible to specify the actual strength of the kiloGauss solar magnetic elements within 10-20 Gauss, and their field orientations within a few degrees.
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The accompanying images from the Advanced Stokes Polarimeter illustrate the vector magnetic field structure in and around a sunspot by displaying the field strength as a grey scale, and the two angles which describe its orientation are coded into colors. The horizontal lines in the intensity image are reference hairlines used for alignment purposes. The magnetic fields in this sunspot are stronger than 2500 Gauss in the dark centers of the large sunspot (the umbrae) -- a strength comparable to that near the end of a strong magnet of the type used in loudspeakers. Near the edge of the sunspot (the penumbra) the field strength drops to to about 800 Gauss. Note that outside the sunspots, magnetic fields are considerably stronger than in the penumbra, 1400-1500 Gauss, but they are very small in extent and, in fact, are not even resolved by these observations. Thus, the instrument is capable of providing reliable field strengths even for unresolved fields.
Two angles describe the field orientation: the azimuth and the inclination. The azimuth is the angle the component of the magnetic field horizontal to the surface of the Sun makes with respect to the solar West direction, as indicated by the color wheel to the right of the azimuth angle image. The inclination is the angle the field makes with the vertical direction, also shown by a color bar. The large sunspot has negative polarity magnetic fields; that is, the field is generally pointing down into the Sun near the centers of the sunspot. The yellow and black contours on the images show the locations where the field orientation passes through the horizontal.
Time sequences of vector magnetic field measurements similar to the one shown here give scientists a new, clear picture of the emergence, evolution, and dissipation of magnetic fields as they pass upward through the solar atmosphere.
-Revised 23 August 1995 by rob@ucar.edu.
Copyright 1995, NCAR.
- Approved by Bruce Lites -
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