Heritage

HAO continues a decades-long presence in the field of solar spectro-polarimetry. In this capacity, HAO has contributed to the community broadly through innovative instrumentation, analysis techniques, and scientific investigations surrounding the precision measurement of magnetic fields in the solar atmosphere. With the Community Spectro-polarimetric Analysis Center (CSAC), HAO and NCAR broaden their community support through development and maintenance of a suite of standardized data reduction and analysis tools.

The developments leading to CSAC at HAO/NCAR date back to the late 1960s and early 1970s when HAO scientists developed two polarimetric instruments for solar studies: the K Emission Line Polarimeter (KELP), and the Stokes I spectro-polarimeter. KELP was developed to provide information on the coronal magnetic field vector as inferred from emission lines observed above the solar limb with a coronagraph. The Stokes I instrument is the original spectro-polarimeter from which many of the modern polarimeters derive their heritage.

Stokes I

Deployed at the National Solar Observatory/Sacramento Peak (NSO/SP), Stokes I was an instrument that scanned a narrow wavelength range around selected spectral lines for a single, small area of the solar disk. This was the first of the class of photometric spectro-polarimeters based upon photoelectric detectors. It provided detailed line profile shapes of spectrum lines in the full state of polarization (Stokes I,Q,U,V), thereby opening up a new world of quantitative analysis that permitted higher precision measurements of the magnetic field vector.

Stokes II

The Stokes II instrument was a refinement of Stokes I. It replaced Stokes I at NSO/SP and was operated for a little more than a year in 1980. It featured many improvements although the most important was a linear array detector for simultaneously measuring a range of wavelengths. However, like Stokes I, images of the solar surface in polarized light were built up by raster scans of the aperture (~5 arc seconds; very poor resolution by modern standards) over the solar surface.

Stokes Inversion Codes in the 1980s

For the Stokes I instrument, an inversion code had been developed at HAO to extract measures of the photospheric magnetic field vector from the polarization spectra. This code did not often succeed in producing good fits to the measured spectra. In the early 1980s, with the better Stokes II data in hand, the HAO inversion code was improved and refined to the point that it became a useful analysis tool. Unique new results for the field structure of some large sunspots mapped with the Stokes II instrument were published as a result of this refined analysis technique. Later in that decade, the code was further refined at HAO to simultaneously fit more than one spectral line. This enabled a much higher precision for the measurement of the field vector, and also allowed estimates of the "fill fraction" of spatially-unresolved structures. This development was done in anticipation of the Advanced Stokes Polarimeter: an instrument that allowed simultaneous measurements of more than one spectral line.

The 1990s - The Advanced Stokes Polarimeter

In the late 1980s HAO, in collaboration with NSO/SP, developed a new instrument that provided precision photometric images of solar polarization spectra. The Advanced Stokes Polarimeter (ASP) was deployed in late 1991, and the first scientifically useful observations were obtained in early 1992. By all accounts, this instrument ushered in a new era of quantitative, high-resolution vector magnetic field observations. Many modern spectro-polarimeters developed by observatories around the world draw their heritage from the ASP. It was deployed at the Dunn Solar Telescope (DST) at NSO/SP, a large aperture instrument capable of very high angular resolution. ASP was the first precision spectro-polarimeter to use Charge-Coupled Device (CCD) detectors.

As a result of the ASP effort, a robust inversion code was developed. Also developed were techniques for measurement of the instrumental polarization, and reduction of the data.

The Modern Era

The success of ASP resulted in the development of new spectro-polarimeters, both ground- and space-based. The accompanying table lists a number of modern instruments, many of which draw their heritage from Stokes I → Stokes II → ASP. Building upon this heritage, it has been possible to refine these techniques and apply them to the greatly-improved data now being gathered by new instruments. The dramatic new results coming from the Hinode Spectro-Polarimeter, for example, owe much to the long heritage of spectro-polarimetry at HAO/NCAR.

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Historical Development of Spectro-Polarimeters

Ground-Based Full Stokes Polarimeters	Type	Developers	Year
Advanced Stokes Polarimeter (ASP)	Spectrograph	HAO, NSO	1992
Imaging Vector Magnetograph (IVM)	F-P Filtergraph	U. Hawaii	1992
Zurich Imaging Stokes Polarimeter (ZIMPOL II)	Flexible	ETH Zurich	1996
THEMIS	Spectrograph	France, Italy	1997
La Palma Stokes Polarimeter (LPSP)	Spectrograph	IAC, Spain	1998
Tenerife Infrared Polarimeter (TIP)	Spectrograph	IAC, Spain	1998
Polarimetric Littrow Spectrograph (POLIS)	Spectrograph	KIS (Germany), HAO	2002
Diffraction-Limited Spectro-Polarimeter (DLSP)	Spectrograph	NSO, HAO	2002
SOLIS-VSM	Spectrograph	NSO	2003
Swedish 1-m Solar Telescope	Spectrograph	ROYAC, HAO	2004
Spectro-Polarimeter for Infrared and Optical Regions (SPINOR)	Spectrograph	HAO	2004
ATST Visible/Near IR Polarimeter	Spectrograph	NSO, HAO	2010
Space-Based Polarimeters	Type	Developers	Year
Hinode	Spectrograph	Japan/US	2006
Sunrise (High Altitude Antarctic Balloon)	Spectrograph, Filter	Germany/US/Spain	2009
Solar Dynamics Observatory HMI (SDO/HMI)	Michelson Filter	Stanford, Lockheed	2008
Solar Orbiter	Fabry-Perot Filtergraph	ESA	2012