Data archive:

Index


Second Solar Spectrum (SS2) atlas

When using these data, please refer to the material as follows:

"The data for this analysis have been provided in electronic form by IRSOL as a compilation by Stenflo (2014), based on the atlas of Gandorfer (2000, 2002, 2005)"


The full atlas (3161.2 - 6987.0 Å) has been assembled (by J.O. Stenflo) from a number of digital sections provided by Achim Gandorfer, which he used for the three volumes of his atlas. In the present reassembly of the atlas (into a single atlas file) the different sections with arbitrary zero points of the polarization scale have been shifted to be consistent with the empirical determination of the continuum polarization in the Stenflo (A&A 429, 713-730, 2005) paper. The level of this continuum polarization is overplotted as a dashed line.

Above 4000 Å it is possible to obtain a well defined continuum polarization level and zero point of the scale this way. Below 4000 Å it is much more problematic because the spectrum is so crowded with no good continuum windows. Therefore no great effort has been invested in trying to define the zero point of the polarization scale in a precise way. The choices made in the atlas should only be seen as crude indications, with room for considerable refinement if one wants to study a particular section in more detail. For the range (over more than 100 Å) covered by the Ca II H and K lines the zero points have been chosen to make the atlas consistent with the recordings published in Stenflo (A&A 84, 68-74, 1980) (which covered a large spectral range in one context, in contrast to the ZIMPOL recordings, which represent a set of separate small (5-6 Å) spectral windows with individual and arbitrary zero points, which need to be pieced together to achieve large spectral coverage).

There are two versions of the full atlas, represented by two pdf files:

A very conservative, mild form of wavelet smoothing has been used, to avoid the risk of generating spurious features or significantly affect the polarization amplitudes. The smoothed version is cleaner and better and the one recommended for use, but the unsmoothed version is provided to allow comparison and verification that there is full consistency between the two versions. No inconsistency between the two versions has yet been identified. If someone detects an inconsistency, please report it (preferably to Stenflo, who is responsible for the wavelet smoothing).

The pdf atlas is presented in the form of 10 Å sections. To optimize the visibility of the structures, the Q/I scale has been maximized for each 10 Å section, between the maximum value and the smaller of zero and the minimum value. This is in contrast to the three printed volumes of the original Gandorfer atlas, which used fixed Q/I scales. A particular consequence of this is that in the long-wavelength portion of the atlas, where the polarization signals are very small, the signals including the noise get greatly magnified. This brings out tiny but still significant polarization features which are hardly noticeable in the printed atlas version, while also magnifying spurious features, like polarized fringes and noise, which occur at levels of a few times 10-5. In general features with Q/I < 3 · 10-5 should not be trusted, but larger amplitudes are most likely significant.

The UV portion 3165.0 - 4231.0 Å is also covered by the old Kitt Peak survey of the Second Solar Spectrum made on October 6-8, 1978, with the vertical grating spectrometer at Kitt Peak/McMath and published in Stenflo et al. (A&A Suppl. Ser., 52, 161-180, 1983). The spectral resolution was low (in contrast to the fully resolved FTS spectra), about 0.1 Å, determined by the use of relatively wide entrance and exit slits to optimize polarimetric sensitivity. Nevertheless this low-resolution atlas is still of interest since it has a better definition of the continuum level and less fluctuations in the zero polarization level, since the recordings were made through scanning with the grating over 185 Å wide spectral sections, while the ZIMPOL atlas of Gandorfer was pieced together from spectral windows of 5-6 Å width each. The slit position was at 10 arcsec inside the limb, corresponding to µ = 0.144 (in contrast to (µ = 0.10 for the Gandorfer ZIMPOL atlas).

The UV1978 SS2 atlas is in the file UV1978.pdf

Data files:


FTS atlases of the Stokes V spectra due to the longitudinal Zeeman effect

When using these data, please refer to the material as follows:

"The data for this analysis have been provided in electronic form by IRSOL as a compilation by Stenflo (2014)"


The recordings were made with the Kitt Peak FTS polarimeter in 1979 in various magnetic regions near disk center and reported in Stenflo et al. (A&A 131, 333-346, 1984), which is the paper that should be referred to when using these data. With the FTS all wavelengths within the range of the prefilter used are recorded simultaneously. The spectrum is fully resolved (no significant spectral smearing at all), and there is no spectral stray light. The typical band width of the prefilters used was 1000 Å. Near the edges of the band pass the spectrum gets noisy due to lack of photons, but apart from this the spectra are amazingly noise-free and clean. The spatial averaging was over a circular area 10 arcsec in diameter.

The representation is in the form of V/Ic, Stokes V in units of the continuum intensity level (and not in the form of fractional polarization V/I).

There are five separate recordings, each covering simultaneously approximately 1000 Å, represented by the following pdf files, where the ones with name FTSatlas have no smoothing, while for the ones with name FTS_vw_atlas a mild form of wavelet smoothing has been applied:

Data files:

Note that since the amplitudes of the narrow V peaks get somewhat reduced by the smoothing, it is recommended to use the v rather than the vw array in the case of precise quantitative analysis of peak amplitudes.

FTS atlas of the Third Solar Spectrum (SS3) and its relation to the Second Solar Spectrum (SS2)

When using these data, please refer to the material as follows:

"The data for this analysis have been provided in electronic form by IRSOL as a compilation by Stenflo (2014)"


The FTS atlas of the Third Solar Spectrum (SS3) represents the ratio between the intensity spectrum at µ = 0.145 and the intensity spectrum at disk center (µ = 1.0), both in units of the intensity of the local continuum level. This ratio spectrum covers the range 4084.29 - 9950.50 Å and can be viewed in three different pdf files, each of which showing a 10 Å wavelength section per plot page. Each file contains a comparison of the ratio atlas with two other atlases, so there are three panels per plot page. The files are also extensively described in the following paper:

Stenflo, J.O. 2015, FTS atlas of the Sun's spectrally resolved center-to-limb variation. A&A 573, A74 (2015)

Due to the exceptional nature of the Kitt Peak/McMath FTS, a powerful instrument constructed by Jim Brault, both the limb and disk center spectra are fully resolved and free from stray light, in addition to being nearly noise-free and having a well-defined continuum level. The three pdf files are:

The model that converts the disk-center spectrum sic into SS3 (simod) is defined as follows (where w is the wavelength):

x=(w-5000.)/5000.

a0=3.3779388

a1=-2.0900198

expon=a0+a1*x

a0=1.9312026

a1=-1.4757004

ampl=a0+a1*x

simod=ampl*(1.-sidc)^expon+1.

The disk center FTS atlas representing the quiet Sun has been taken from the compilation of the Kitt Peak FTS-Spectral-Atlas in the digital form provided by H. Neckel, Hamburg. It is given here over the full range 3290-12508 Å in file DCatlas.pdf

The limb FTS atlas was recorded on October 2-3, 1978, and April 27-28, 1979, as part of the first survey of the Second Solar Spectrum that was published in Stenflo et al. (A&A Suppl. Ser., 54, 505-514, 1983). The spatial field of view with the Kitt Peak FTS polarimeter was a rectangular 17.5" x 10" aperture centered 10 arcsec inside the solar limb near one of the heliographic poles (to minimize the effect of magnetic fields). The 17.5 arcsec side was parallel to the limb, so the limb distance interval covered is 5-15 arcsec from the limb. At 10 arcsec limb distance, µ = 0.144. If we use as an example the limb darkening function valid for the continuum at 5000 Å (the limb darkening varies with wavelength and within the line profiles), then we find an intensity-weighted average µ of 0.145. This demonstrates that this average is insensitive to the choice of limb darkening function, and the effects are generally smaller than the errors in the positioning of the aperture on the Sun. Therefore we adopt µ = 0.145 as representing the disk position to which the limb spectrum refers.

Since the disk and limb spectra were recorded at different times they can be affected by small differential Doppler shifts due to different velocities between the telescope and the Sun (in particular due to the rotation and orbital elliptical motion of the Earth). Therefore we have used the disk center FTS spectrum as a reference and made the wavelength scale of the limb spectrum conform to the scale of this disk center spectrum by iterative least squares fitting of the two spectra, using shift and stretch as the two free parameters of the fit.

The SS2 atlas has been described in a previous section. In the representation of SS2 in the third panel of the SS312.pdf plots, a mild form of smoothing of SS2 based on wavelets has been used. No smoothing has been applied to the FTS limb, disk center, or ratio spectra.

Data files:


 

Published: 29 July 2014, Revised: 24 December 2014, Author: J.O. Stenflo, Edited by: R. Ramelli

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