Level 1 (Calibrated SOT-SP Data)

Overview

SOT-SP Level 1 calibrated polarization spectra are produced from the Level 0 FITS files by applying the sp_prep.pro routine (written in IDL, publicly available through SolarSoft), which applies the set of calibration algorithms that are described in detail in Lites & Ichimoto (2013). The source code for sp_prep.pro can be found at this link.

As listed in the introduction to Lites & Ichimoto (2013), the calibration process involves the following tasks:

• accounting for the digital wrap-around of the Stokes \(I\) profiles and restoration of the spectra for on-board bit-shifting,
• applying the dark- and flat-field corrections,
• removal of instrumentally induced polarization,
• rectification of spectra so that the dispersion is along a pixel row,
• aligning the spectra vertically between the two beams of the dual-beam polarimeter,
• merging of the two polarization beams from the dual-beam polarimeter,
• correcting the spectral-line curvature, the thermal drift of the spectrum in the wavelength direction, and orbital Doppler shift,
• rotation of the polarization frame of reference to the standard frame (\(+Q\) along solar East–West),
• reversal of the spectrum direction so that increasing spectral pixel corresponds to the direction toward longer wavelengths,
• compensation for residual crosstalk between \(I\) and \(Q\), \(U\), and \(V\),
• shifting the spectrum along pixel columns to correct for thermal flexure of the instrument in that direction,
• correction for the slowly varying intensity response of the instrument (vignetting of the image plane) in the slit-scan direction, and
• conversion of the data back to the same format as the unprocessed data (integers, possibly bit-shifted).

Quick analyses of the Level 1 SOT-SP data are also used to produce estimates of some of the Level 2 (inverted) data products, including estimates of the longitudinal and transverse field, Doppler velocity, continuum intensity, and several other parameters. These so-called "quick-look" data are discussed further in Section 3.2.13 of Lites & Ichimoto (2013). Estimates of the following physical quantities are available, as listed in Table 2 of that paper:

• empirical location of 6301.5 Å line center [pixels]
• longitudinal apparent flux density [Mx cm⁻²]
• transverse apparent flux density [Mx cm⁻²]
• continuum intensity [DN]
• wavelength of minimum intensity of 6301.5 Å and 6302.5 Å [pixels]
• solar north-south disk position, north of disk center [pixels]
• orbital Doppler shift, from header [pixels]
• cosine of heliocentric angle \(\mu\)
• smoothed empirical spectral drift correction [pixels]
• smoothed predicted spectral drift correction [pixels]
• smoothed empirical thermal drift along slit [pixels]
• Universal Time from start of operation [minutes]
• \( I/I_c \) integrated over lines
• \( \sqrt{Q^2+U^2}/I_c \) integrated over continuum wavelengths
• \( \sqrt{Q^2+U^2}/I_c \) integrated over lines
• measure of fractional linear polarization
• \( Q/I_c \)integrated over lines
• \( \sqrt{Q^2+U^2+V^2}/I_c \) (total polarization fraction) integrated over lines
• \( U/I_c \)integrated over lines
• signed mean \( V/I_c \) integrated over lines
• solar east-west disk position, west of disk center [pixels]
• estimate for field azimuth, counterclockwise from solar west [radians]
• scattered light profile [DN]
• solar Carrington latitude [heliographic degrees]
• solar Carrington longitude [Stonyhurst degrees]
• estimate of the thermal drift along the slit [pixels]
• estimate of the thermal spectral drift [pixels]
• Universal Time from start of day [hr]
• Stokes \(V\) zero-crossing wavelength, 6301.5 Å and 6302.5 Å [pixels]
• empirical thermal drift along slit [pixels]

Level 1 File Format

Level 1 SOT-SP data are available as groups of FITS files that are organized by each commanded scan program. The individual FITS files have the same format as the corresponding Level 0 FITS files, with pertinent details regarding the calibration process added to the list of keywords in the FITS header.

Level 1 "Quick-Look" File Format

The quick-look data are found as tags in the stks structure after running the sotsp_stks2struct.pro function. These quantities were generated from the Level 1 SOT-SP data using the stksimages_sbsp.pro routine, and are listed in the table below. In the "Array Size" column in the table, \(nx\) is the number of slit positions, \(ny\) is the number of pixels in the \(y\) direction along the slit, and \(n\lambda\) is the number of wavelength indices in each spectrum. Units of each quantity are given in square brackets.