Description of Hinode SOT/SP Level2 processing and products

Bruce Lites , January 22, 2009, Last Revision April 9, 2015

SOT/SP LEVEL2 Data Description

The SOT/SP level2 data are outputs from "inversions" using the HAO "MERLIN" inversion code developed under the Community Spectro-polarimetric Analysis Center (CSAC) initiative (http://www.csac.hao.ucar.edu/) at the National Center for Atmospheric Research, Boulder, Colorado, USA.

SOME BASIC INFORMATION ABOUT MERLIN
MERLIN is a flexible code that may be used on data from a variety of sources. It performs a least-squares fitting of the Stokes profiles using the Milne-Eddington atmospheric approximation that allows for a linear variation of the source function along the line-of-sight, but holds the magnetic field vector, line strength, Doppler shift, line broadening, magnetic fill fraction (or scattered light fraction) constant along the line-of-sight. The Levenberg-Marquardt algorithm least-squares fitting is preceeded by an estimate of global initial values for [field strength, Doppler shift, and magnetic fill fraction] based upon the genetic algorithm. The other fit parameters are initialized with simpler procedures.

MERLIN APPLICATION TO HINODE SOT/SP DATA
Routine inversions are performed on Hinode SOT/SP with the following characteristics:

Stokes profiles for both Fe I 630nm line are fit. Profiles are fit for only the wavelength range +/-0.30 Angstroms from the respective line centers.
Macroturbulence is held fixed at 0.7083 km/s, corresponding to the nearly Gaussian SP spectral response function (FWHM = 24.8 mA). No additional macroturbulence of solar origin is assumed.
The scattered light profile for each map is computed from the average of low polarization profiles included in that map. Profiles whose net polarization [sqrt(Q^2 + U^2 + V^2)/Ic] integrated over both lines falls below 0.35% are included in this average.
The scattered light profile is allowed to shift in wavelength through a least-squares minimization.
The SOT/SP inversions solve for the fill fraction f, where the observed Stokes I profile I_obs is fitted with [f*I_mag +(1-f)*I_scatt], where I_mag is the magetized component and I_scatt is the scattered light profile. The quantity f is output as the variable SALPHA.
Inversions are limited to a maximum field strength of 5000 Gauss.
No attempt is made to resolve the 180-degree azimuth ambiguity. Azimuths, as well as field inclinations, are reported between 0 and 180 degrees. The azimuths are positive counterclockwise from solar west.
Doppler broadening of the line is limited to 20 < Dopplerwidth < 90 mA.
Line damping is limited to 0 < damping < 1.5*Dopplerwidth.
Line strength relative to continuum opacity is limited to 0.01 < eta0 < 100.
Source function surface value (B_zero) and gradient (B_1) are given in data units.
Shifts of the 630.15 and 630.25 nm lines, as well as the shift of the scattered light profile (SDELTA) are given in km/sec. No absolute wavelength reference is assumed for either line. All shifts are given as increasing with increasing wavelength (redshifts).
The scattered light shift is relative to the unshifted mean scattered light spectral profile. it is limited to -10 < SDELTA < 10 km/sec.
The continuum intensity in the inversion results (ICONT) corresponds to the continuum of the fitted profile. The fits are restricted to +/-300 mA from line center, so ICONT can depart substantially from the actual continuum. The measures of continuum intensity from the Level1 processing should be used instead of ICONT for most scientific analysis purposes.
Standard errors of the inversion are included for each pixel for each of the 12 inversion parameters, BUT AT THIS TIME THESE STANDARD ERRORS ARE INCORRECT AND SHOULD NOT BE USED.
The inversion reports its final status in the index RCONV, and also the number of iterations (NITER), and various measures of the chisquare of the fit. The status index RCONV indicates the mode for exit from the iteration:

UNSET_INVERSION_NOT_PERFORMED = 0
REACHED_MAX_ITERATIONS = 1
REACHED_MAX_FAILURES = 2
REACH_CONVERGENCE_CRITERIA = 3
LAMBDA_GREATER_THAM_MAX_ALLOWED = 4
LAMBDA_SMALLER_THAM_MIN_ALLOWED = 5
NOTHING_TO_DO = 6

(Refer to the MERLIN documentation for more information about these parameters).

LEVEL2 DATA STRUCTURE
Following Hinode program guidelines, the MERLIN inversion results for Hinode SOT/SP data are available in extended FITS binary form. They may be downloaded via commands that may be cut-and-pasted into an SSW session, for example:

SSWIDL> sotsp_stks2index,'20061201_032125',index,data,/LEVEL2
where "index" is the fits header, and "data" is the data array [NX,NY,36]
There are 36 parameters output from the inversion. The descriptions of these parameters may be had by the SSW command, e.g.:
SSWIDL> more,get_infox(index,'xcen,ycen,fitstags,ftype,cunit1')
resulting in the following output that indicates the order of the parameters:

    2.3891 -160.5100 SFLD Magnetic Field Gauss
    2.3891 -160.5100 SPSI Magnetic Field Inclination Degrees
    2.3891 -160.5100 SAZM Magnetic Field Azimuth Degrees
    2.3891 -160.5100 SCEN1 Doppler Shift of 630.15nm km/s, + to red
    2.3891 -160.5100 SCEN2 Doppler Shift of 630.25nm km/s, + to red 
    2.3891 -160.5100 SDOP Doppler Width mA
    2.3891 -160.5100 SETA0 SETA0 Line opacity relative to continuum 
    2.3891 -160.5100 SDMP Damping Parameter
    2.3891 -160.5100 SBZERO Source Function, data units
    2.3891 -160.5100 SB1MU Source Function Gradient, data units
    2.3891 -160.5100 VMAC Macroturbulence km/s
    2.3891 -160.5100 SALPHA Stray Light Fraction
    2.3891 -160.5100 SDELTA Stray Light Shift km/s, to red
    2.3891 -160.5100 SFLD_ERR Magnetic Field Error
    2.3891 -160.5100 SPSI_ERR Magnetic Field Inclination Error
    2.3891 -160.5100 SAZM_ERR Magnetic Field Azimuth Error
    2.3891 -160.5100 SCEN1_ERR Doppler Shift of 630.15nm Error
    2.3891 -160.5100 SCEN2_ERR Doppler Shift of 630.25nm Error
    2.3891 -160.5100 SDOP_ERR Doppler Width Error
    2.3891 -160.5100 SETA0_ERR Line Strength Error
    2.3891 -160.5100 SDMP_ERR Damping Parameter Error
    2.3891 -160.5100 SBZERO_ERR Source Function Error
    2.3891 -160.5100 SB1MU_ERR Source Function Gradient Error
    2.3891 -160.5100 VMAC_ERR Macroturbulence Error
    2.3891 -160.5100 SALPHA_ERR Stray Light Fraction Error
    2.3891 -160.5100 SDELTA_ERR Stray Light Shift Error
    2.3891 -160.5100 CHISQ_I Chi-Squared for Stokes I
    2.3891 -160.5100 CHISQ_Q Chi-Squared for Stokes Q
    2.3891 -160.5100 CHISQ_U Chi-Squared for Stokes U
    2.3891 -160.5100 CHISQ_V Chi-Squared for Stokes V
    2.3891 -160.5100 CHISQ_TOT Total Chi-Squared [I, Q, U & V]
    2.3891 -160.5100 ICONT Continuum Intensity Data Number
    2.3891 -160.5100 NET_POL Polarization Degree
    2.3891 -160.5100 STOKESV Magnitude of Stokes V Percent
    2.3891 -160.5100 RCONV Final Status of Inversion Value
    2.3891 -160.5100 NITER Number of Iterations Iterations

lites@ucar.edu