MWO MAGNETOGRAMS L. Bertello, R.K. Ulrich, J. Boyden and T. Tran Magnetogram Workshop, UCLA, April 2-4, 2007

OUTLINE DATA ACQUISITION DATA CALIBRATION See Ulrich et al (ApJS, 139, ) RESULT

The Babcock magnetograph uses a moving stage to track the spectral line. Alternating circular polarization analyzer detects the shift of the Zeeman components through two spectral samples. The four variables: Phase 1 is LCP (solid line, shown), Phase 2 is RCP (shifted to the dashed line). Observables

Fundamental Quantities I is the intensity Z/I is the magnetic signal (Stokes V parameter) E/I is a measure of the Doppler servo error C/I is a measure of the circular polarization introduced by the telescope with its coelostat mirrors

Temporal Integration The KD*P crystal is modulated with a square wave at frequency of 400 Hz s integration time for slowgrams s integration time for fastgrams values acquired

Scanning The Solar Image The solar image is built up by back and forth scans. The entrance aperture size is indicated by the rectangle to the left of the arrows. The coordinates for each point are recorded along with the data in a raw data file. The blue circle shows the deduced solar limb position. The superposed line is a typical intensity trace with measured points indicated.

MWO Magnetograms SLOWGRAMS (High resolution)  12-arcsec-square aperture  ~40 minutes to complete a scan  Typically 2 magnetograms a day FASTGRAMS (Low resolution)  20-arcsec-square aperture  ~20 minutes to complete a scan  Up to magnetograms a day

Spectral Samplings λ5250 Å FeI (±39 mÅ) λ5237 Å CrII (±26 mÅ) λ6768 Å NiI (±29 mÅ, ±71 mÅ, ±115 mÅ ±329 mÅ, ±423 mÅ) λ5896 Å NaD (±10 mÅ, ±32 mÅ, ±93 mÅ ±114 mÅ, ±198 mÅ)

Spectral line profiles – sampling bandpass

Heights of formation

Advantages of Multiple Spectral Samplings Study the magnetic field as a function of height in the solar atmosphere. Opportunity to create and test different synoptic maps used as lower boundary condition in models to predict the SWS and IMF.

Calibration of Observables The line scan intensities are converted to the observable E/I as a function of displacement. This parameter yields the magnetic separation when the displacement is modulated by the circular polarization.

Fitted Calibration Curves as a Function of Center-to-Limb Angle

How often we calibrate? Every 3 fastgrams (~ 1 hour) Before each slowgram Calibration done only at center disk (ρ = 0 o ) Center-to-limb (ρ  0 o ) dependence is fix

From E/I to Gauss (5250 case) 1. Y = x– x x 5 where Y = microns and x = E/I (ρ = 0 degrees) 2. Y=Y xz x 2 z x 5 z where z = 1 – cos(ρ), ρ = center-to-limb angle 3.Δλ(microns)=Y 2 –Y 1,Δλ(Å)=Δλ(microns)/ B(Gauss) = 2(4.67x g λ 2 ) Δλ(Å) where g = 3.0 and λ =

Magnetic saturation for 5250 Because the inner edges of the sampling band are so close to the line core, even a week field is sufficient to cause saturation. Saturation effects occur when B is in excess of ~200 Gauss. Cross-calibration technique to rescale the 5250 magnetograms on a 5233 magnetic field scale.

5233Å – A Non-Saturating Line This line of Fe I at Angstroms has broad and nearly linear line wings. It provides reference magnetic measurements that are not influenced by magnetic saturation effects.

Center-to-limb Dependence Å The calibration curve for 5233Å shows little center-to-limb change.

Magnetic Fields are compared point by point to get a saturation factor As a special setup, both 5250 and 5233 were measured over the fill solar disk. The two sets of magnetic field are well correlated and define a conversion factor as a function of . SaF = (1-cos 2 ρ) B 5250 = B 5250 (obs)  SaF

Result from high-resolution maps MDI data (from 1997 to 2006) 1-degree resolution synoptic maps We do not assume B θ = 0 The spherical harmonic expansion coefficients are computed using the method described in Altschuler et al (Solar Physics 51, 345)

|B(r=1AU,year,rss)|