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. 2002 (ApJS, 139, 259-279)

• 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

• 0.135 s integration time for slowgrams

• 0.115 s integration time for fastgrams

• 46-54 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 15-16 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 = 0o)

• Center-to-limb (ρ 0o) dependence is fix

From E/I to Gauss (5250 case)

1. Y = 933.1692x–34.70916x2+9629.775x5

where Y = microns and x = E/I (ρ = 0 degrees)

2. Y=Y+576.4066xz+114.6506x2z+29628.58x5z

where z = 1 – cos(ρ), ρ = center-to-limb angle

3. Δλ(microns)=Y2 –Y1,Δλ(Å)=Δλ(microns)/15219.271

4. B(Gauss) = 2(4.67x10-13g λ2) Δλ(Å)

where g = 3.0 and λ = 5250.2

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 5232.9 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 -- 5233Å

• 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 = 4.5-2.5(1-cos2ρ)

B5250 = B5250 (obs)SaF

Result from high-resolution maps

• MDI data (from 1997 to 2006)

• 1-degree resolution synoptic maps

• We do not assume Bθ = 0

• The spherical harmonic expansion coefficients are computed using the method described in Altschuler et al. 1977 (Solar Physics 51, 345)

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