1. MDI Level 1.8 Magnetograms
Yang Liu, J. T. Hoeksema
Stanford University
9/12/2018
SHINE 2008

2. MDI magnetograms sa = F1 - F3, sc = F2 - F4, sum = sa + sc,
α = sum / sa, or
α = sum / sc, if sum ≤ 0.
α velocity
B_los=c*(velocity_lcp - velocity_rcp)
15-bit lookup table F1 F3
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SHINE 2008 F2 F4

3. Corrections and issues
Corrections made for level 1.8 magnetograms.
Zero offset;
Under-estimate of field flux density.
Corrections proposed to be made.
MDI saturation;
Cosmic rays.
Issues still remain in level 1.8 magnetograms.
Non-uniform noise level over the disk;
Magnetic field reversal during some major flares.
9/12/2018
SHINE 2008

4. Zero offset correction
Zero offset: due mainly to shutter noise that adds a nearly constant value over the whole disk of a magnetogram.
Correction: using a Gaussian function to fit the distribution of low flux density pixels of a MDI magnetogram. The shift of the gaussian center is deemed to be the offset of this magnetogram. The offset is subtracted.
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SHINE 2008

5. Zero offset correction
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6. Correction: underestimate of flux density
Underestimate of magnetic flux: corrected by multiplied by a rescaling factor derived by Tran et al. (2005).
9/12/2018
SHINE 2008

7. Proposed correction: saturation
Saturation: measurement is saturated in some sunspot’s umbrae.
9/12/2018
SHINE 2008

8. Proposed correction: saturation
Saturation: correction using I-B relationship: log(B)=a+b*log(I/Ic)
Period a b
2.9
-0.426
-0.326
-0.402
-0.307
-0.291
-0.275
-0.430
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SHINE 2008

9. Proposed correction: saturation
Saturation correction: one example in AR10882 near the disk center at 17:36 of 2005 November 18.
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10. Proposed correction: cosmic rays
9/12/2018
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11. Remaining issues Non-uniform noise over solar disk.
Distortion difference for LCP and RCP images. It allows granulation and oscillation to leak into the magnetic signal as noise.
9/12/2018
SHINE 2008

12. Remaining issues Magnetic field reversal during some major flares.
9/12/2018
SHINE 2008

13. Comparison with other observations
Comparison with magnetograms taken by SOLIS, MWO, and WSO.
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14. Comparison with other observations
Comparison with WSO magnetograms.
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15. Synoptic maps of magnetic field
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16. Conclusions
MDI level 1.8 magnetograms have made following corrections:
Zero offset;
Under-estimation of magnetic flux density;
An empirical method is proposed to correct saturation.
MDI level 1.8 magnetograms still have following issues needed to be addressed:
Non-uniform noise level over the disk;
Magnetic field reversal during some major flares;
Cosmic rays.
From the new MDI magnetograms from 1996 to 2008, we found
a strong correlation between solar mean field and interplanetary magnetic field;
a correlation between sunspot area and total flux, indicating magnetic field from sunspots make a significant contribution to the total flux;
no significant correlation between solar mean field and sunspot area, indicating that most IMF comes from areas outside of sunspots.