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MDI Level 1.8 Magnetograms

MDI Level 1.8 Magnetograms. Yang Liu, J. T. Hoeksema Stanford University. 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.

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MDI Level 1.8 Magnetograms

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  1. MDI Level 1.8 Magnetograms Yang Liu, J. T. Hoeksema Stanford University 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 SHINE 2008 F2 F4

  3. Corrections and issues SHINE 2008 • 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.

  4. Zero offset correction SHINE 2008 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.

  5. Zero offset correction SHINE 2008

  6. Correction: underestimate of flux density SHINE 2008 Underestimate of magnetic flux: corrected by multiplied by a rescaling factor derived by Tran et al. (2005).

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

  8. Proposed correction: saturation SHINE 2008 Saturation: correction using I-B relationship: log(B)=a+b*log(I/Ic)

  9. Proposed correction: saturation SHINE 2008 Saturation correction: one example in AR10882 near the disk center at 17:36 of 2005 November 18.

  10. Proposed correction: cosmic rays Cosmic ray SHINE 2008

  11. Remaining issues Distortion difference for LCP and RCP images. It allows granulation and oscillation to leak into the magnetic signal as noise. SHINE 2008 Non-uniform noise over solar disk.

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

  13. Comparison with other observations SHINE 2008 Comparison with magnetograms taken by SOLIS, MWO, and WSO.

  14. Comparison with other observations SHINE 2008 Comparison with WSO magnetograms.

  15. Synoptic maps of magnetic field SHINE 2008

  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.

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