MDI Level 1.8 Magnetograms Yang Liu Stanford University 6/23/ SHINE 2008

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

MDI on-board computation for Doppler velocity sa=F1-F3, sc=F2-F4, sa=sa×2^4, sc=sc×2^4, sum=sa+sc. sa (sc) reciprocal pseudo_α=sum×reciprocal pseudo_α= pseudo_α×2^2. pseudo_α α α Doppler velocity 15-bit lookup table 15-bit recip-lookup table Produce this 15-bit recip-lookup table table=big_number/sa big_number=3×2^24 if table > then table= Thus, when sa (or sc) is lower than 96, the table saturates. The reciprocal for this sa is simply set to be extract high 15-bit value

Calibration and corrections Offset: using a Gaussian function to fit the distribution of low 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. Offset: using a Gaussian function to fit the distribution of low 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. 6/23/2008 4SHINE 2008

Calibration and corrections Offset Offset 6/23/2008 5SHINE 2008

Calibration and corrections Saturation: measurement is saturated in some sunspot’s umbrae. Saturation: measurement is saturated in some sunspot’s umbrae. 6/23/2008 6SHINE 2008

Calibration and corrections Saturation: correction. Saturation: correction. 6/23/2008 7SHINE 2008

Calibration and corrections Saturation: correction using I-B relationship: log(B)=a+b*log(I/Ic) Saturation: correction using I-B relationship: log(B)=a+b*log(I/Ic) 6/23/2008 8SHINE 2008 Periodab

Calibration and corrections Saturation correction: one example in AR10882 near the disk center at 17:36 of 2005 November 18. Saturation correction: one example in AR10882 near the disk center at 17:36 of 2005 November 18. 6/23/2008 9SHINE 2008

Calibration and corrections Under-estimate of magnetic flux: corrected by multiplied by a rescaling factor derived by Tran et al. (2005). Under-estimate of magnetic flux: corrected by multiplied by a rescaling factor derived by Tran et al. (2005). 6/23/ SHINE 2008

Remaining quirks Non-uniform noise over solar disk. Non-uniform noise over solar disk. 6/23/ SHINE 2008 Distortion difference for LCP and RCP images. It allows granulation and oscillation to leak into the magnetic signal as noise.

Remaining quirks Magnetic field reversal during some major flares. Magnetic field reversal during some major flares. 6/23/ SHINE 2008 Field reversal during an x-class flare

Remaining quirks Cosmic rays. Cosmic rays. 6/23/ SHINE 2008 Cosmic ray

Comparison with other observations Comparison with magnetograms taken by SOLIS, MWO, and WSO. Comparison with magnetograms taken by SOLIS, MWO, and WSO. 6/23/ SHINE 2008

Comparison with other observations Comparison with WSO magnetograms. Comparison with WSO magnetograms. 6/23/ SHINE 2008

MDI magnetograms in cycle 23 Interplanetary magnetic field versus mean field: they are correlated (cc=0.79 after removing solar cycle trend). Interplanetary magnetic field versus mean field: they are correlated (cc=0.79 after removing solar cycle trend). 6/23/ SHINE 2008

MDI magnetograms in cycle23 Sunspot areas, total flux, and unsigned mean field after removing solar cycle trend: magnetic field in sunspots doesn’t make significant comtribution to IMF. Sunspot areas, total flux, and unsigned mean field after removing solar cycle trend: magnetic field in sunspots doesn’t make significant comtribution to IMF. 6/23/ SHINE 2008 Sunspot area vs total flux (cc=0.60) Mean field vs total flux (cc=0.23) Sunspot area vs mean field (cc=0.03)

Conclusions 6/23/ SHINE 2008 MDI level 1.8 magnetograms have made following corrections: MDI level 1.8 magnetograms have made following corrections: Zero offset; Zero offset; Under-estimation of magnetic flux density; Under-estimation of magnetic flux density; An empirical method is proposed to correct saturation. An empirical method is proposed to correct saturation. MDI level 1.8 magnetograms still have following issues needed to be addressed: MDI level 1.8 magnetograms still have following issues needed to be addressed: Non-uniform noise level over the disk; Non-uniform noise level over the disk; Magnetic field reversal during some major flares; Magnetic field reversal during some major flares; Cosmic rays. Cosmic rays. From the new MDI magnetograms from 1996 to 2008, we found From the new MDI magnetograms from 1996 to 2008, we found a strong correlation between solar mean field and interplanetary magnetic field; 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; 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. no significant correlation between solar mean field and sunspot area, indicating that most IMF comes from areas outside of sunspots.