MDI Level 1.8 MagnetogramsMDI Level 1.8 Magnetograms

Yang LiuYang LiuStanford UniversityStanford University

6/23/2008 1SHINE 2008

MDI magnetogramsMDI magnetogramssa = 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 MDI on-board computation for computation for Doppler velocityDoppler velocity

sa=F1-F3, sc=F2-F4, sa=F1-F3, sc=F2-F4, sa=sa×2^4, sc=sc×2^4,sa=sa×2^4, sc=sc×2^4,sum=sa+sc.sum=sa+sc.sa (sc) sa (sc)

reciprocalreciprocalpseudo_pseudo_αα=sum×=sum×reciprocalreciprocal pseudo_pseudo_αα= pseudo_= pseudo_αα×2^2.×2^2.pseudo_pseudo_αα

αα

αα Doppler velocity Doppler velocity15-bit lookup table

15-bit recip-lookup table

Produce this 15-bit recip-lookup table

table=big_number/sabig_number=3×2^24

if table > 32767 thentable= 32767

Thus, when sa (or sc) is lower than 96, the table saturates. The reciprocal for this sa is simply set to be 32767.

extract high 15-bit value

Calibration and Calibration and correctionscorrections

Offset: using a Gaussian function to fit the Offset: using a Gaussian function to fit the distribution of low density pixels of a MDI distribution of low density pixels of a MDI magnetogram. The shift of the gaussian magnetogram. The shift of the gaussian center is deemed to be the offset of this center is deemed to be the offset of this magnetogram. The offset is subtracted. magnetogram. The offset is subtracted.

6/23/2008 4SHINE 2008

Calibration and Calibration and correctionscorrections

Offset Offset

6/23/2008 5SHINE 2008

Calibration and Calibration and correctionscorrections

Saturation: Saturation: measurement is measurement is saturated in some saturated in some sunspot’s umbrae. sunspot’s umbrae.

6/23/2008 6SHINE 2008

Calibration and Calibration and correctionscorrections

Saturation: correction. Saturation: correction.

6/23/2008 7SHINE 2008

.4/)(

,)()2

2/2

4321

242

23

0

IIIII

III(II

IIII

average

1depth

averagedepthc

3

40

102.2

,96

,700

c

ji

I

II

I

Calibration and Calibration and correctionscorrections

Saturation: Saturation: correction using correction using I-B relationship: I-B relationship: log(B)=a+b*log(Ilog(B)=a+b*log(I/Ic) /Ic)

6/23/2008 8SHINE 2008

Period a b

1996-1999

2.9 -0.426

2000-2000

2.9 -0.326

2001-2001

2.9 -0.402

2002-2002

2.9 -0.307

2003-2003

2.9 -0.291

2004-2004

2.9 -0.275

2005-2007

2.9 -0.430

Calibration and Calibration and correctionscorrections

Saturation correction: one example in Saturation correction: one example in AR10882 near the disk center at 17:36 AR10882 near the disk center at 17:36 of 2005 November 18. of 2005 November 18.

6/23/2008 9SHINE 2008

Calibration and Calibration and correctionscorrections

Under-Under-estimate of estimate of magnetic flux: magnetic flux: corrected by corrected by multiplied by multiplied by a rescaling a rescaling factor derived factor derived by Tran et al. by Tran et al. (2005).(2005).

6/23/2008 10SHINE 2008

Remaining quirksRemaining quirks Non-uniform noise over solar disk.Non-uniform noise over solar disk.

6/23/2008 11SHINE 2008

Distortion difference for LCP and RCP images. It allows granulation and oscillation to leak into the magnetic signal as noise.

Remaining quirksRemaining quirks

Magnetic field reversal during some Magnetic field reversal during some major flares.major flares.

6/23/2008 12SHINE 2008

Field reversal during an x-class flare

Remaining quirksRemaining quirks

Cosmic rays.Cosmic rays.

6/23/2008 13SHINE 2008

Cosmic ray

Comparison with Comparison with other observationsother observations

Comparison with magnetograms taken by Comparison with magnetograms taken by SOLIS, MWO, and WSO.SOLIS, MWO, and WSO.

6/23/2008 14SHINE 2008

Comparison with Comparison with other observationsother observations

Comparison with WSO Comparison with WSO magnetograms.magnetograms.

6/23/2008 15SHINE 2008

MDI magnetograms MDI magnetograms in cycle 23in cycle 23

Interplanetary magnetic field versus Interplanetary magnetic field versus mean field: they are correlated mean field: they are correlated (cc=0.79 after removing solar cycle (cc=0.79 after removing solar cycle trend).trend).

6/23/2008 16SHINE 2008

MDI magnetograms in MDI magnetograms in cycle23cycle23

Sunspot areas, total flux, Sunspot areas, total flux, and unsigned mean field and unsigned mean field after removing solar cycle after removing solar cycle trend: magnetic field in trend: magnetic field in sunspots doesn’t make sunspots doesn’t make significant comtribution to significant comtribution to IMF.IMF.

6/23/2008 17SHINE 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 Conclusions

6/23/2008 18SHINE 2008

MDI level 1.8 magnetograms have made following MDI level 1.8 magnetograms have made following corrections: 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 MDI level 1.8 magnetograms still have following issues needed to be addressed: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 From the new MDI magnetograms from 1996 to 2008, we foundfound a strong correlation between solar mean field and interplanetary a strong correlation between solar mean field and interplanetary

magnetic field;magnetic field; a correlation between sunspot area and total flux, indicating magnetic a correlation between sunspot area and total flux, indicating magnetic

field from sunspots make a significant contribution to the total flux;field from sunspots make a significant contribution to the total flux; no significant correlation between solar mean field and sunspot area, no significant correlation between solar mean field and sunspot area,

indicating that most IMF comes from areas outside of sunspots.indicating that most IMF comes from areas outside of sunspots.