1. A particularly obvious example of daily changing background noise level Constructing the BEST High-Resolution Synoptic Maps from MDI J.T. Hoeksema, Y. Liu, X.P. Zhao, A. Amezcua Stanford University Synoptic maps provide a global view of the solar magnetic field. However, what constitutes the 'best' possible synoptic map depends on the application. Traditional charts are compiled from data observed close to central meridian from magnetograms observed over a 27-day solar rotation. But with higher resolution data, a whole new set of details must be addressed when assembling such maps. Small-scale features move and evolve on the time scale over which the maps are constructed. The fluctuating background noise level is comparable to the smallest features. Projection effects and sensitivity variations of different sorts complicate the effort. And what about the polar field? (See accompanying poster by Liu et al.) Parts of the Sun cannot be seen. We describe methods for assembling the best maps possible by accounting for image sensitivity, image scale, corrupt pixels, differential rotation, geometric field projection, zero offset, varying noise characteristics of MDI magnetograms, and polar field interpolation. Process for Constructing Synoptic Chart Announcing New Level 1.8 MDI Magnetograms For each 96-minute MDI magnetogram, we correct for Uniform Zero Offset due to Shutter Jitter (Existing) Bad Cosmic Ray Pixels (New, rare) MDI saturation (Not yet implemented) Calibration using UCLA rescaling mask (New) In heliographic remapping processing Convert from Line-of-sight to radial (assumption). Adjust Carrington mapping for differential rotation Ulrich & Boyden (Solar Phys. 235, 17, 2006); Zhao et al., in prep, 2006/7) Ulrich & Boyden (Solar Phys. 235, 17, 2006); Zhao et al., in prep, 2006/7) Radial Field Synoptic Chart Construction Combine remapped radial magnetograms Select for uniform effective integration time (equivalent to 20 one-minute magnetograms) Use data closest to central meridian Statistically test and eliminate cosmic ray corruption Options: Polar field interpolation (See adjacent poster) Variance at Each LongitudeVariance at Each Latitude Top: Standard Synoptic Chart; Middle: Differential Chart; Bottom: Constant Integration Time Differential Chart Variance depends on the noise level. In CR 2041, more 5-minute integrations contributed near 360 degrees, lowering the noise level at the left edge. Requiring 20-minutes of observations at each longitude lowered and made the variance more uniform. Rows: Different areas of radial field synoptic chart CR 1988 shown in each row at high latitude and equator. Columns: Left: standard synoptic chart; some polar features blur. Differentially corrected mapping improves poles, center. Right column shows best equal integration time chart. Saturation of strong field regions in MDI occurs because of the on-board algorithm for computing the field strength. Identification and approximate correction for saturation is possible using associated intensity images, but this correction not yet implemented (Liu, et al.) Scale correction map for MDI magnetograms derived by Tran, Bertello, Ulrich, &Evans, ( Ap J Supp, 145:259, 2005 ) from Mt Wilson observations. A smoothed version is used to calibrate individual magnetograms Data from 96-minute magnetograms combine to make synoptic chart. Some 1-minute and some 5-minute observations are collected. Sensitivity and instrumental noise vary across the disk. Calculation of open field foot-point locations with a potential- field – source surface model provides a sensitive test of large- scale data quality. The calculation is highly sensitive to way in which the polar field is interpolated (see accompanying poster by Liu et al.) WSO & MDI computations agreed well in CR 2029 in May, 2005. Carrington Rotation 2055 – April 2007 RADIAL FIELD