MDI Magnetic Field and Intensity Synoptic Charts

The Carrington Rotation synoptic charts are generated from the definitive level 1.8 magnetograms and intensitygrams. The charts are generally averages of several observations of each location that have been collected over the course of a 27+ day solar rotation.

• All synoptic maps have been reconstructed with magnetogram data recalibrated in December 2008. Data files and plots made after 26 January, 2009 have been recalibrated.

• Magnetic Field - MDI full-disk magnetograms were re-calibrated in December 2008 using an improved procedure.

  Full-disk 2-arc second MDI magnetograms are observed every 96 minutes. The calibrated disk images are remapped to a high resolution Carrington coordinate grid and converted to a radial field. Projection corrections to the field strengths have been made assuming that MDI makes line-of-sight measurements of a radial magnetic field.

  The field strength measured at each synoptic grid point is averaged, after accounting for differential rotation, to form a map of the entire solar surface. Standard synoptic charts are constructed from the ~20 magnetograms providing the nearest central meridian data. We provide both a radial and a reconstructed line-of-sight version of the maps.

  The magnetograms have been interpolated to disk-center resolution, resulting in a 3600 * 1080 pixel synoptic map. The axes are linear in Carrington longitude (0.1 degree intervals) and in sine latitude. In this projection each pixel represents the same surface area on the solar surface. Note that pixels near the poles are not independent and they do overlap.

  The effects of cosmic rays have been reduced by discarding outlying values during the averaging process.

  The poles may not be visible due to the inclination of the solar equator.

  Field strengths above ~3400 G are saturated because of the on-board processing and appear as lower field values. Corrected values can be reconstructed if intensity images are available.

  Two kinds of synoptic magnetograms are provided by MDI - single-minute and 5-minute averages. The five-minute averages have lower noise, at the few gauss level. We combine a varying number of the two types of magnetograms at each longitude to produce a uniform noise level, equivalent to averaging 20 single-minute observations. The noise level for the recalibrated charts is approximately 5 Gauss. The noise level in the original magnetograms increases with distace from the center of the disk and is higher toward the bottom right of the disk due to velocity signal leakage caused by MDI filter non-uniformity. Thus the noise characteristics change somewhat depending on the orientation of the spacecraft.

  Ordinarily synoptic maps are constructed using data observed near central meridian. Additional charts are constructed using data from other disk meridians with the disk longitude offset specified (60E, 45E, 30E, 15E, 15W, 30W, 45W, or 60W) in the file name. These show how the diachronic synoptic field would be constructed if observed from different vantage points (e.g. the STEREO spacecraft) at the same time as the ordinary chart constructed for Earth. Note that in all of these standard diachronic synoptic charts the data at each longitude is observed at a different time. Specifically in the case of the standard central meridian chart when the Carrington longitude rotates beneath the spacecraft (or is offset as indicated above).

• Intensity - Photograms are usually observed 2-4 times per day. After correction for limb darkening, and for flat field, orbital and instrument variations, remapped images are combined in a similar way as the magnetic fields to form the synoptic chart. Near central meridian data from 3 or 5 images (indicated in the file name) are combined by taking the median value at each location. The resolution is 3600 steps in Carrington longitude by 1080 steps in sine latitude.