MDI Polar-Interpolated Synoptic Magnetic Field Data

Due to the inclination of the Earth's orbit to the Sun's equator, one solar pole or the other is not visible for several months each year. As a result synoptic maps typically have an inconvenient, but unavoidable, small data gap at high latitudes.

During the Carrington Rotation each year when the pole is tipped most toward the Earth, a fairly good observation of the polar field can be made. This happens about March 7 for the south pole and about September 7 for the north. From year to year the large-scale polar field changes relatively slowly, so a reasonable interpolation can be made between the annual views. Extrapolation for recent rotations can be (and is) done, but is less reliable.

Determining the polar field correction is a multi-step process that preserves as much of the high quality observed data as possible. First, using the annual observations of the visible pole we interpolate in time the slowly varying smoothed polar field for that rotation. Next, to find the smoothed polar field for that rotation, the observations poleward of 75 degrees are replaced by the interpolated annual value and a 7th order polynomial is then fit to the entire region above 55 degrees. Finally that smoothed one-rotation fit is cleanly merged with the full-resolution observations between 62 and 75 degrees; a smooth transition is made from 100% observed high-resolution MDI field to 100% interpolated smooth polar field. The field above 75 degrees in the interpolated maps is 100% the smoothed one-rotation fit.

For more details, see "A New Methond For Polar Field Interpolation" by Xudog Sun, et al., 2011, Solar Physics, 270, 9.
Resulting synoptic charts are now availabe from the synoptic map request page.