A New Method for Polar Field Interpolation

The photospheric magnetic field in the Sun's polar region is not well observed compared to the low-latitude regions. Data are periodically missing due to the Sun's tilt angle, and the noise level is high due to the projection effect on the line-of-sight (LOS) measurement. However, the large-scale characteristics of the polar magnetic field data are known to be important for global modeling. This report describes a new method for interpolating the photospheric field in polar regions that has been tested on MDI synoptic maps (1996 - 2009). This technique, based on a two-dimensional spatial/temporal interpolation and a simple version of the flux transport model, uses a multi-year series of well-observed, smoothed north (south) pole observations from each September (March) to interpolate for missing pixels at any time of interest. It is refined by using a spatial smoothing scheme to seamlessly incorporate this filled-in data into the original observation starting from lower latitudes. For recent observations, an extrapolated polar field correction is required. Scaling the average flux density from the prior observations of slightly lower latitudes is found to be a good proxy of the future polar field. This new method has several advantages over some existing methods. It is demonstrated to improve the results of global models such as the Wang-Sheeley-Arge (WSA) model and MHD simulation, especially during the sunspot minimum phase.