  The Sun has a polar magnetic field which oscillates with the 11 year sunspot
cycle. This polar magnetic field is an important component of the dynamo
process which is operating in the solar convection zone and produces the
sunspot cycle. We have systematic direct measurements of the Sun's polar
magnetic field only from about mid 1970s. There are, however, indirect proxies
which give us information about this field at earlier times. The Ca K
spectroheliograms taken in Kodaikanal Solar Observatory during 1904 - 2007 have
now been digitized with the 4k x 4k CCD and have higher resolution (0.86
arcsec) than the other available historical datasets. From these Ca-K
spectroheliograms, we have developed a completely new proxy (Polar Network
Index, PNI) for the Sun's polar magnetic field. We calculate the PNI from the
digitized images using an automated algorithm and calibrate our measured PNI
against the polar field as measured by the Wilcox Solar Observatory for the
period of 1976 - 1990. This calibration allows us to estimate polar fields for
the earlier period up to 1904. The dynamo calculations done with this proxy as
input data reproduce the Sun's magnetic behavior for the past century
reasonably well.
