Investigating the Magnetic Field in Sun’s Polar Regions in the Context of Understanding and Predicting the Solar Wind at 1 AU

Understanding the origin of the solar wind requires its source region to be well determined. Presently, there are considerable uncertainties in locating the source region, which prevents us from testing proposed solar wind acceleration models against in-situ plasma and magnetic field data. For example, in the declining phase of a solar cycle as we are in that tends to show coronal holes scattered in low-latitude regions as well as around the poles, it is not easy to determine the source region of not only the slow solar wind but also the fast solar wind. We need to rely on models to calculate where on the Sun we are connected to, and these models make use of synoptic maps of radial magnetic field at the photosphere. The inadequacy of such synoptic maps has often been pointed out. Here we address the questions of how realistic the portion of the polar region is in the synoptic maps and what impact it may give on the computation of the heliospheric magnetic field. The magnetic field in the polar regions is intrinsically difficult to measure, and it must be eventually observed more extensively and routinely by the next-generation polar or high-inclination missions and constellations. In the meantime, however, the Hinode SOT/SP gives the best reference of the magnetic field in these almost inaccessible regions. Here, we investigate how different inversion codes and different observing modes may affect the SP measurement of the radial magnetic field component in the polar regions. We also compare the SP and SDO/HMI measurements of the polar field.