On the Coordinate System of Space-Weather HMI Active Region Patches (SHARPs): A Technical Note

We describe the coordinate systems of two streams of HMI active region vector data. A distinction is made between (a) the 2D grid on which the field vector is measured (or sampled), and (b) the 3D coordinate established at each grid point, in which the field vector is presented. The HMI data reduction can involve coordinate changes on both, with those performed on the former termed “remapping”, the latter “vector transformation”. Relevant pipeline procedures are described. Useful examples are given for data analysis.