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

TL;DR

This paper clarifies the coordinate systems used in HMI active region vector data, detailing the differences between the measurement grid and the 3D vector presentation, and explaining the data reduction procedures involved.

Contribution

It provides a detailed description of the coordinate systems and data processing steps for HMI active region vector data, aiding accurate data analysis.

Findings

Clarifies the distinction between 2D measurement grid and 3D vector coordinate systems.

Describes the data reduction procedures including remapping and vector transformation.

Provides practical examples for data analysis.

Abstract

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.