Large-Scale Spatial Cross-Calibration of Hinode/SOT-SP and SDO/HMI

TL;DR

This paper presents a decade-long cross-calibration of Hinode/SOT-SP and SDO/HMI data, correcting pixel size and pointing errors to improve magnetic field measurements and reduce biases in solar physics research.

Contribution

It introduces a robust geometric fitting method for instrument calibration, revealing significant pointing errors and proposing practical correction solutions.

Findings

Hinode/SOT-SP pixel size is off by about 1%.

Pointing errors often exceed dozens of arcseconds with a bias.

Corrected calibration reduces hemispheric bias in magnetic field estimates.

Abstract

We investigate the cross-calibration of the Hinode/SOT-SP and SDO/HMI instrument meta-data, specifically the correspondence of the scaling and pointing information. Accurate calibration of these datasets gives the correspondence needed by inter-instrument studies and learning-based magnetogram systems, and is required for physically-meaningful photospheric magnetic field vectors. We approach the problem by robustly fitting geometric models on correspondences between images from each instrument's pipeline. This technique is common in computer vision, but several critical details are required when using scanning slit spectrograph data like Hinode/SOT-SP. We apply this technique to data spanning a decade of the Hinode mission. Our results suggest corrections to the published Level 2 Hinode/SOT-SP data. First, an analysis on approximately 2,700 scans suggests that the reported pixel size in Hinode/SOT-SP Level 2 data is incorrect by around 1%. Second, analysis of over 12,000 scans show that the pointing information is often incorrect by dozens of arcseconds with a strong bias. Regression of these corrections indicates that thermal effects have caused secular and cyclic drift in Hinode/SOT-SP pointing data over its mission. We offer two solutions. First, direct co-alignment with SDO/HMI data via our procedure can improve alignments for many Hinode/SOT-SP scans. Second, since the pointing errors are predictable, simple post-hoc corrections can substantially improve the pointing. We conclude by illustrating the impact of this updated calibration on derived physical data products needed for research and interpretation. Among other things, our results suggest that the pointing errors induce a hemispheric bias in estimates of radial current density.