A comparison of co-temporal vector magnetograms obtained with HMI/SDO and SP/Hinode

TL;DR

This study compares vector magnetograms from HMI/SDO and SP/Hinode, demonstrating that HMI/SDO provides measurements closely matching SP/Hinode with minimal systematic variations, validating its use for high-temporal-resolution solar magnetic field studies.

Contribution

The paper provides a direct comparison showing HMI/SDO magnetograms closely match SP/Hinode measurements, confirming the effectiveness of filter-based magnetographs for accurate, high-temporal-resolution solar magnetic field observations.

Findings

HMI/SDO magnetograms closely match SP/Hinode measurements.

Systematic center-to-limb variations are minimal in HMI/SDO data.

Filter-based magnetographs can effectively combine high temporal resolution with measurement accuracy.

Abstract

An accurate measurement of magnetic field is very important for understanding the formation and evolution of solar magnetic fields. Currently there are two types of solar magnetic field measurement instruments: the filter-based magnetographs and the Stokes polarimeters. The former gives high temporal resolution magnetograms and the latter provides more accurate measurements of magnetic fields. Calibrating the magnetograms obtained by filter-based magnetographs with those obtained by Stokes polarimeters is a good way to combine the advantages of the two types. Our previous studies have shown that, compared to the magnetograms obtained by the Spectro-Polarimeter (SP) on board Hinode, those magnetograms obtained by both the filter-based Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station (HSOS) and by the filter-based Michelson Doppler Imager (MDI) aboard SOHO have underestimated the flux densities in their magnetograms and systematic center-to-limb variations present in the magnetograms of both instruments. Here, using a sample of 75 vector magnetograms of stable alpha sunspots, we compare the vector magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) aboard SDO with co-temporal vector magnetograms obtained by SP/Hinode. Our analysis shows that both the longitudinal and transverse flux densities in the HMI/SDO magnetograms are very close to those in the SP/Hinode magnetograms and the systematic center-to-limb variations in the HMI/SDO magnetograms are very minor. Our study suggests that using the filter-based magnetograph to construct a low spectral resolution Stokes profile, as done by HMI/SDO, can largely remove the disadvantages of the filter-type measurements and yet still possess the advantage of high temporal resolution.