Comparison of Ground- and Space-based Longitudinal Magnetograms

TL;DR

This study compares ground-based and space-based solar magnetograms, demonstrating high agreement and analyzing factors affecting their scaling relationships, thereby validating the consistency of different observational instruments.

Contribution

It provides a detailed comparison and calibration of VSM magnetograms with space-based data, highlighting their agreement and flux-dependent scaling factors.

Findings

VSM and space-based magnetograms agree well in flux measurements.

VSM mean flux density time series has high signal-to-noise with no zero-offsets.

Scaling factors between VSM and other instruments depend on flux density.

Abstract

We compare photospheric line-of-sight magnetograms from the Synoptic Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM) instrument with observations from the 150-foot Solar Tower at Mt. Wilson (MWO), Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO), and Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SOHO). We find very good agreement between VSM and the other data sources for both disk-averaged flux densities and pixel-by-pixel measurements. We show that the VSM mean flux density time series is of consistently high signal-to-noise with no significant zero-offsets. We discuss in detail some of the factors -spatial resolution, flux dependence and position on the solar disk- affecting the determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The VSM flux densities agree well with spatially smoothed data from MDI and HMI, although the scaling factors show clear dependence on flux density. The factor to convert VSM to HMI increases with increasing flux density (from $\approx$1 to $\approx$1.5). The nonlinearity is smaller for the VSM vs. ~SOHO/MDI scaling factor (from $\approx$1 to $\approx$1.2).