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A Multi-Observatory Inter-Comparison of Line-of-Sight Synoptic Solar Magnetograms

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Abstract

The observed photospheric magnetic field is a crucial parameter for understanding a range of fundamental solar and heliospheric phenomena. Synoptic maps, in particular, which are derived from the observed line-of-sight photospheric magnetic field and built up over a period of 27 days, are the main driver for global numerical models of the solar corona and inner heliosphere. Yet, in spite of 60 years of measurements, quantitative estimates remain elusive. In this study, we compare maps from seven solar observatories (Stanford/WSO, NSO/KPVT, NSO/SOLIS, NSO/GONG, SOHO/MDI, UCLA/MWO, and SDO /HMI) to identify consistencies and differences among them. We find that while there is a general qualitative consensus, there are also some significant differences. We compute conversion factors that relate measurements made by one observatory to another using both synoptic map pixel-by-pixel and histogram-equating techniques, and we also estimate the correlation between datasets. For example, Wilcox Solar Observatory (WSO) synoptic maps must be multiplied by a factor of 3 – 4 to match Mount Wilson Observatory (MWO) estimates. Additionally, we find no evidence that the MWO saturation correction factor should be applied to WSO data, as has been done in previous studies. Finally, we explore the relationship between these datasets over more than a solar cycle, demonstrating that, with a few notable exceptions, the conversion factors remain relatively constant. While our study was able to quantitatively describe the relationship between the datasets, it did not uncover any obvious “ground truth.” We offer several suggestions for how this may be addressed in the future.

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Notes

  1. Since SOLIS replaced KPVT, we collectively refer to these data as “SOLIS” when we present long-term analyses, acknowledging that prior to Carrington rotation (CR) 2008, the data were obtained by KPVT.

  2. Strictly speaking, the 5124 Å line does have a slight Zeeman sensitivity – g=−0.013 (Landi Degl’Innocenti 1982).

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Acknowledgements

The authors gratefully acknowledge the support of the LWS Strategic Capabilities Program (NASA, NSF, and AFOSR), the NSF Center for Integrated Space Weather Modeling (CISM), NASA’s Heliophysics Theory Program (HTP), the Causes and Consequences of the Minimum of Solar Cycle 24 program, the STEREO/IMPACT, STEREO/SECCHI, and SDO/HMI instrument teams. Wilcox Solar Observatory data used in this study were obtained via the web site http://wso.stanford.edu courtesy of J.T. Hoeksema. The Wilcox Solar Observatory is currently supported by NASA. The SOLIS, GONG and KPVT programs are managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. SOHO/MDI is a project of international cooperation between ESA and NASA. The MWO data included in this study are from the synoptic program at the 150-Foot Solar Tower of the Mt. Wilson Observatory. The Mt. Wilson 150-Foot Solar Tower is operated by UCLA, with funding from NASA, ONR and NSF, under agreement with the Mt. Wilson Institute. HMI data are courtesy of the Joint Science Operations Center (JSOC) Science Data Processing team at Stanford University.

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Authors and Affiliations

  1. Predictive Science, 9990 Mesa Rim Road, Suite 170, San Diego, CA, USA

    P. Riley, M. Ben-Nun, J. A. Linker & Z. Mikic

  2. Stanford University, Stanford, CA, USA

    L. Svalgaard, T. Hoeksema & Y. Liu

  3. NSO, Kitt Peak, Tucson, AZ, USA

    J. Harvey & L. Bertello

  4. UCLA, Los Angeles, CA, USA

    R. Ulrich

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  1. P. Riley
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  2. M. Ben-Nun
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  3. J. A. Linker
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  8. T. Hoeksema
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  9. Y. Liu
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Correspondence to P. Riley.

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Riley, P., Ben-Nun, M., Linker, J.A. et al. A Multi-Observatory Inter-Comparison of Line-of-Sight Synoptic Solar Magnetograms. Sol Phys 289, 769–792 (2014). https://doi.org/10.1007/s11207-013-0353-1

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