Reconstructing solar magnetic fields from historical observations

V. Sunspot magnetic field measurements at Mount Wilson Observatory^⋆

¹ National Solar Observatory, Boulder, CO 80303, USA
e-mail: apevtsov@nso.edu, lbertello@nso.edu, aapevtsov@nso.edu
² ReSoLVE Centre of Excellence, Astronomy and Space Physics Research Unit, University of Oulu, PO Box 3000, 90014 Oulu, Finland
³ Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, 196140 Saint Petersburg, Russia
⁴ Uzbekistan Academy of Sciences, Ulugh Beg Astronomical Institute, Tashkent, Uzbekistan
⁵ Kislovodsk Mountain Astronomical Station of Pulkovo Observatory, Kislovodsk, Russia
⁶ Kalmyk State University, Elista, Russia
⁷ University of California at Los Angeles (UCLA), Los Angeles, CA, USA

Received: 27 December 2018
Accepted: 2 July 2019

Abstract

Context. Systematic observations of magnetic field strength and polarity in sunspots began at Mount Wilson Observatory (MWO), USA in early 1917. Except for a few brief interruptions, this historical dataset has continued until the present.

Aims. Sunspot field strength and polarity observations are critical in our project of reconstructing the solar magnetic field over the last hundred years. We provide a detailed description of the newly digitized dataset of drawings of sunspot magnetic field observations.

Methods. The digitization of MWO drawings is based on a software package that we developed. It includes a semiautomatic selection of solar limbs and other features of the drawing, and a manual entry of the time of observations, measured field strength, and other notes handwritten on each drawing. The data are preserved in an MySQL database.

Results. We provide a brief history of the project and describe the results from digitizing this historical dataset. We also provide a summary of the final dataset and describe its known limitations. Finally, we compare the sunspot magnetic field measurements with those from other instruments, and demonstrate that, if needed, the dataset could be continued using modern observations such as, for example, the Vector Stokes Magnetograph on the Synoptic Optical Long-term Investigations of the Sun platform.