Reconstructing solar magnetic fields from historical observations

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.

Thaddäus Derfflinger’s Sunspot Observations during 1802–1824: A Primary Reference to Understand the Dalton Minimum

“…The Astrophysical Journal, doi: 10.3847/1538Journal, doi: 10.3847/ -4357/ab65c9 et al, 2007Journal, doi: 10.3847/ , 2011Arlt, 2008Arlt, , 2009Arlt, , 2018Hayakawa et al, 2018aHayakawa et al, , 2018bCarrasco et al, 2015Carrasco et al, , 2016Carrasco et al, , 2018Carrasco et al, , 2019bDenig and McVaugh, 2017), remove apparent continuous spotless days (Vaquero, 2007;Vaquero et al, 2016), and revise observations based on modern viewpoints Svalgaard, 2017;Hayakawa et al, 2018d;Arlt et al, 2013Arlt et al, , 2016Fujiyama et al, 2019;Karoff et al, 2019;Jørgensen et al, 2019;Pevtsov et al 2019). Based on these revisions, a long-term variation of solar activity was evaluated utilising multiple methodologies (Svalgaard and Schatten, 2016;Usoskin et al, 2016;Clette and Lefevre, 2016;Chatzistergos et al, 2017).…”

Sunspot observations by Hisako Koyama: 1945–1996

“…Therefore, a thorough revision of the calibration of both series is currently ongoing on the basis of original observations and using new modern methodologies (Clette et al, 2014Vaquero et al, 2016;Svalgaard and Schatten, 2016;Usoskin et al, 2016;Cliver and Ling, 2016;Chatzistergos et al, 2017;Muñoz-Jaramillo and Vaquero, 2019). While observations were conducted over many decades in various institutional observatories such as Greenwich (Willis et al, 2013(Willis et al, , 2016a(Willis et al, , 2016b, Debrecen (Baranyi et al, 2016;Győri et al, 2017), or Mount Wilson (Lefèvre et al, 2005Pevtsov et al, 2019), long-term observations at smaller observatories by individual astronomers -frequently skilled amateur observers -are of particular importance in terms of stability. Indeed, such sunspot time series have been derived from a single observer, often with a unique telescope, in contrast with professional teams working in shifts and with a staff changing over the years (e.g., Clette et al, 2014;Carrasco et al, 2019).…”

Магнитные Поля Солнечных Пятен: Сравнение Данных КрАО И SDO/HMI

“…2), что свидетельствует о некотором завышении HMI-измерений на слабых полях и, напротив, занижении на сильных. В работе (Pevtsov et al, 2019) показано, что при измерении магнитного поля методом, подобным тому, что используется в КрАО, появляется нелинейность, связанная с использованием стеклянной пластинки для измерения сдвига в направлении дисперсии. Расщепление сигма-компонент ∆x (которое прямо пропорционально полю, см.…”

“…Расщепление сигма-компонент ∆x (которое прямо пропорционально полю, см. выражение (1) в работе Pevtsov et al, 2019) нелинейно связано с измеряемым углом поворота пластинки α (выражение (2) в работе Pevtsov et al, 2019):…”

“…Однако здесь присутствует одна важная особенность, которая не позволяет нам считать этот вид аппроксимации наилучшим и вполне отражающим действительность. А именно, нелинейность между расщеплением и углом поворота характеризуется тем, что она весьма слабая при слабых полях и усиливается при сильных (Pevtsov et al, 2019). Однако распределение данных показывает обратную картину (см.…”