Sunspot areas and tilt angles for solar cycles 7–10

Abstract

Aims. Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims to estimate areas of sunspots observed by Schwabe in 1825−1867 and to calculate the tilt angles of sunspot groups. Methods. The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century. Results. Umbral areas for about 130 000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, average to 5.• 85 ± 0.• 25, with the leading polarity located closer to the equator, in good agreement with tilt angles obtained from 20th century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided at the CDS.

A history of solar activity over millennia

“…2.2.1). Sunspot areas can be reconstructed even before that using drawing of the sun by H. Schwabe for the years 1826-1867 Senthamizh Pavai et al 2015) and images by Spörer for the period 1861-1894 (Diercke et al 2015). In addition, some fragmentary solar drawings exist even for earlier periods, including the Maunder minimum in the seventeenth century (Ribes and Nesme-Ribes 1993;Vaquero et al 2004;Arlt 2008).…”

Wings of the butterfly: Sunspot groups for 1826–2015

“…For each sunspot group we calculated the mean latitude, weighted by the sunspot sizes (umbral area). We used the revised version of the Schwabe data with sunspot groups defined according to modern understanding (for details see Senthamizh Pavai et al 2015). This sunspot grouping is somewhat different from that made originally by Schwabe and later used by Wolf, but it ensures consistency with the RGO dataset.…”

Properties of sunspot cycles and hemispheric wings since the 19th century

“…The fraction of revisited sunspot groups in the total number of sunspot groups is on average less than 10% in the first half of observations, but can be up to 35% in the second half. A detailed description of the group definition and the homogeneity of the data set is given elsewhere by Senthamizh Pavai et al (2015). They noted that the change in the spots-per-group ratio around 1835-1836 might be due to a combined effect of a change in drawing style around 1830-1831, and the solar minimum in 1833, since an increase in the number of spots per group can be seen after other minima as well.…”

“…With the scaling coefficient 12.08, the ratio between WSN-S and GSN-S should be unity throughout the whole series, but it in fact increases above one in the rising phase of cycle 8. This increase is probably related to the sudden increase in spots/group ratio in Schwabe data in 1836 (Senthamizh Pavai et al 2015). The difference between WSN-S and GSN-S over most of the Schwabe time interval indicates a change in the relative normalization of WSN and GSN between the Schwabe data and the sunspot series in the 20th century, which was used to normalize the GSN to WSN.…”

“…This is another indication of the change in spots/group ratio around 1836 within Schwabe's observations. Senthamizh Pavai et al (2015) stated that the change is most likely because of a superposition of two effects: the change in drawing style around 1830-1831 and the recovery of solar activity from the 1833 minimum.…”