70 years of Sunspot Observations at Kanzelh\"ohe Observatory: systematic study of parameters affecting the derivation of the relative sunspot number

TL;DR

This paper reviews 70 years of sunspot observations at Kanzelhöhe Observatory, analyzing how instrumental and personnel changes impacted the derivation of relative sunspot numbers and their agreement with international standards.

Contribution

It provides a systematic study of historical sunspot data, identifying factors affecting the accuracy of sunspot number derivation and long-term observational trends.

Findings

Relative sunspot numbers at KSO agree within 10% with the International Sunspot Number.

Instrumental changes and observer experience significantly influence sunspot data accuracy.

Long-term improvements in image quality and seeing conditions have been observed since 2000.

Abstract

Kanzelh\"ohe Observatory (KSO) was founded during World War II by the "Deutsche Luftwaffe" (German Airforces) as one station of a network of observatories, which should provide information on solar activity in order to better assess the actual conditions of the Earth's ionosphere in terms of radio wave propagation. The solar observations began in 1943 with photographs of the photosphere, drawings of sunspots, plage regions and faculae, as well as patrol observations of the solar corona. At the beginning all data was sent to Freiburg (Germany). After WWII international cooperation was established and the data was sent to Zurich, Paris, Moscow and Greenwich. Relative sunspot numbers are derived since 1944. The agreement between relative sunspot numbers derived at KSO and the new International Sunspot Number (ISN) \citep{SIDC} lies within $\sim10\%$. However, revisiting the historical data, we also find periods with larger deviations. The reasons for the deviations were twofold: (1) On the one hand a major instrumental change took place during which a relocation and modification of the instrument happened. (2) On the other hand, a period of frequent replacement of personnel caused significant deviations, i.e. stressing the importance of experienced observers. On long-term, the instrumental improvements led to better image quality. Additionally we find a long term trend towards better seeing conditions since the year 2000. We may speculate that this is related to climatic changes.