Differences in heliographic positions and rotation velocities of sunspot groups from various observatories

Abstract

Measured positions of sunspot groups that differ in format, precision and observing procedure are collected from various data sets: GPR (Greenwich Photoheliographic Results), SOON/USAF/NOAA (Solar Optical Observing Network/United States Air Force/National Oceanic and Atmospheric Administration), as well as from the Kodaikanal and Debrecen observatories. Kanzelh&{uml;o}he and Kandilli Observatory currently provide the digitized sunspot drawings, from which the positions of selected sunspot groups are determined with a special software Sungrabber. The rotation velocities are calculated from the position data. The aim of this work is to compare and to check the precision of the mentioned data sets using the Kanzelh&{uml;o}he Observatory data set as the reference basis of sunspot position measurements. The selected groups (about 40% consist of single sunspots Z&{uml;u}rich types H and J) are from the years 1972 and 1993 belonging to similar declining phases of two solar activity cycles. The occurrence of some systematic differences of the sunspot group positions and rotation velocities suggests the need for a more detailed analysis of the data accumulation procedures.